Groups > linux.kernel > #1701535 > unrolled thread

Re: RCU stall when using function_graph

Started by	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
First post	2017-08-02 00:10 +0200
Last post	2017-08-02 14:50 +0200
Articles	20 on this page of 21 — 4 participants

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  Re: RCU stall when using function_graph "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-02 00:10 +0200
    Re: RCU stall when using function_graph Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-02 00:20 +0200
      Re: RCU stall when using function_graph Steven Rostedt <rostedt@goodmis.org> - 2017-08-02 02:20 +0200
        Re: RCU stall when using function_graph Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-02 14:50 +0200
          Re: RCU stall when using function_graph Steven Rostedt <rostedt@goodmis.org> - 2017-08-02 15:10 +0200
            Re: RCU stall when using function_graph "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-03 04:50 +0200
            Re: RCU stall when using function_graph Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-03 13:50 +0200
              Re: RCU stall when using function_graph "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-03 14:50 +0200
                Re: RCU stall when using function_graph Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-03 16:40 +0200
                  Re: RCU stall when using function_graph "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-06 19:10 +0200
                    Re: RCU stall when using function_graph Pratyush Anand <panand@redhat.com> - 2017-08-09 11:20 +0200
                      Re: RCU stall when using function_graph "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-09 15:00 +0200
                        Re: RCU stall when using function_graph Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-09 15:30 +0200
                          Re: RCU stall when using function_graph "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-09 16:50 +0200
                            Re: RCU stall when using function_graph Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-09 18:00 +0200
                              Re: RCU stall when using function_graph "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-09 19:30 +0200
                                Re: RCU stall when using function_graph Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-10 11:50 +0200
                                  Re: RCU stall when using function_graph "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-10 23:40 +0200
                                    Re: RCU stall when using function_graph Daniel Lezcano <daniel.lezcano@linaro.org> - 2017-08-11 11:40 +0200
          Re: RCU stall when using function_graph "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-02 19:00 +0200
    Re: RCU stall when using function_graph "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-02 14:50 +0200

Page 1 of 2 [1] 2 Next page →

#1701535 — Re: RCU stall when using function_graph

From	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date	2017-08-02 00:10 +0200
Subject	Re: RCU stall when using function_graph
Message-ID	<u9Nwt-1j6-5@gated-at.bofh.it>

> Hi Paul,
> 
> I have been trying to set the function_graph tracer for ftrace and each time I
> get a CPU stall.
> 
> How to reproduce:
> -----------------
> 
> 		 echo function_graph > /sys/kernel/debug/tracing/current_tracer
> 
> This error appears with v4.13-rc3 and v4.12-rc6.
> 
> Is it something already reported ?

I have seen this sort of thing, but only when actually dumping the trace
out, and I though those got fixed.  You are seeing this just accumulating
the trace?

These RCU CPU stall warnings usually occur when something grabs hold of
a CPU for too long, as in 21 seconds or so.  One way that they can happen
is excessive lock contention, another is having the kernel run through
too much data at one shot.

Adding Steven Rostedt on CC for his thoughts.

							Thanx, Paul

> [ ... ]
> 
> [  472.934986] INFO: rcu_preempt detected stalls on CPUs/tasks:
> [  472.941302]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=3195
> [  472.948982]  (detected by 3, t=6502 jiffies, g=549, c=548, q=10)
> [  472.955180] Task dump for CPU 0:
> [  472.958513] swapper/0       R  running task        0     0      0 0x00000002
> [  550.954786] INFO: rcu_preempt detected stalls on CPUs/tasks:
> [  550.960657]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=12926
> [  550.967897]  (detected by 3, t=26007 jiffies, g=549, c=548, q=27)
> [  550.974181] Task dump for CPU 0:
> [  550.977513] swapper/0       R  running task        0     0      0 0x00000002
> [  628.974788] INFO: rcu_preempt detected stalls on CPUs/tasks:
> [  628.980659]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=22666
> [  628.987900]  (detected by 3, t=45512 jiffies, g=549, c=548, q=43)
> [  628.994184] Task dump for CPU 0:
> [  628.997518] swapper/0       R  running task        0     0      0 0x00000002
> [  706.994787] INFO: rcu_preempt detected stalls on CPUs/tasks:
> [  707.000658]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=31785
> [  707.007899]  (detected by 3, t=65017 jiffies, g=549, c=548, q=74)
> [  707.014183] Task dump for CPU 0:
> [  707.017516] swapper/0       R  running task        0     0      0 0x00000002
> [  707.024902] rcu_preempt kthread starved for 1261 jiffies! g549 c548 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
> [  707.034548] rcu_preempt     S    0     8      2 0x00000000
> [  726.049583] INFO: task jbd2/mmcblk1p2-:1120 blocked for more than 120 seconds.
> [  726.061772]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> [  726.074294] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [  726.089027] jbd2/mmcblk1p2- D    0  1120      2 0x00000000
> [  726.126737] INFO: task rs:main Q:Reg:2272 blocked for more than 120 seconds.
> [  726.140484]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> [  726.154515] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [  726.170140] rs:main Q:Reg   D    0  2272      1 0x00000000
> [  726.205626]
> [  726.205626] Showing all locks held in the system:
> [  726.272952]
> [  726.278194] =============================================
> [  726.278194]
> [  785.014779] INFO: rcu_preempt detected stalls on CPUs/tasks:
> [  785.020658]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=41387
> [  785.027905]  (detected by 3, t=84524 jiffies, g=549, c=548, q=94)
> [  785.034194] Task dump for CPU 0:
> [  785.037532] swapper/0       R  running task        0     0      0 0x00000002
> [  846.879753] INFO: task jbd2/mmcblk1p2-:1120 blocked for more than 120 seconds.
> [  846.892966]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> [  846.909597] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [  846.926726] jbd2/mmcblk1p2- D    0  1120      2 0x00000000
> [  846.981356] INFO: task rs:main Q:Reg:2272 blocked for more than 120 seconds.
> [  847.001593]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> [  847.017642] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [  847.034135] rs:main Q:Reg   D    0  2272      1 0x00000000
> [  847.077209]
> [  847.077209] Showing all locks held in the system:
> [  847.180736]
> [  847.186753] =============================================
> [  847.186753]
> [  863.034778] INFO: rcu_preempt detected stalls on CPUs/tasks:
> [  863.040660]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=50881
> [  863.047907]  (detected by 4, t=104030 jiffies, g=549, c=548, q=114)
> [  863.054373] Task dump for CPU 0:
> [  863.057711] swapper/0       R  running task        0     0      0 0x00000002
> [  941.054780] INFO: rcu_preempt detected stalls on CPUs/tasks:
> [  941.060659]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=59726
> [  941.067903]  (detected by 7, t=123535 jiffies, g=549, c=548, q=303)
> [  941.074370] Task dump for CPU 0:
> [  941.077708] swapper/0       R  running task        0     0      0 0x00000002
> [  967.711682] INFO: task jbd2/mmcblk1p2-:1120 blocked for more than 120 seconds.
> [  967.725731]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> [  967.743347] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [  967.765658] jbd2/mmcblk1p2- D    0  1120      2 0x00000000
> [  967.810595] INFO: task rs:main Q:Reg:2272 blocked for more than 120 seconds.
> [  967.825360]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> [  967.838595] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [  967.854460] rs:main Q:Reg   D    0  2272      1 0x00000000
> [  967.897804]
> [  967.897804] Showing all locks held in the system:
> [  967.997067]
> [  968.002570] =============================================
> 
> 
> [ ... ]
> 
> etc ...

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#1701541

From	Daniel Lezcano <daniel.lezcano@linaro.org>
Date	2017-08-02 00:20 +0200
Message-ID	<u9NGa-1oh-15@gated-at.bofh.it>
In reply to	#1701535

On 02/08/2017 00:04, Paul E. McKenney wrote:
>> Hi Paul,
>>
>> I have been trying to set the function_graph tracer for ftrace and each time I
>> get a CPU stall.
>>
>> How to reproduce:
>> -----------------
>>
>> 		 echo function_graph > /sys/kernel/debug/tracing/current_tracer
>>
>> This error appears with v4.13-rc3 and v4.12-rc6.
>>
>> Is it something already reported ?
> 
> I have seen this sort of thing, but only when actually dumping the trace
> out, and I though those got fixed.  You are seeing this just accumulating
> the trace?

No, just by changing the tracer. It is the first operation I do after
rebooting and it is reproducible each time. That happens on an ARM64
platform.

