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[tip:locking/core] sched/cputime: Fix invalid gtime in proc

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  [tip:locking/core] sched/cputime: Fix invalid gtime in proc tip-bot for Hiroshi Shimamoto <tipbot@zytor.com> - 2015-12-04 13:00 +0100
    Re: [tip:locking/core] sched/cputime: Fix invalid gtime in proc Frederic Weisbecker <fweisbec@gmail.com> - 2015-12-07 17:30 +0100
      Re: [tip:locking/core] sched/cputime: Fix invalid gtime in proc Ingo Molnar <mingo@kernel.org> - 2015-12-08 06:40 +0100

#1283728 — [tip:locking/core] sched/cputime: Fix invalid gtime in proc

Commit-ID:  2541117b0cf79977fa11a0d6e17d61010677bd7b
Gitweb:     http://git.kernel.org/tip/2541117b0cf79977fa11a0d6e17d61010677bd7b
Author:     Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
AuthorDate: Thu, 19 Nov 2015 16:47:28 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 4 Dec 2015 10:18:49 +0100

sched/cputime: Fix invalid gtime in proc

/proc/stats shows invalid gtime when the thread is running in guest.
When vtime accounting is not enabled, we cannot get a valid delta.
The delta is calculated with now - tsk->vtime_snap, but tsk->vtime_snap
is only updated when vtime accounting is runtime enabled.

This patch makes task_gtime() just return gtime without computing the
buggy non-existing tickless delta when vtime accounting is not enabled.

Use context_tracking_is_enabled() to check if vtime is accounting on
some cpu, in which case only we need to check the tickless delta. This
way we fix the gtime value regression on machines not running nohz full.

The kernel config contains CONFIG_VIRT_CPU_ACCOUNTING_GEN=y and
CONFIG_NO_HZ_FULL_ALL=n and boot without nohz_full.

I ran and stop a busy loop in VM and see the gtime in host.
Dump the 43rd field which shows the gtime in every second:

	 # while :; do awk '{print $3" "$43}' /proc/3955/task/4014/stat; sleep 1; done
	S 4348
	R 7064566
	R 7064766
	R 7064967
	R 7065168
	S 4759
	S 4759

During running busy loop, it returns large value.

After applying this patch, we can see right gtime.

	 # while :; do awk '{print $3" "$43}' /proc/10913/task/10956/stat; sleep 1; done
	S 5338
	R 5365
	R 5465
	R 5566
	R 5666
	S 5726
	S 5726

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1447948054-28668-2-git-send-email-fweisbec@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/cputime.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 26a5446..05de80b 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -788,6 +788,9 @@ cputime_t task_gtime(struct task_struct *t)
 	unsigned int seq;
 	cputime_t gtime;
 
+	if (!context_tracking_is_enabled())
+		return t->gtime;
+
 	do {
 		seq = read_seqbegin(&t->vtime_seqlock);
 
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#1285770

On Fri, Dec 04, 2015 at 03:53:13AM -0800, tip-bot for Hiroshi Shimamoto wrote:
> Commit-ID:  2541117b0cf79977fa11a0d6e17d61010677bd7b
> Gitweb:     http://git.kernel.org/tip/2541117b0cf79977fa11a0d6e17d61010677bd7b
> Author:     Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> AuthorDate: Thu, 19 Nov 2015 16:47:28 +0100
> Committer:  Ingo Molnar <mingo@kernel.org>
> CommitDate: Fri, 4 Dec 2015 10:18:49 +0100
> 
> sched/cputime: Fix invalid gtime in proc
> 
> /proc/stats shows invalid gtime when the thread is running in guest.
> When vtime accounting is not enabled, we cannot get a valid delta.
> The delta is calculated with now - tsk->vtime_snap, but tsk->vtime_snap
> is only updated when vtime accounting is runtime enabled.
> 
> This patch makes task_gtime() just return gtime without computing the
> buggy non-existing tickless delta when vtime accounting is not enabled.
> 
> Use context_tracking_is_enabled() to check if vtime is accounting on
> some cpu, in which case only we need to check the tickless delta. This
> way we fix the gtime value regression on machines not running nohz full.
> 
> The kernel config contains CONFIG_VIRT_CPU_ACCOUNTING_GEN=y and
> CONFIG_NO_HZ_FULL_ALL=n and boot without nohz_full.
> 
> I ran and stop a busy loop in VM and see the gtime in host.
> Dump the 43rd field which shows the gtime in every second:
> 
> 	 # while :; do awk '{print $3" "$43}' /proc/3955/task/4014/stat; sleep 1; done
> 	S 4348
> 	R 7064566
> 	R 7064766
> 	R 7064967
> 	R 7065168
> 	S 4759
> 	S 4759
> 
> During running busy loop, it returns large value.
> 
> After applying this patch, we can see right gtime.
> 
> 	 # while :; do awk '{print $3" "$43}' /proc/10913/task/10956/stat; sleep 1; done
> 	S 5338
> 	R 5365
> 	R 5465
> 	R 5566
> 	R 5666
> 	S 5726
> 	S 5726
> 
> Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Chris Metcalf <cmetcalf@ezchip.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Luiz Capitulino <lcapitulino@redhat.com>
> Cc: Mike Galbraith <efault@gmx.de>
> Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Link: http://lkml.kernel.org/r/1447948054-28668-2-git-send-email-fweisbec@gmail.com
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---