> These RCU CPU stall warnings usually occur when something grabs hold of
> a CPU for too long, as in 21 seconds or so.  One way that they can happen
> is excessive lock contention, another is having the kernel run through
> too much data at one shot.
> 
> Adding Steven Rostedt on CC for his thoughts.
> 
> 							Thanx, Paul
> 
>> [ ... ]
>>
>> [  472.934986] INFO: rcu_preempt detected stalls on CPUs/tasks:
>> [  472.941302]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=3195
>> [  472.948982]  (detected by 3, t=6502 jiffies, g=549, c=548, q=10)
>> [  472.955180] Task dump for CPU 0:
>> [  472.958513] swapper/0       R  running task        0     0      0 0x00000002
>> [  550.954786] INFO: rcu_preempt detected stalls on CPUs/tasks:
>> [  550.960657]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=12926
>> [  550.967897]  (detected by 3, t=26007 jiffies, g=549, c=548, q=27)
>> [  550.974181] Task dump for CPU 0:
>> [  550.977513] swapper/0       R  running task        0     0      0 0x00000002
>> [  628.974788] INFO: rcu_preempt detected stalls on CPUs/tasks:
>> [  628.980659]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=22666
>> [  628.987900]  (detected by 3, t=45512 jiffies, g=549, c=548, q=43)
>> [  628.994184] Task dump for CPU 0:
>> [  628.997518] swapper/0       R  running task        0     0      0 0x00000002
>> [  706.994787] INFO: rcu_preempt detected stalls on CPUs/tasks:
>> [  707.000658]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=31785
>> [  707.007899]  (detected by 3, t=65017 jiffies, g=549, c=548, q=74)
>> [  707.014183] Task dump for CPU 0:
>> [  707.017516] swapper/0       R  running task        0     0      0 0x00000002
>> [  707.024902] rcu_preempt kthread starved for 1261 jiffies! g549 c548 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
>> [  707.034548] rcu_preempt     S    0     8      2 0x00000000
>> [  726.049583] INFO: task jbd2/mmcblk1p2-:1120 blocked for more than 120 seconds.
>> [  726.061772]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
>> [  726.074294] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [  726.089027] jbd2/mmcblk1p2- D    0  1120      2 0x00000000
>> [  726.126737] INFO: task rs:main Q:Reg:2272 blocked for more than 120 seconds.
>> [  726.140484]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
>> [  726.154515] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [  726.170140] rs:main Q:Reg   D    0  2272      1 0x00000000
>> [  726.205626]
>> [  726.205626] Showing all locks held in the system:
>> [  726.272952]
>> [  726.278194] =============================================
>> [  726.278194]
>> [  785.014779] INFO: rcu_preempt detected stalls on CPUs/tasks:
>> [  785.020658]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=41387
>> [  785.027905]  (detected by 3, t=84524 jiffies, g=549, c=548, q=94)
>> [  785.034194] Task dump for CPU 0:
>> [  785.037532] swapper/0       R  running task        0     0      0 0x00000002
>> [  846.879753] INFO: task jbd2/mmcblk1p2-:1120 blocked for more than 120 seconds.
>> [  846.892966]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
>> [  846.909597] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [  846.926726] jbd2/mmcblk1p2- D    0  1120      2 0x00000000
>> [  846.981356] INFO: task rs:main Q:Reg:2272 blocked for more than 120 seconds.
>> [  847.001593]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
>> [  847.017642] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [  847.034135] rs:main Q:Reg   D    0  2272      1 0x00000000
>> [  847.077209]
>> [  847.077209] Showing all locks held in the system:
>> [  847.180736]
>> [  847.186753] =============================================
>> [  847.186753]
>> [  863.034778] INFO: rcu_preempt detected stalls on CPUs/tasks:
>> [  863.040660]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=50881
>> [  863.047907]  (detected by 4, t=104030 jiffies, g=549, c=548, q=114)
>> [  863.054373] Task dump for CPU 0:
>> [  863.057711] swapper/0       R  running task        0     0      0 0x00000002
>> [  941.054780] INFO: rcu_preempt detected stalls on CPUs/tasks:
>> [  941.060659]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=59726
>> [  941.067903]  (detected by 7, t=123535 jiffies, g=549, c=548, q=303)
>> [  941.074370] Task dump for CPU 0:
>> [  941.077708] swapper/0       R  running task        0     0      0 0x00000002
>> [  967.711682] INFO: task jbd2/mmcblk1p2-:1120 blocked for more than 120 seconds.
>> [  967.725731]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
>> [  967.743347] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [  967.765658] jbd2/mmcblk1p2- D    0  1120      2 0x00000000
>> [  967.810595] INFO: task rs:main Q:Reg:2272 blocked for more than 120 seconds.
>> [  967.825360]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
>> [  967.838595] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [  967.854460] rs:main Q:Reg   D    0  2272      1 0x00000000
>> [  967.897804]
>> [  967.897804] Showing all locks held in the system:
>> [  967.997067]
>> [  968.002570] =============================================
>>
>>
>> [ ... ]
>>
>> etc ...
> 


-- 
 <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
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[toc] | [prev] | [next] | [standalone]

#1701599

From	Steven Rostedt <rostedt@goodmis.org>
Date	2017-08-02 02:20 +0200
Message-ID	<u9Pyh-2wR-3@gated-at.bofh.it>
In reply to	#1701541

On Wed, 2 Aug 2017 00:15:44 +0200
Daniel Lezcano <daniel.lezcano@linaro.org> wrote:

> On 02/08/2017 00:04, Paul E. McKenney wrote:
> >> Hi Paul,
> >>
> >> I have been trying to set the function_graph tracer for ftrace and each time I
> >> get a CPU stall.
> >>
> >> How to reproduce:
> >> -----------------
> >>
> >> 		 echo function_graph > /sys/kernel/debug/tracing/current_tracer
> >>
> >> This error appears with v4.13-rc3 and v4.12-rc6.

Can you bisect this? It may be due to this commit:

0598e4f08 ("ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines")

-- Steve

> >>
> >> Is it something already reported ?  
> > 
> > I have seen this sort of thing, but only when actually dumping the trace
> > out, and I though those got fixed.  You are seeing this just accumulating
> > the trace?  
> 
> No, just by changing the tracer. It is the first operation I do after
> rebooting and it is reproducible each time. That happens on an ARM64
> platform.
> 
> > These RCU CPU stall warnings usually occur when something grabs hold of
> > a CPU for too long, as in 21 seconds or so.  One way that they can happen
> > is excessive lock contention, another is having the kernel run through
> > too much data at one shot.
> > 
> > Adding Steven Rostedt on CC for his thoughts.
> > 
>

[toc] | [prev] | [next] | [standalone]

#1702086

From	Daniel Lezcano <daniel.lezcano@linaro.org>
Date	2017-08-02 14:50 +0200
Message-ID	<ua1g6-1vG-7@gated-at.bofh.it>
In reply to	#1701599

On Tue, Aug 01, 2017 at 08:12:14PM -0400, Steven Rostedt wrote:
> On Wed, 2 Aug 2017 00:15:44 +0200
> Daniel Lezcano <daniel.lezcano@linaro.org> wrote:
> 
> > On 02/08/2017 00:04, Paul E. McKenney wrote:
> > >> Hi Paul,
> > >>
> > >> I have been trying to set the function_graph tracer for ftrace and each time I
> > >> get a CPU stall.
> > >>
> > >> How to reproduce:
> > >> -----------------
> > >>
> > >> 		 echo function_graph > /sys/kernel/debug/tracing/current_tracer
> > >>
> > >> This error appears with v4.13-rc3 and v4.12-rc6.
> 
> Can you bisect this? It may be due to this commit:
> 
> 0598e4f08 ("ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines")

Hi Steve,

I git bisected but each time the issue occured. I went through the different
version down to v4.4 where the board was not fully supported and it ended up to
have the same issue.

Finally, I had the intuition it could be related to the wall time (there is no
RTC clock with battery on the board and the wall time is Jan 1st, 1970).

Setting up the with ntpdate solved the problem.

Even if it is rarely the case to have the time not set, is it normal to have a
RCU cpu stall ?

> > >>
> > >> Is it something already reported ?  
> > > 
> > > I have seen this sort of thing, but only when actually dumping the trace
> > > out, and I though those got fixed.  You are seeing this just accumulating
> > > the trace?  
> > 
> > No, just by changing the tracer. It is the first operation I do after
> > rebooting and it is reproducible each time. That happens on an ARM64
> > platform.
> > 
> > > These RCU CPU stall warnings usually occur when something grabs hold of
> > > a CPU for too long, as in 21 seconds or so.  One way that they can happen
> > > is excessive lock contention, another is having the kernel run through
> > > too much data at one shot.
> > > 
> > > Adding Steven Rostedt on CC for his thoughts.
> > > 
> >

-- 

 <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog

[toc] | [prev] | [next] | [standalone]

#1702095

From	Steven Rostedt <rostedt@goodmis.org>
Date	2017-08-02 15:10 +0200
Message-ID	<ua1zs-1Rq-19@gated-at.bofh.it>
In reply to	#1702086

On Wed, 2 Aug 2017 14:42:39 +0200
Daniel Lezcano <daniel.lezcano@linaro.org> wrote:

> On Tue, Aug 01, 2017 at 08:12:14PM -0400, Steven Rostedt wrote:
> > On Wed, 2 Aug 2017 00:15:44 +0200
> > Daniel Lezcano <daniel.lezcano@linaro.org> wrote:
> >   
> > > On 02/08/2017 00:04, Paul E. McKenney wrote:  
> > > >> Hi Paul,
> > > >>
> > > >> I have been trying to set the function_graph tracer for ftrace and each time I
> > > >> get a CPU stall.
> > > >>
> > > >> How to reproduce:
> > > >> -----------------
> > > >>
> > > >> 		 echo function_graph > /sys/kernel/debug/tracing/current_tracer
> > > >>
> > > >> This error appears with v4.13-rc3 and v4.12-rc6.  
> > 
> > Can you bisect this? It may be due to this commit:
> > 
> > 0598e4f08 ("ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines")  
> 
> Hi Steve,
> 
> I git bisected but each time the issue occured. I went through the different
> version down to v4.4 where the board was not fully supported and it ended up to
> have the same issue.
> 
> Finally, I had the intuition it could be related to the wall time (there is no
> RTC clock with battery on the board and the wall time is Jan 1st, 1970).
> 
> Setting up the with ntpdate solved the problem.
> 
> Even if it is rarely the case to have the time not set, is it normal to have a
> RCU cpu stall ?
> 
>

BTW, function_graph tracer is the most invasive of the tracers. It's 4x
slower than function tracer. I'm wondering if the tracer isn't the
cause, but just slows things down enough to cause a some other race
condition that triggers the bug.

-- Steve

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#1702634

From	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date	2017-08-03 04:50 +0200
Message-ID	<uaen0-1Y5-5@gated-at.bofh.it>
In reply to	#1702095

On Wed, Aug 02, 2017 at 09:07:44AM -0400, Steven Rostedt wrote:
> On Wed, 2 Aug 2017 14:42:39 +0200
> Daniel Lezcano <daniel.lezcano@linaro.org> wrote:
> 
> > On Tue, Aug 01, 2017 at 08:12:14PM -0400, Steven Rostedt wrote:
> > > On Wed, 2 Aug 2017 00:15:44 +0200
> > > Daniel Lezcano <daniel.lezcano@linaro.org> wrote:
> > >   
> > > > On 02/08/2017 00:04, Paul E. McKenney wrote:  
> > > > >> Hi Paul,
> > > > >>
> > > > >> I have been trying to set the function_graph tracer for ftrace and each time I
> > > > >> get a CPU stall.
> > > > >>
> > > > >> How to reproduce:
> > > > >> -----------------
> > > > >>
> > > > >> 		 echo function_graph > /sys/kernel/debug/tracing/current_tracer
> > > > >>
> > > > >> This error appears with v4.13-rc3 and v4.12-rc6.  
> > > 
> > > Can you bisect this? It may be due to this commit:
> > > 
> > > 0598e4f08 ("ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines")  
> > 
> > Hi Steve,
> > 
> > I git bisected but each time the issue occured. I went through the different
> > version down to v4.4 where the board was not fully supported and it ended up to
> > have the same issue.
> > 
> > Finally, I had the intuition it could be related to the wall time (there is no
> > RTC clock with battery on the board and the wall time is Jan 1st, 1970).
> > 
> > Setting up the with ntpdate solved the problem.
> > 
> > Even if it is rarely the case to have the time not set, is it normal to have a
> > RCU cpu stall ?
> > 
> >
> 
> BTW, function_graph tracer is the most invasive of the tracers. It's 4x
> slower than function tracer. I'm wondering if the tracer isn't the
> cause, but just slows things down enough to cause a some other race
> condition that triggers the bug.

Easy to check!  Use the rcupdate.rcu_cpu_stall_timeout kernel boot
parameter to increase this timeout by a factor of four.  Mainline
default is 21 seconds, but many distros set it to 60 seconds.
You can always check sysfs to find the value for your system, or
CONFIG_RCU_CPU_STALL_TIMEOUT in your .config file.

								Thanx, Paul

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#1702977

From	Daniel Lezcano <daniel.lezcano@linaro.org>
Date	2017-08-03 13:50 +0200
Message-ID	<uamNA-7Pj-11@gated-at.bofh.it>
In reply to	#1702095

On 02/08/2017 15:07, Steven Rostedt wrote:
> On Wed, 2 Aug 2017 14:42:39 +0200
> Daniel Lezcano <daniel.lezcano@linaro.org> wrote:
> 
>> On Tue, Aug 01, 2017 at 08:12:14PM -0400, Steven Rostedt wrote:
>>> On Wed, 2 Aug 2017 00:15:44 +0200
>>> Daniel Lezcano <daniel.lezcano@linaro.org> wrote:
>>>   
>>>> On 02/08/2017 00:04, Paul E. McKenney wrote:  
>>>>>> Hi Paul,
>>>>>>
>>>>>> I have been trying to set the function_graph tracer for ftrace and each time I
>>>>>> get a CPU stall.
>>>>>>
>>>>>> How to reproduce:
>>>>>> -----------------
>>>>>>
>>>>>> 		 echo function_graph > /sys/kernel/debug/tracing/current_tracer
>>>>>>
>>>>>> This error appears with v4.13-rc3 and v4.12-rc6.  
>>>
>>> Can you bisect this? It may be due to this commit:
>>>
>>> 0598e4f08 ("ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines")  
>>
>> Hi Steve,
>>
>> I git bisected but each time the issue occured. I went through the different
>> version down to v4.4 where the board was not fully supported and it ended up to
>> have the same issue.
>>
>> Finally, I had the intuition it could be related to the wall time (there is no
>> RTC clock with battery on the board and the wall time is Jan 1st, 1970).
>>
>> Setting up the with ntpdate solved the problem.

Actually, it did not solve the problem. The function_graph trace is set,
I can use the system but after awhile (no tracing enabled at anytime),
the stall appears.

>> Even if it is rarely the case to have the time not set, is it normal to have a
>> RCU cpu stall ?
>>
>>
> 
> BTW, function_graph tracer is the most invasive of the tracers. It's 4x
> slower than function tracer. I'm wondering if the tracer isn't the
> cause, but just slows things down enough to cause a some other race
> condition that triggers the bug.

Yes, that could be true.

I tried the following scenario:

 - cpufreq governor => userspace + max_freq (1.2GHz)
   - function_graph set ==> OK

 - cpufreq governor => userspace + min_freq (200MHz)
   - function_graph set ==> RCU stall

Beside that, I realize the board is constantly processing SOF interrupts
every 124us, so that adds more overhead.

Removing the USB support, thus the associated processing for the SOF
interrupts, I don't see anymore the RCU stall.

Is it the expected behavior to have the system hang after a RCU stall
raises ?

-- 
 <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog

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#1703078

From	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date	2017-08-03 14:50 +0200
Message-ID	<uanJF-8s6-29@gated-at.bofh.it>
In reply to	#1702977

On Thu, Aug 03, 2017 at 01:41:11PM +0200, Daniel Lezcano wrote:
> On 02/08/2017 15:07, Steven Rostedt wrote:
> > On Wed, 2 Aug 2017 14:42:39 +0200
> > Daniel Lezcano <daniel.lezcano@linaro.org> wrote:
> > 
> >> On Tue, Aug 01, 2017 at 08:12:14PM -0400, Steven Rostedt wrote:
> >>> On Wed, 2 Aug 2017 00:15:44 +0200
> >>> Daniel Lezcano <daniel.lezcano@linaro.org> wrote:
> >>>   
> >>>> On 02/08/2017 00:04, Paul E. McKenney wrote:  
> >>>>>> Hi Paul,
> >>>>>>
> >>>>>> I have been trying to set the function_graph tracer for ftrace and each time I
> >>>>>> get a CPU stall.
> >>>>>>
> >>>>>> How to reproduce:
> >>>>>> -----------------
> >>>>>>
> >>>>>> 		 echo function_graph > /sys/kernel/debug/tracing/current_tracer
> >>>>>>
> >>>>>> This error appears with v4.13-rc3 and v4.12-rc6.  
> >>>
> >>> Can you bisect this? It may be due to this commit:
> >>>
> >>> 0598e4f08 ("ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines")  
> >>
> >> Hi Steve,
> >>
> >> I git bisected but each time the issue occured. I went through the different
> >> version down to v4.4 where the board was not fully supported and it ended up to
> >> have the same issue.
> >>
> >> Finally, I had the intuition it could be related to the wall time (there is no
> >> RTC clock with battery on the board and the wall time is Jan 1st, 1970).
> >>
> >> Setting up the with ntpdate solved the problem.
> 
> Actually, it did not solve the problem. The function_graph trace is set,
> I can use the system but after awhile (no tracing enabled at anytime),
> the stall appears.
> 
> >> Even if it is rarely the case to have the time not set, is it normal to have a
> >> RCU cpu stall ?
> >>
> >>
> > 
> > BTW, function_graph tracer is the most invasive of the tracers. It's 4x
> > slower than function tracer. I'm wondering if the tracer isn't the
> > cause, but just slows things down enough to cause a some other race
> > condition that triggers the bug.
> 
> Yes, that could be true.
> 
> I tried the following scenario:
> 
>  - cpufreq governor => userspace + max_freq (1.2GHz)
>    - function_graph set ==> OK
> 
>  - cpufreq governor => userspace + min_freq (200MHz)
>    - function_graph set ==> RCU stall
> 
> Beside that, I realize the board is constantly processing SOF interrupts
> every 124us, so that adds more overhead.
> 
> Removing the USB support, thus the associated processing for the SOF
> interrupts, I don't see anymore the RCU stall.

Looks like Steve called this one!  ;-)

> Is it the expected behavior to have the system hang after a RCU stall
> raises ?

No, but if NMI stack traces are enabled and there are any NMI problems,
bad things can happen.  In addition, the bulk of output can cause problems
if you have a slow console connection.