Thanks for applying the patchset!

However we may want to backport this one, it's a regression fix affecting
CONFIG_NO_HZ_FULL=y with nohz_full off (99% of many distros defaults).

Thanks.

>  kernel/sched/cputime.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 26a5446..05de80b 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -788,6 +788,9 @@ cputime_t task_gtime(struct task_struct *t)
>  	unsigned int seq;
>  	cputime_t gtime;
>  
> +	if (!context_tracking_is_enabled())
> +		return t->gtime;
> +
>  	do {
>  		seq = read_seqbegin(&t->vtime_seqlock);
>  
--
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#1286249

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> On Fri, Dec 04, 2015 at 03:53:13AM -0800, tip-bot for Hiroshi Shimamoto wrote:
> > Commit-ID:  2541117b0cf79977fa11a0d6e17d61010677bd7b
> > Gitweb:     http://git.kernel.org/tip/2541117b0cf79977fa11a0d6e17d61010677bd7b
> > Author:     Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> > AuthorDate: Thu, 19 Nov 2015 16:47:28 +0100
> > Committer:  Ingo Molnar <mingo@kernel.org>
> > CommitDate: Fri, 4 Dec 2015 10:18:49 +0100
> > 
> > sched/cputime: Fix invalid gtime in proc
> > 
> > /proc/stats shows invalid gtime when the thread is running in guest.
> > When vtime accounting is not enabled, we cannot get a valid delta.
> > The delta is calculated with now - tsk->vtime_snap, but tsk->vtime_snap
> > is only updated when vtime accounting is runtime enabled.
> > 
> > This patch makes task_gtime() just return gtime without computing the
> > buggy non-existing tickless delta when vtime accounting is not enabled.
> > 
> > Use context_tracking_is_enabled() to check if vtime is accounting on
> > some cpu, in which case only we need to check the tickless delta. This
> > way we fix the gtime value regression on machines not running nohz full.
> > 
> > The kernel config contains CONFIG_VIRT_CPU_ACCOUNTING_GEN=y and
> > CONFIG_NO_HZ_FULL_ALL=n and boot without nohz_full.
> > 
> > I ran and stop a busy loop in VM and see the gtime in host.
> > Dump the 43rd field which shows the gtime in every second:
> > 
> > 	 # while :; do awk '{print $3" "$43}' /proc/3955/task/4014/stat; sleep 1; done
> > 	S 4348
> > 	R 7064566
> > 	R 7064766
> > 	R 7064967
> > 	R 7065168
> > 	S 4759
> > 	S 4759
> > 
> > During running busy loop, it returns large value.
> > 
> > After applying this patch, we can see right gtime.
> > 
> > 	 # while :; do awk '{print $3" "$43}' /proc/10913/task/10956/stat; sleep 1; done
> > 	S 5338
> > 	R 5365
> > 	R 5465
> > 	R 5566
> > 	R 5666
> > 	S 5726
> > 	S 5726
> > 
> > Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Chris Metcalf <cmetcalf@ezchip.com>
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Luiz Capitulino <lcapitulino@redhat.com>
> > Cc: Mike Galbraith <efault@gmx.de>
> > Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Link: http://lkml.kernel.org/r/1447948054-28668-2-git-send-email-fweisbec@gmail.com
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > ---
> 
> Thanks for applying the patchset!
> 
> However we may want to backport this one, it's a regression fix affecting
> CONFIG_NO_HZ_FULL=y with nohz_full off (99% of many distros defaults).
> 
> Thanks.

Absolutely, not sure why I forgot that tag - I added it for other recent fixes.

Greg, please consider applying this upstream fix to -stable as well:

  2541117b0cf7 ("sched/cputime: Fix invalid gtime in proc")

Note, the commit will cherry-pick cleanly all the way back to v3.9 (released 2.5+ 
years ago), but it will only build on v3.14 and later kernels.

Thanks,

	Ingo
--
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