							Thanx, Paul

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#1703164

From	Daniel Lezcano <daniel.lezcano@linaro.org>
Date	2017-08-03 16:40 +0200
Message-ID	<uaps5-1gd-7@gated-at.bofh.it>
In reply to	#1703078

On Thu, Aug 03, 2017 at 05:44:21AM -0700, Paul E. McKenney wrote:

[ ... ]

> > > BTW, function_graph tracer is the most invasive of the tracers. It's 4x
> > > slower than function tracer. I'm wondering if the tracer isn't the
> > > cause, but just slows things down enough to cause a some other race
> > > condition that triggers the bug.
> > 
> > Yes, that could be true.
> > 
> > I tried the following scenario:
> > 
> >  - cpufreq governor => userspace + max_freq (1.2GHz)
> >    - function_graph set ==> OK
> > 
> >  - cpufreq governor => userspace + min_freq (200MHz)
> >    - function_graph set ==> RCU stall
> > 
> > Beside that, I realize the board is constantly processing SOF interrupts
> > every 124us, so that adds more overhead.
> > 
> > Removing the USB support, thus the associated processing for the SOF
> > interrupts, I don't see anymore the RCU stall.
> 
> Looks like Steve called this one!  ;-)

Yep :)

> > Is it the expected behavior to have the system hang after a RCU stall
> > raises ?
> 
> No, but if NMI stack traces are enabled and there are any NMI problems,
> bad things can happen.  In addition, the bulk of output can cause problems
> if you have a slow console connection.

Ok, thanks.

  -- Daniel

-- 

 <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog

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#1704894

From	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date	2017-08-06 19:10 +0200
Message-ID	<ubxdU-50T-33@gated-at.bofh.it>
In reply to	#1703164

On Sat, Aug 05, 2017 at 02:24:21PM +0900, 김동현 wrote:
> Dear All
> 
> As for me, after configuring function_graph as below, crash disappears.
> "echo 0 > d/tracing/tracing_on"
> "sleep 1"
> 
> "echo function_graph > d/tracing/current_tracer"
> "sleep 1"
> 
> "echo smp_call_function_single > d/tracing/set_ftrace_filter"
> adb shell "sleep 1"
> 
> "echo 1 > d/tracing/tracing_on"
> adb shell "sleep 1"
> 
> Right after function_graph is enabled, too many logs are traced upon IRQ
> transaction which many times eventually causes stall.

That would do it!

Hmmm...

Steven, would it be helpful if RCU were to inform tracing (say) halfway
through the RCU CPU stall interval, allowing the tracer to do something
like cond_resched_rcu_qs()?  I can imagine all sorts of reasons why this
wouldn't work, for example, if all the tracing was with irqs disabled
or some such, but figured I should ask.

Does Guillermo's approach work for others?

							Thanx, Paul

> BR,
> Guillermo Austin Kim
> 
> 2017. 8. 3. 오후 11:38에 "Daniel Lezcano" <daniel.lezcano@linaro.org>님이 작성:
> 
> On Thu, Aug 03, 2017 at 05:44:21AM -0700, Paul E. McKenney wrote:
> 
> [ ... ]
> 
> > > > BTW, function_graph tracer is the most invasive of the tracers. It's
> 4x
> > > > slower than function tracer. I'm wondering if the tracer isn't the
> > > > cause, but just slows things down enough to cause a some other race
> > > > condition that triggers the bug.
> > >
> > > Yes, that could be true.
> > >
> > > I tried the following scenario:
> > >
> > >  - cpufreq governor => userspace + max_freq (1.2GHz)
> > >    - function_graph set ==> OK
> > >
> > >  - cpufreq governor => userspace + min_freq (200MHz)
> > >    - function_graph set ==> RCU stall
> > >
> > > Beside that, I realize the board is constantly processing SOF interrupts
> > > every 124us, so that adds more overhead.
> > >
> > > Removing the USB support, thus the associated processing for the SOF
> > > interrupts, I don't see anymore the RCU stall.
> >
> > Looks like Steve called this one!  ;-)
> 
> Yep :)
> 
> > > Is it the expected behavior to have the system hang after a RCU stall
> > > raises ?
> >
> > No, but if NMI stack traces are enabled and there are any NMI problems,
> > bad things can happen.  In addition, the bulk of output can cause problems
> > if you have a slow console connection.
> 
> Ok, thanks.
> 
>   -- Daniel
> 
> --
> 
>  <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs
> 
> Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
> <http://twitter.com/#!/linaroorg> Twitter |
> <http://www.linaro.org/linaro-blog/> Blog

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#1707173

From	Pratyush Anand <panand@redhat.com>
Date	2017-08-09 11:20 +0200
Message-ID	<ucvjI-59r-5@gated-at.bofh.it>
In reply to	#1704894


On Sunday 06 August 2017 10:32 PM, Paul E. McKenney wrote:
> On Sat, Aug 05, 2017 at 02:24:21PM +0900, 김동현 wrote:
>> Dear All
>>
>> As for me, after configuring function_graph as below, crash disappears.
>> "echo 0 > d/tracing/tracing_on"
>> "sleep 1"
>>
>> "echo function_graph > d/tracing/current_tracer"
>> "sleep 1"
>>
>> "echo smp_call_function_single > d/tracing/set_ftrace_filter"

It will limit trace output to only for the filtered function 
(smp_call_function_single).

>> adb shell "sleep 1"
>>
>> "echo 1 > d/tracing/tracing_on"
>> adb shell "sleep 1"
>>
>> Right after function_graph is enabled, too many logs are traced upon IRQ
>> transaction which many times eventually causes stall.
> 
> That would do it!
> 
> Hmmm...
> 
> Steven, would it be helpful if RCU were to inform tracing (say) halfway
> through the RCU CPU stall interval, allowing the tracer to do something
> like cond_resched_rcu_qs()?  I can imagine all sorts of reasons why this
> wouldn't work, for example, if all the tracing was with irqs disabled
> or some such, but figured I should ask.
> 
> Does Guillermo's approach work for others?

Limited output with a couple of filtered function will definitely not cause 
RCU schedule stall. But the question is whether we should expect a full 
function graph trace working on every platform or not (specially the one which 
generates high interrupts)?

-- 
Regards
Pratyush

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#1707326

From	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date	2017-08-09 15:00 +0200
Message-ID	<ucyKB-7af-1@gated-at.bofh.it>
In reply to	#1707173

On Wed, Aug 09, 2017 at 02:43:49PM +0530, Pratyush Anand wrote:
> 
> 
> On Sunday 06 August 2017 10:32 PM, Paul E. McKenney wrote:
> >On Sat, Aug 05, 2017 at 02:24:21PM +0900, 김동현 wrote:
> >>Dear All
> >>
> >>As for me, after configuring function_graph as below, crash disappears.
> >>"echo 0 > d/tracing/tracing_on"
> >>"sleep 1"
> >>
> >>"echo function_graph > d/tracing/current_tracer"
> >>"sleep 1"
> >>
> >>"echo smp_call_function_single > d/tracing/set_ftrace_filter"
> 
> It will limit trace output to only for the filtered function
> (smp_call_function_single).
> 
> >>adb shell "sleep 1"
> >>
> >>"echo 1 > d/tracing/tracing_on"
> >>adb shell "sleep 1"
> >>
> >>Right after function_graph is enabled, too many logs are traced upon IRQ
> >>transaction which many times eventually causes stall.
> >
> >That would do it!
> >
> >Hmmm...
> >
> >Steven, would it be helpful if RCU were to inform tracing (say) halfway
> >through the RCU CPU stall interval, allowing the tracer to do something
> >like cond_resched_rcu_qs()?  I can imagine all sorts of reasons why this
> >wouldn't work, for example, if all the tracing was with irqs disabled
> >or some such, but figured I should ask.
> >
> >Does Guillermo's approach work for others?
> 
> Limited output with a couple of filtered function will definitely
> not cause RCU schedule stall. But the question is whether we should
> expect a full function graph trace working on every platform or not
> (specially the one which generates high interrupts)?

It might well be that the user must disable RCU CPU stall warnings via
the rcu_cpu_stall_suppress sysfs entry (or increase their timeout via the
rcu_cpu_stall_timeout sysfs entry) before doing something that greatly
increases overhead.  Like enabling large quantities of tracing.  ;-)

It -might- be possible to do this automatically, but reliable
automation would require that tracing understand how often each
function was called, which sounds to me to be a bit of a stretch.

Thoughts?

							Thanx, Paul

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#1707335

From	Daniel Lezcano <daniel.lezcano@linaro.org>
Date	2017-08-09 15:30 +0200
Message-ID	<uczdD-7A3-3@gated-at.bofh.it>
In reply to	#1707326

On 09/08/2017 14:58, Paul E. McKenney wrote:
> On Wed, Aug 09, 2017 at 02:43:49PM +0530, Pratyush Anand wrote:
>>
>>
>> On Sunday 06 August 2017 10:32 PM, Paul E. McKenney wrote:
>>> On Sat, Aug 05, 2017 at 02:24:21PM +0900, 김동현 wrote:
>>>> Dear All
>>>>
>>>> As for me, after configuring function_graph as below, crash disappears.
>>>> "echo 0 > d/tracing/tracing_on"
>>>> "sleep 1"
>>>>
>>>> "echo function_graph > d/tracing/current_tracer"
>>>> "sleep 1"
>>>>
>>>> "echo smp_call_function_single > d/tracing/set_ftrace_filter"
>>
>> It will limit trace output to only for the filtered function
>> (smp_call_function_single).
>>
>>>> adb shell "sleep 1"
>>>>
>>>> "echo 1 > d/tracing/tracing_on"
>>>> adb shell "sleep 1"
>>>>
>>>> Right after function_graph is enabled, too many logs are traced upon IRQ
>>>> transaction which many times eventually causes stall.
>>>
>>> That would do it!
>>>
>>> Hmmm...
>>>
>>> Steven, would it be helpful if RCU were to inform tracing (say) halfway
>>> through the RCU CPU stall interval, allowing the tracer to do something
>>> like cond_resched_rcu_qs()?  I can imagine all sorts of reasons why this
>>> wouldn't work, for example, if all the tracing was with irqs disabled
>>> or some such, but figured I should ask.
>>>
>>> Does Guillermo's approach work for others?
>>
>> Limited output with a couple of filtered function will definitely
>> not cause RCU schedule stall. But the question is whether we should
>> expect a full function graph trace working on every platform or not
>> (specially the one which generates high interrupts)?
> 
> It might well be that the user must disable RCU CPU stall warnings via
> the rcu_cpu_stall_suppress sysfs entry (or increase their timeout via th
> rcu_cpu_stall_timeout sysfs entry) before doing something that greatly
> increases overhead.  Like enabling large quantities of tracing.  ;-)
> 
> It -might- be possible to do this automatically, but reliable
> automation would require that tracing understand how often each
> function was called, which sounds to me to be a bit of a stretch.
> 
> Thoughts?

A random thought:

Is it possible to have a mid-timeout happening and store some
information like the instruction pointer, so when the timeout happen we
can compare if there was some progress, if yes, very likely, system
performance collapsed and we are not fast enough.

-- 
 <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog

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#1707382

From	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date	2017-08-09 16:50 +0200
Message-ID	<ucAt4-fu-5@gated-at.bofh.it>
In reply to	#1707335

On Wed, Aug 09, 2017 at 03:28:05PM +0200, Daniel Lezcano wrote:
> On 09/08/2017 14:58, Paul E. McKenney wrote:
> > On Wed, Aug 09, 2017 at 02:43:49PM +0530, Pratyush Anand wrote:
> >>
> >>
> >> On Sunday 06 August 2017 10:32 PM, Paul E. McKenney wrote:
> >>> On Sat, Aug 05, 2017 at 02:24:21PM +0900, 김동현 wrote:
> >>>> Dear All
> >>>>
> >>>> As for me, after configuring function_graph as below, crash disappears.
> >>>> "echo 0 > d/tracing/tracing_on"
> >>>> "sleep 1"
> >>>>
> >>>> "echo function_graph > d/tracing/current_tracer"
> >>>> "sleep 1"
> >>>>
> >>>> "echo smp_call_function_single > d/tracing/set_ftrace_filter"
> >>
> >> It will limit trace output to only for the filtered function
> >> (smp_call_function_single).
> >>
> >>>> adb shell "sleep 1"
> >>>>
> >>>> "echo 1 > d/tracing/tracing_on"
> >>>> adb shell "sleep 1"
> >>>>
> >>>> Right after function_graph is enabled, too many logs are traced upon IRQ
> >>>> transaction which many times eventually causes stall.
> >>>
> >>> That would do it!
> >>>
> >>> Hmmm...
> >>>
> >>> Steven, would it be helpful if RCU were to inform tracing (say) halfway
> >>> through the RCU CPU stall interval, allowing the tracer to do something
> >>> like cond_resched_rcu_qs()?  I can imagine all sorts of reasons why this
> >>> wouldn't work, for example, if all the tracing was with irqs disabled
> >>> or some such, but figured I should ask.
> >>>
> >>> Does Guillermo's approach work for others?
> >>
> >> Limited output with a couple of filtered function will definitely
> >> not cause RCU schedule stall. But the question is whether we should
> >> expect a full function graph trace working on every platform or not
> >> (specially the one which generates high interrupts)?
> > 
> > It might well be that the user must disable RCU CPU stall warnings via
> > the rcu_cpu_stall_suppress sysfs entry (or increase their timeout via th
> > rcu_cpu_stall_timeout sysfs entry) before doing something that greatly
> > increases overhead.  Like enabling large quantities of tracing.  ;-)
> > 
> > It -might- be possible to do this automatically, but reliable
> > automation would require that tracing understand how often each
> > function was called, which sounds to me to be a bit of a stretch.
> > 
> > Thoughts?
> 
> A random thought:
> 
> Is it possible to have a mid-timeout happening and store some
> information like the instruction pointer, so when the timeout happen we
> can compare if there was some progress, if yes, very likely, system
> performance collapsed and we are not fast enough.

RCU already does take various actions for an impending stall, so something
could be done.  But in most slowdowns, the instruction pointer will be
changing rapidly, just not as rapidly as it would normally.  So exactly
how would the forward-progress comparison be carried out?

It would be easy to set up a notifier, so that if any notifier in the
chain returned an error, stall warnings would be suppressed.  It would
be harder to figure out when to re-enable them, though I suppose that
they could be suppressed only for the duration of the current grace
period or some such.

But what exactly would you use such a notifier for?

Or am I misunderstanding your suggestion?

							Thanx, Paul

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#1707439

From	Daniel Lezcano <daniel.lezcano@linaro.org>
Date	2017-08-09 18:00 +0200
Message-ID	<ucByO-T5-13@gated-at.bofh.it>
In reply to	#1707382

On 09/08/2017 16:40, Paul E. McKenney wrote:
> On Wed, Aug 09, 2017 at 03:28:05PM +0200, Daniel Lezcano wrote:
>> On 09/08/2017 14:58, Paul E. McKenney wrote:
>>> On Wed, Aug 09, 2017 at 02:43:49PM +0530, Pratyush Anand wrote:
>>>>
>>>>
>>>> On Sunday 06 August 2017 10:32 PM, Paul E. McKenney wrote:
>>>>> On Sat, Aug 05, 2017 at 02:24:21PM +0900, 김동현 wrote:
>>>>>> Dear All
>>>>>>
>>>>>> As for me, after configuring function_graph as below, crash disappears.
>>>>>> "echo 0 > d/tracing/tracing_on"
>>>>>> "sleep 1"
>>>>>>
>>>>>> "echo function_graph > d/tracing/current_tracer"
>>>>>> "sleep 1"
>>>>>>
>>>>>> "echo smp_call_function_single > d/tracing/set_ftrace_filter"
>>>>
>>>> It will limit trace output to only for the filtered function
>>>> (smp_call_function_single).
>>>>
>>>>>> adb shell "sleep 1"
>>>>>>
>>>>>> "echo 1 > d/tracing/tracing_on"
>>>>>> adb shell "sleep 1"
>>>>>>
>>>>>> Right after function_graph is enabled, too many logs are traced upon IRQ
>>>>>> transaction which many times eventually causes stall.
>>>>>
>>>>> That would do it!
>>>>>
>>>>> Hmmm...
>>>>>
>>>>> Steven, would it be helpful if RCU were to inform tracing (say) halfway
>>>>> through the RCU CPU stall interval, allowing the tracer to do something
>>>>> like cond_resched_rcu_qs()?  I can imagine all sorts of reasons why this
>>>>> wouldn't work, for example, if all the tracing was with irqs disabled
>>>>> or some such, but figured I should ask.
>>>>>
>>>>> Does Guillermo's approach work for others?
>>>>
>>>> Limited output with a couple of filtered function will definitely
>>>> not cause RCU schedule stall. But the question is whether we should
>>>> expect a full function graph trace working on every platform or not
>>>> (specially the one which generates high interrupts)?
>>>
>>> It might well be that the user must disable RCU CPU stall warnings via
>>> the rcu_cpu_stall_suppress sysfs entry (or increase their timeout via th
>>> rcu_cpu_stall_timeout sysfs entry) before doing something that greatly
>>> increases overhead.  Like enabling large quantities of tracing.  ;-)
>>>
>>> It -might- be possible to do this automatically, but reliable
>>> automation would require that tracing understand how often each
>>> function was called, which sounds to me to be a bit of a stretch.
>>>
>>> Thoughts?
>>
>> A random thought:
>>
>> Is it possible to have a mid-timeout happening and store some
>> information like the instruction pointer, so when the timeout happen we
>> can compare if there was some progress, if yes, very likely, system
>> performance collapsed and we are not fast enough.
> 
> RCU already does take various actions for an impending stall, so something
> could be done.  But in most slowdowns, the instruction pointer will be
> changing rapidly, just not as rapidly as it would normally.  So exactly
> how would the forward-progress comparison be carried out?
> 
> It would be easy to set up a notifier, so that if any notifier in the
> chain returned an error, stall warnings would be suppressed.  It would
> be harder to figure out when to re-enable them, though I suppose that
> they could be suppressed only for the duration of the current grace
> period or some such.
> 
> But what exactly would you use such a notifier for?
> 
> Or am I misunderstanding your suggestion?

Well, may be the instruction pointer thing is not a good idea.

I learnt from this experience, an overloaded kernel with a lot of
interrupts can hang the console and issue RCU stall.

However, someone else can face the same situation. Even if he reads the
RCU/stallwarn.txt documentation, it will be hard to figure out the issue.

A message telling the grace period can't be reached because we are too
busy processing interrupts would have helped but I understand it is not
easy to implement.

Perhaps, adding a new bullet in the documentation can help:

"If the interrupt processing time is longer than the interval between
each interrupt, the CPU will keep processing the interrupts without
allowing the RCU's grace period kthread. This situation can happen if
there is a highly rated number of interrupts and the function_graph
tracer is enabled".




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#1707622

From	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date	2017-08-09 19:30 +0200
Message-ID	<ucCXV-1Uf-45@gated-at.bofh.it>
In reply to	#1707439

On Wed, Aug 09, 2017 at 05:51:33PM +0200, Daniel Lezcano wrote:
> On 09/08/2017 16:40, Paul E. McKenney wrote:
> > On Wed, Aug 09, 2017 at 03:28:05PM +0200, Daniel Lezcano wrote:
> >> On 09/08/2017 14:58, Paul E. McKenney wrote:
> >>> On Wed, Aug 09, 2017 at 02:43:49PM +0530, Pratyush Anand wrote:
> >>>>
> >>>>
> >>>> On Sunday 06 August 2017 10:32 PM, Paul E. McKenney wrote:
> >>>>> On Sat, Aug 05, 2017 at 02:24:21PM +0900, 김동현 wrote:
> >>>>>> Dear All
> >>>>>>
> >>>>>> As for me, after configuring function_graph as below, crash disappears.
> >>>>>> "echo 0 > d/tracing/tracing_on"
> >>>>>> "sleep 1"
> >>>>>>
> >>>>>> "echo function_graph > d/tracing/current_tracer"
> >>>>>> "sleep 1"
> >>>>>>
> >>>>>> "echo smp_call_function_single > d/tracing/set_ftrace_filter"
> >>>>
> >>>> It will limit trace output to only for the filtered function
> >>>> (smp_call_function_single).
> >>>>
> >>>>>> adb shell "sleep 1"
> >>>>>>
> >>>>>> "echo 1 > d/tracing/tracing_on"
> >>>>>> adb shell "sleep 1"
> >>>>>>
> >>>>>> Right after function_graph is enabled, too many logs are traced upon IRQ
> >>>>>> transaction which many times eventually causes stall.
> >>>>>
> >>>>> That would do it!
> >>>>>
> >>>>> Hmmm...
> >>>>>
> >>>>> Steven, would it be helpful if RCU were to inform tracing (say) halfway
> >>>>> through the RCU CPU stall interval, allowing the tracer to do something
> >>>>> like cond_resched_rcu_qs()?  I can imagine all sorts of reasons why this
> >>>>> wouldn't work, for example, if all the tracing was with irqs disabled
> >>>>> or some such, but figured I should ask.
> >>>>>
> >>>>> Does Guillermo's approach work for others?
> >>>>
> >>>> Limited output with a couple of filtered function will definitely
> >>>> not cause RCU schedule stall. But the question is whether we should
> >>>> expect a full function graph trace working on every platform or not
> >>>> (specially the one which generates high interrupts)?
> >>>
> >>> It might well be that the user must disable RCU CPU stall warnings via
> >>> the rcu_cpu_stall_suppress sysfs entry (or increase their timeout via th
> >>> rcu_cpu_stall_timeout sysfs entry) before doing something that greatly
> >>> increases overhead.  Like enabling large quantities of tracing.  ;-)
> >>>
> >>> It -might- be possible to do this automatically, but reliable
> >>> automation would require that tracing understand how often each
> >>> function was called, which sounds to me to be a bit of a stretch.
> >>>
> >>> Thoughts?
> >>
> >> A random thought:
> >>
> >> Is it possible to have a mid-timeout happening and store some
> >> information like the instruction pointer, so when the timeout happen we
> >> can compare if there was some progress, if yes, very likely, system
> >> performance collapsed and we are not fast enough.
> > 
> > RCU already does take various actions for an impending stall, so something
> > could be done.  But in most slowdowns, the instruction pointer will be
> > changing rapidly, just not as rapidly as it would normally.  So exactly
> > how would the forward-progress comparison be carried out?
> > 
> > It would be easy to set up a notifier, so that if any notifier in the
> > chain returned an error, stall warnings would be suppressed.  It would
> > be harder to figure out when to re-enable them, though I suppose that
> > they could be suppressed only for the duration of the current grace
> > period or some such.
> > 
> > But what exactly would you use such a notifier for?
> > 
> > Or am I misunderstanding your suggestion?
> 
> Well, may be the instruction pointer thing is not a good idea.
> 
> I learnt from this experience, an overloaded kernel with a lot of
> interrupts can hang the console and issue RCU stall.
> 
> However, someone else can face the same situation. Even if he reads the
> RCU/stallwarn.txt documentation, it will be hard to figure out the issue.
> 
> A message telling the grace period can't be reached because we are too
> busy processing interrupts would have helped but I understand it is not
> easy to implement.
> 
> Perhaps, adding a new bullet in the documentation can help:
> 
> "If the interrupt processing time is longer than the interval between
> each interrupt, the CPU will keep processing the interrupts without
> allowing the RCU's grace period kthread. This situation can happen if
> there is a highly rated number of interrupts and the function_graph
> tracer is enabled".

How about this?

Any other debugging options that should be called out?  I bet that
the function_graph tracer isn't the only way to make this happen.

							Thanx, Paul

------------------------------------------------------------------------

commit 8b12d9919f59fe6855429e2eacc1c2e03ecdfe96
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Wed Aug 9 10:16:29 2017 -0700

    documentation: Long-running irq handlers can stall RCU grace periods
    
    If a periodic interrupt's handler takes longer to execute than the period
    between successive interrupts, RCU's kthreads and softirq handlers can
    be prevented from executing, resulting in otherwise inexplicable RCU
    CPU stall warnings.  This commit therefore calls out this possibility
    in Documentation/RCU/stallwarn.txt.
    
    Reported-by: Daniel Lezcano <daniel.lezcano@linaro.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 96a3d81837e1..21b8913acbdf 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -40,7 +40,9 @@ o	Booting Linux using a console connection that is too slow to
 o	Anything that prevents RCU's grace-period kthreads from running.
 	This can result in the "All QSes seen" console-log message.
 	This message will include information on when the kthread last
-	ran and how often it should be expected to run.
+	ran and how often it should be expected to run.  It can also
+	result in the "rcu_.*kthread starved for" console-log message,
+	which will include additional debugging information.
 
 o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
 	happen to preempt a low-priority task in the middle of an RCU
@@ -60,6 +62,14 @@ o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
 	CONFIG_PREEMPT_RCU case, you might see stall-warning
 	messages.
 
+o	A periodic interrupt whose handler takes longer than the time
+	interval between successive pairs of interrupts.  This can
+	prevent RCU's kthreads and softirq handlers from running.
+	Note that certain high-overhead debugging options, for example
+	the function_graph tracer, can result in interrupt handler taking
+	considerably longer than normal, which can in turn result in
+	RCU CPU stall warnings.
+
 o	A hardware or software issue shuts off the scheduler-clock
 	interrupt on a CPU that is not in dyntick-idle mode.  This
 	problem really has happened, and seems to be most likely to

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#1708382

From	Daniel Lezcano <daniel.lezcano@linaro.org>
Date	2017-08-10 11:50 +0200
Message-ID	<ucSgi-3P5-15@gated-at.bofh.it>
In reply to	#1707622

On 09/08/2017 19:22, Paul E. McKenney wrote:
> On Wed, Aug 09, 2017 at 05:51:33PM +0200, Daniel Lezcano wrote:
>> On 09/08/2017 16:40, Paul E. McKenney wrote:
>>> On Wed, Aug 09, 2017 at 03:28:05PM +0200, Daniel Lezcano wrote:
>>>> On 09/08/2017 14:58, Paul E. McKenney wrote:
>>>>> On Wed, Aug 09, 2017 at 02:43:49PM +0530, Pratyush Anand wrote:
>>>>>>
>>>>>>
>>>>>> On Sunday 06 August 2017 10:32 PM, Paul E. McKenney wrote:
>>>>>>> On Sat, Aug 05, 2017 at 02:24:21PM +0900, 김동현 wrote:
>>>>>>>> Dear All
>>>>>>>>
>>>>>>>> As for me, after configuring function_graph as below, crash disappears.
>>>>>>>> "echo 0 > d/tracing/tracing_on"
>>>>>>>> "sleep 1"
>>>>>>>>
>>>>>>>> "echo function_graph > d/tracing/current_tracer"
>>>>>>>> "sleep 1"
>>>>>>>>
>>>>>>>> "echo smp_call_function_single > d/tracing/set_ftrace_filter"
>>>>>>
>>>>>> It will limit trace output to only for the filtered function
>>>>>> (smp_call_function_single).
>>>>>>
>>>>>>>> adb shell "sleep 1"
>>>>>>>>
>>>>>>>> "echo 1 > d/tracing/tracing_on"
>>>>>>>> adb shell "sleep 1"
>>>>>>>>
>>>>>>>> Right after function_graph is enabled, too many logs are traced upon IRQ
>>>>>>>> transaction which many times eventually causes stall.
>>>>>>>
>>>>>>> That would do it!
>>>>>>>
>>>>>>> Hmmm...
>>>>>>>
>>>>>>> Steven, would it be helpful if RCU were to inform tracing (say) halfway
>>>>>>> through the RCU CPU stall interval, allowing the tracer to do something
>>>>>>> like cond_resched_rcu_qs()?  I can imagine all sorts of reasons why this
>>>>>>> wouldn't work, for example, if all the tracing was with irqs disabled
>>>>>>> or some such, but figured I should ask.
>>>>>>>
>>>>>>> Does Guillermo's approach work for others?
>>>>>>
>>>>>> Limited output with a couple of filtered function will definitely
>>>>>> not cause RCU schedule stall. But the question is whether we should
>>>>>> expect a full function graph trace working on every platform or not
>>>>>> (specially the one which generates high interrupts)?
>>>>>
>>>>> It might well be that the user must disable RCU CPU stall warnings via
>>>>> the rcu_cpu_stall_suppress sysfs entry (or increase their timeout via th
>>>>> rcu_cpu_stall_timeout sysfs entry) before doing something that greatly
>>>>> increases overhead.  Like enabling large quantities of tracing.  ;-)
>>>>>
>>>>> It -might- be possible to do this automatically, but reliable
>>>>> automation would require that tracing understand how often each
>>>>> function was called, which sounds to me to be a bit of a stretch.
>>>>>
>>>>> Thoughts?
>>>>
>>>> A random thought:
>>>>
>>>> Is it possible to have a mid-timeout happening and store some
>>>> information like the instruction pointer, so when the timeout happen we
>>>> can compare if there was some progress, if yes, very likely, system
>>>> performance collapsed and we are not fast enough.
>>>
>>> RCU already does take various actions for an impending stall, so something
>>> could be done.  But in most slowdowns, the instruction pointer will be
>>> changing rapidly, just not as rapidly as it would normally.  So exactly
>>> how would the forward-progress comparison be carried out?
>>>
>>> It would be easy to set up a notifier, so that if any notifier in the
>>> chain returned an error, stall warnings would be suppressed.  It would
>>> be harder to figure out when to re-enable them, though I suppose that
>>> they could be suppressed only for the duration of the current grace
>>> period or some such.
>>>
>>> But what exactly would you use such a notifier for?
>>>
>>> Or am I misunderstanding your suggestion?
>>
>> Well, may be the instruction pointer thing is not a good idea.
>>
>> I learnt from this experience, an overloaded kernel with a lot of
>> interrupts can hang the console and issue RCU stall.
>>
>> However, someone else can face the same situation. Even if he reads the
>> RCU/stallwarn.txt documentation, it will be hard to figure out the issue.
>>
>> A message telling the grace period can't be reached because we are too
>> busy processing interrupts would have helped but I understand it is not
>> easy to implement.
>>
>> Perhaps, adding a new bullet in the documentation can help:
>>
>> "If the interrupt processing time is longer than the interval between
>> each interrupt, the CPU will keep processing the interrupts without
>> allowing the RCU's grace period kthread. This situation can happen if
>> there is a highly rated number of interrupts and the function_graph
>> tracer is enabled".
> 
> How about this?

Yes, it is clear. Thanks for reformulating it.

> Any other debugging options that should be called out?  I bet that
> the function_graph tracer isn't the only way to make this happen.

Nothing coming in mind but may be worth to mention the slowness of the
CPU is the aggravating factor. In particular I was able to reproduce the
issue by setting to the min CPU frequency. With the ondemand governor,
we can have the frequency high (hence enough CPU power) at the moment we
set the function_graph because another CPU is loaded (and both CPUs are
sharing the same clock line). The system became stuck at the moment the
other CPU went idle with the lowest frequency. That introduced
randomness in the issue and made hard to figure out why the RCU stall
was happening.

	-- Daniel

> ------------------------------------------------------------------------
> 
> commit 8b12d9919f59fe6855429e2eacc1c2e03ecdfe96
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Date:   Wed Aug 9 10:16:29 2017 -0700
> 
>     documentation: Long-running irq handlers can stall RCU grace periods
>     
>     If a periodic interrupt's handler takes longer to execute than the period
>     between successive interrupts, RCU's kthreads and softirq handlers can
>     be prevented from executing, resulting in otherwise inexplicable RCU
>     CPU stall warnings.  This commit therefore calls out this possibility
>     in Documentation/RCU/stallwarn.txt.
>     
>     Reported-by: Daniel Lezcano <daniel.lezcano@linaro.org>
>     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
> index 96a3d81837e1..21b8913acbdf 100644
> --- a/Documentation/RCU/stallwarn.txt
> +++ b/Documentation/RCU/stallwarn.txt
> @@ -40,7 +40,9 @@ o	Booting Linux using a console connection that is too slow to
>  o	Anything that prevents RCU's grace-period kthreads from running.
>  	This can result in the "All QSes seen" console-log message.
>  	This message will include information on when the kthread last
> -	ran and how often it should be expected to run.
> +	ran and how often it should be expected to run.  It can also
> +	result in the "rcu_.*kthread starved for" console-log message,
> +	which will include additional debugging information.
>  
>  o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
>  	happen to preempt a low-priority task in the middle of an RCU
> @@ -60,6 +62,14 @@ o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
>  	CONFIG_PREEMPT_RCU case, you might see stall-warning
>  	messages.
>  
> +o	A periodic interrupt whose handler takes longer than the time
> +	interval between successive pairs of interrupts.  This can
> +	prevent RCU's kthreads and softirq handlers from running.
> +	Note that certain high-overhead debugging options, for example
> +	the function_graph tracer, can result in interrupt handler taking
> +	considerably longer than normal, which can in turn result in
> +	RCU CPU stall warnings.
> +
>  o	A hardware or software issue shuts off the scheduler-clock
>  	interrupt on a CPU that is not in dyntick-idle mode.  This
>  	problem really has happened, and seems to be most likely to
> 


-- 
 <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
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#1709038

From	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date	2017-08-10 23:40 +0200
Message-ID	<ud3lo-31s-23@gated-at.bofh.it>
In reply to	#1708382

On Thu, Aug 10, 2017 at 11:45:09AM +0200, Daniel Lezcano wrote:
> On 09/08/2017 19:22, Paul E. McKenney wrote:
> > On Wed, Aug 09, 2017 at 05:51:33PM +0200, Daniel Lezcano wrote:
> >> On 09/08/2017 16:40, Paul E. McKenney wrote:
> >>> On Wed, Aug 09, 2017 at 03:28:05PM +0200, Daniel Lezcano wrote:
> >>>> On 09/08/2017 14:58, Paul E. McKenney wrote:
> >>>>> On Wed, Aug 09, 2017 at 02:43:49PM +0530, Pratyush Anand wrote:
> >>>>>>
> >>>>>>
> >>>>>> On Sunday 06 August 2017 10:32 PM, Paul E. McKenney wrote:
> >>>>>>> On Sat, Aug 05, 2017 at 02:24:21PM +0900, 김동현 wrote:
> >>>>>>>> Dear All
> >>>>>>>>
> >>>>>>>> As for me, after configuring function_graph as below, crash disappears.
> >>>>>>>> "echo 0 > d/tracing/tracing_on"
> >>>>>>>> "sleep 1"
> >>>>>>>>
> >>>>>>>> "echo function_graph > d/tracing/current_tracer"
> >>>>>>>> "sleep 1"
> >>>>>>>>
> >>>>>>>> "echo smp_call_function_single > d/tracing/set_ftrace_filter"
> >>>>>>
> >>>>>> It will limit trace output to only for the filtered function
> >>>>>> (smp_call_function_single).
> >>>>>>
> >>>>>>>> adb shell "sleep 1"
> >>>>>>>>
> >>>>>>>> "echo 1 > d/tracing/tracing_on"
> >>>>>>>> adb shell "sleep 1"
> >>>>>>>>
> >>>>>>>> Right after function_graph is enabled, too many logs are traced upon IRQ
> >>>>>>>> transaction which many times eventually causes stall.
> >>>>>>>
> >>>>>>> That would do it!
> >>>>>>>
> >>>>>>> Hmmm...
> >>>>>>>
> >>>>>>> Steven, would it be helpful if RCU were to inform tracing (say) halfway
> >>>>>>> through the RCU CPU stall interval, allowing the tracer to do something
> >>>>>>> like cond_resched_rcu_qs()?  I can imagine all sorts of reasons why this
> >>>>>>> wouldn't work, for example, if all the tracing was with irqs disabled
> >>>>>>> or some such, but figured I should ask.
> >>>>>>>
> >>>>>>> Does Guillermo's approach work for others?
> >>>>>>
> >>>>>> Limited output with a couple of filtered function will definitely
> >>>>>> not cause RCU schedule stall. But the question is whether we should
> >>>>>> expect a full function graph trace working on every platform or not
> >>>>>> (specially the one which generates high interrupts)?
> >>>>>
> >>>>> It might well be that the user must disable RCU CPU stall warnings via
> >>>>> the rcu_cpu_stall_suppress sysfs entry (or increase their timeout via th
> >>>>> rcu_cpu_stall_timeout sysfs entry) before doing something that greatly
> >>>>> increases overhead.  Like enabling large quantities of tracing.  ;-)
> >>>>>
> >>>>> It -might- be possible to do this automatically, but reliable
> >>>>> automation would require that tracing understand how often each
> >>>>> function was called, which sounds to me to be a bit of a stretch.
> >>>>>
> >>>>> Thoughts?
> >>>>
> >>>> A random thought:
> >>>>
> >>>> Is it possible to have a mid-timeout happening and store some
> >>>> information like the instruction pointer, so when the timeout happen we
> >>>> can compare if there was some progress, if yes, very likely, system
> >>>> performance collapsed and we are not fast enough.
> >>>
> >>> RCU already does take various actions for an impending stall, so something
> >>> could be done.  But in most slowdowns, the instruction pointer will be
> >>> changing rapidly, just not as rapidly as it would normally.  So exactly
> >>> how would the forward-progress comparison be carried out?
> >>>
> >>> It would be easy to set up a notifier, so that if any notifier in the
> >>> chain returned an error, stall warnings would be suppressed.  It would
> >>> be harder to figure out when to re-enable them, though I suppose that
> >>> they could be suppressed only for the duration of the current grace
> >>> period or some such.
> >>>
> >>> But what exactly would you use such a notifier for?
> >>>
> >>> Or am I misunderstanding your suggestion?
> >>
> >> Well, may be the instruction pointer thing is not a good idea.
> >>
> >> I learnt from this experience, an overloaded kernel with a lot of
> >> interrupts can hang the console and issue RCU stall.
> >>
> >> However, someone else can face the same situation. Even if he reads the
> >> RCU/stallwarn.txt documentation, it will be hard to figure out the issue.
> >>
> >> A message telling the grace period can't be reached because we are too
> >> busy processing interrupts would have helped but I understand it is not
> >> easy to implement.
> >>
> >> Perhaps, adding a new bullet in the documentation can help:
> >>
> >> "If the interrupt processing time is longer than the interval between
> >> each interrupt, the CPU will keep processing the interrupts without
> >> allowing the RCU's grace period kthread. This situation can happen if
> >> there is a highly rated number of interrupts and the function_graph
> >> tracer is enabled".
> > 
> > How about this?
> 
> Yes, it is clear. Thanks for reformulating it.
> 
> > Any other debugging options that should be called out?  I bet that
> > the function_graph tracer isn't the only way to make this happen.
> 
> Nothing coming in mind but may be worth to mention the slowness of the
> CPU is the aggravating factor. In particular I was able to reproduce the
> issue by setting to the min CPU frequency. With the ondemand governor,
> we can have the frequency high (hence enough CPU power) at the moment we
> set the function_graph because another CPU is loaded (and both CPUs are
> sharing the same clock line). The system became stuck at the moment the
> other CPU went idle with the lowest frequency. That introduced
> randomness in the issue and made hard to figure out why the RCU stall
> was happening.

Adding this, then?

							Thanx, Paul

------------------------------------------------------------------------

commit f7d9ce95064f76be583c775fac32076fa59f1617
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Thu Aug 10 14:33:17 2017 -0700

    documentation: Slow systems can stall RCU grace periods
    
    If a fast system has a worst-case grace-period duration of (say) ten
    seconds, then running the same workload on a system ten times as slow
    will get you an RCU CPU stall warning given default stall-warning
    timeout settings.  This commit therefore adds this possibility to
    stallwarn.txt.
    
    Reported-by: Daniel Lezcano <daniel.lezcano@linaro.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 21b8913acbdf..238acbd94917 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -70,6 +70,12 @@ o	A periodic interrupt whose handler takes longer than the time
 	considerably longer than normal, which can in turn result in
 	RCU CPU stall warnings.
 
+o	Testing a workload on a fast system, tuning the stall-warning
+	timeout down to just barely avoid RCU CPU stall warnings, and then
+	running the same workload with the same stall-warning timeout on a
+	slow system.  Note that thermal throttling and on-demand governors
+	can cause a single system to be sometimes fast and sometimes slow!
+
 o	A hardware or software issue shuts off the scheduler-clock
 	interrupt on a CPU that is not in dyntick-idle mode.  This
 	problem really has happened, and seems to be most likely to

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#1709410

From	Daniel Lezcano <daniel.lezcano@linaro.org>
Date	2017-08-11 11:40 +0200
Message-ID	<udeAa-1Md-21@gated-at.bofh.it>
In reply to	#1709038

On 10/08/2017 23:39, Paul E. McKenney wrote:
> On Thu, Aug 10, 2017 at 11:45:09AM +0200, Daniel Lezcano wrote:

[ ... ]

>> Nothing coming in mind but may be worth to mention the slowness of the
>> CPU is the aggravating factor. In particular I was able to reproduce the
>> issue by setting to the min CPU frequency. With the ondemand governor,
>> we can have the frequency high (hence enough CPU power) at the moment we
>> set the function_graph because another CPU is loaded (and both CPUs are
>> sharing the same clock line). The system became stuck at the moment the
>> other CPU went idle with the lowest frequency. That introduced
>> randomness in the issue and made hard to figure out why the RCU stall
>> was happening.
> 
> Adding this, then?

Yes, sure.

Thanks Paul.

  -- Daniel

> ------------------------------------------------------------------------
> 
> commit f7d9ce95064f76be583c775fac32076fa59f1617
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Date:   Thu Aug 10 14:33:17 2017 -0700
> 
>     documentation: Slow systems can stall RCU grace periods
>     
>     If a fast system has a worst-case grace-period duration of (say) ten
>     seconds, then running the same workload on a system ten times as slow
>     will get you an RCU CPU stall warning given default stall-warning
>     timeout settings.  This commit therefore adds this possibility to
>     stallwarn.txt.
>     
>     Reported-by: Daniel Lezcano <daniel.lezcano@linaro.org>
>     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
> index 21b8913acbdf..238acbd94917 100644
> --- a/Documentation/RCU/stallwarn.txt
> +++ b/Documentation/RCU/stallwarn.txt
> @@ -70,6 +70,12 @@ o	A periodic interrupt whose handler takes longer than the time
>  	considerably longer than normal, which can in turn result in
>  	RCU CPU stall warnings.
>  
> +o	Testing a workload on a fast system, tuning the stall-warning
> +	timeout down to just barely avoid RCU CPU stall warnings, and then
> +	running the same workload with the same stall-warning timeout on a
> +	slow system.  Note that thermal throttling and on-demand governors
> +	can cause a single system to be sometimes fast and sometimes slow!
> +
>  o	A hardware or software issue shuts off the scheduler-clock
>  	interrupt on a CPU that is not in dyntick-idle mode.  This
>  	problem really has happened, and seems to be most likely to
> 


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#1702270

From	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date	2017-08-02 19:00 +0200
Message-ID	<ua5a2-3XB-3@gated-at.bofh.it>
In reply to	#1702086

On Wed, Aug 02, 2017 at 02:42:39PM +0200, Daniel Lezcano wrote:
> On Tue, Aug 01, 2017 at 08:12:14PM -0400, Steven Rostedt wrote:
> > On Wed, 2 Aug 2017 00:15:44 +0200
> > Daniel Lezcano <daniel.lezcano@linaro.org> wrote:
> > 
> > > On 02/08/2017 00:04, Paul E. McKenney wrote:
> > > >> Hi Paul,
> > > >>
> > > >> I have been trying to set the function_graph tracer for ftrace and each time I
> > > >> get a CPU stall.
> > > >>
> > > >> How to reproduce:
> > > >> -----------------
> > > >>
> > > >> 		 echo function_graph > /sys/kernel/debug/tracing/current_tracer
> > > >>
> > > >> This error appears with v4.13-rc3 and v4.12-rc6.
> > 
> > Can you bisect this? It may be due to this commit:
> > 
> > 0598e4f08 ("ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines")
> 
> Hi Steve,
> 
> I git bisected but each time the issue occured. I went through the different
> version down to v4.4 where the board was not fully supported and it ended up to
> have the same issue.
> 
> Finally, I had the intuition it could be related to the wall time (there is no
> RTC clock with battery on the board and the wall time is Jan 1st, 1970).
> 
> Setting up the with ntpdate solved the problem.
> 
> Even if it is rarely the case to have the time not set, is it normal to have a
> RCU cpu stall ?

If the system is sufficiently confused about the time, you can indeed
get RCU CPU stall warnings.

In one memorable case, a pair of CPUs had a multi-minute disagreement
as to the current time, which meant that when one of the started an RCU
grace period, the other would immediately issue an RCU CPU stall warning.

							Thanx, Paul

> > > >>
> > > >> Is it something already reported ?  
> > > > 
> > > > I have seen this sort of thing, but only when actually dumping the trace
> > > > out, and I though those got fixed.  You are seeing this just accumulating
> > > > the trace?  
> > > 
> > > No, just by changing the tracer. It is the first operation I do after
> > > rebooting and it is reproducible each time. That happens on an ARM64
> > > platform.
> > > 
> > > > These RCU CPU stall warnings usually occur when something grabs hold of
> > > > a CPU for too long, as in 21 seconds or so.  One way that they can happen
> > > > is excessive lock contention, another is having the kernel run through
> > > > too much data at one shot.
> > > > 
> > > > Adding Steven Rostedt on CC for his thoughts.
> > > > 
> > >
> 
> -- 
> 
>  <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs
> 
> Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
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Re: RCU stall when using function_graph

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#1701535 — Re: RCU stall when using function_graph

#1701541

#1701599

#1702086

#1702095

#1702634

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#1703078

#1703164

#1704894

#1707173

#1707326

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#1709038

#1709410

#1702270