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Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

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  Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-18 17:30 +0200
    Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods Arjan van de Ven <arjan@linux.intel.com> - 2017-07-18 17:30 +0200
      Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods Arjan van de Ven <arjan@linux.intel.com> - 2017-07-18 18:40 +0200
        Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods Peter Zijlstra <peterz@infradead.org> - 2017-07-18 19:10 +0200
      Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods Peter Zijlstra <peterz@infradead.org> - 2017-07-18 18:40 +0200
    Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods "Li, Aubrey" <aubrey.li@linux.intel.com> - 2017-07-19 07:50 +0200
      Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-19 16:50 +0200
        Re: [RFC PATCH v1 00/11] Create fast idle path for short idle  periods Christopher Lameter <cl@linux.com> - 2017-07-19 17:10 +0200
          Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-19 19:00 +0200
        Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods "Li, Aubrey" <aubrey.li@linux.intel.com> - 2017-07-20 03:50 +0200
          Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-20 15:00 +0200
            Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods Arjan van de Ven <arjan@linux.intel.com> - 2017-07-20 15:50 +0200
            Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods Peter Zijlstra <peterz@infradead.org> - 2017-07-20 16:30 +0200
              Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-20 18:10 +0200

#1690369 — Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

On Tue, Jul 18, 2017 at 02:56:47PM +0800, Li, Aubrey wrote:
> On 2017/7/18 14:43, Thomas Gleixner wrote:
> > On Mon, 17 Jul 2017, Andi Kleen wrote:
> > 
> >>> We need a tradeoff here IMHO. I'll check Daniel's work to understand how/if
> >>> it's better than menu governor.
> >>
> >> I still would like to see how the fast path without the C1 heuristic works.
> >>
> >> Fast pathing is a different concept from a better predictor. IMHO we need
> >> both, but the first is likely lower hanging fruit.
> > 
> > Hacking something on the side is always the lower hanging fruit as it
> > avoids solving the hard problems. As Peter said already, that's not going
> > to happen unless there is a real technical reason why the general path
> > cannot be fixed. So far there is no proof for that.
> > 
> Let me try to make a summary, please correct me if I was wrong.
> 
> 1) for quiet_vmstat, we are agreed to move to another place where tick is
> really stopped.
> 
> 2) for rcu idle enter/exit, I measured the details which Paul provided, and
> the result matches with what I have measured before, nothing notable found.
> But it still makes more sense if we can make rcu idle enter/exit hooked with
> tick off. (it's possible other workloads behave differently)

Again, assuming that RCU is informed of CPUs in the kernel, regardless
of whether or not the tick is on that that point in time.

							Thanx, Paul

> 3) for tick nohz idle, we want to skip if the coming idle is short. If we can
> skip the tick nohz idle, we then skip all the items depending on it. But, there
> are two hard points:
> 
> 3.1) how to compute the period of the coming idle. My current proposal is to
> use two factors in the current idle menu governor. There are two possible
> options from Peter and Thomas, the one is to use scheduler idle estimate, which
> is task activity based, the other is to use the statistics generated from irq
> timings work.
> 
> 3.2) how to determine if the idle is short or long. My current proposal is to
> use a tunable value via /sys, while Peter prefers an auto-adjust mechanism. I
> didn't get the details of an auto-adjust mechanism yet
> 
> 4) for idle loop, my proposal introduces a simple one to use default idle
> routine directly, while Peter and Thomas suggest we fix c-state selection
> in the existing idle path.
> 
> Thanks,
> -Aubrey
> 

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#1690371

On 7/18/2017 8:20 AM, Paul E. McKenney wrote:
> 3.2) how to determine if the idle is short or long. My current proposal is to
> use a tunable value via /sys, while Peter prefers an auto-adjust mechanism. I
> didn't get the details of an auto-adjust mechanism yet


the most obvious way to do this (for me, maybe I'm naive) is to add another
C state, lets call it "C1-lite" with its own thresholds and power levels etc,
and just let that be picked naturally based on the heuristics.
(if we want to improve the heuristics, that's fine and always welcome but that
is completely orthogonal in my mind)

this C1-lite would then skip some of the idle steps like the nohz logic. How we
plumb that ... might end up being a flag or whatever, we'll figure that out easily.

as long as "real C1" has a break even time that is appropriate compared to C1-lite,
we'll only pick C1-lite for very very short idles like is desired...
but we don't end up creating a parallel infra for picking states, that part just does
not make sense to me tbh.... I have yet to see any reason why C1-lite couldn't be just
another C-state for everything except the actual place where we do the "go idle" last
bit of logic.

(Also note that for extreme short idles, today we just spinloop (C0), so by this argument
we should also do a C0-lite.. or make this C0 always the lite variant)

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#1690430

On 7/18/2017 9:36 AM, Peter Zijlstra wrote:
> On Tue, Jul 18, 2017 at 08:29:40AM -0700, Arjan van de Ven wrote:
>>
>> the most obvious way to do this (for me, maybe I'm naive) is to add another
>> C state, lets call it "C1-lite" with its own thresholds and power levels etc,
>> and just let that be picked naturally based on the heuristics.
>> (if we want to improve the heuristics, that's fine and always welcome but that
>> is completely orthogonal in my mind)
>
> C1-lite would then have a threshold < C1, whereas I understood the
> desire to be for the fast-idle crud to have a larger threshold than C1
> currently has.
>
> That is, from what I understood, they want C1 selected *longer*.

that's just a matter of fixing the C1 and later thresholds to line up right.

shrug that's the most trivial thing to do, it's a number in a table.

some distros do those tunings anyway when they don't like the upstream tunings

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#1690475

On Tue, Jul 18, 2017 at 09:37:57AM -0700, Arjan van de Ven wrote:

> that's just a matter of fixing the C1 and later thresholds to line up right.
> shrug that's the most trivial thing to do, it's a number in a table.

Well, they represent a physical measure, namely the break-even-time. If
you go muck with them you don't have anything left.  This is tinkering
of the worst possible kind.

Fix the estimator if you want behavioural changes.

> some distros do those tunings anyway when they don't like the upstream tunings

*shudder*

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#1690435

On Tue, Jul 18, 2017 at 08:29:40AM -0700, Arjan van de Ven wrote:
> 
> the most obvious way to do this (for me, maybe I'm naive) is to add another
> C state, lets call it "C1-lite" with its own thresholds and power levels etc,
> and just let that be picked naturally based on the heuristics.
> (if we want to improve the heuristics, that's fine and always welcome but that
> is completely orthogonal in my mind)

C1-lite would then have a threshold < C1, whereas I understood the
desire to be for the fast-idle crud to have a larger threshold than C1
currently has.

That is, from what I understood, they want C1 selected *longer*.

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#1690985

On 2017/7/18 23:20, Paul E. McKenney wrote:

>> 2) for rcu idle enter/exit, I measured the details which Paul provided, and
>> the result matches with what I have measured before, nothing notable found.
>> But it still makes more sense if we can make rcu idle enter/exit hooked with
>> tick off. (it's possible other workloads behave differently)
> 
> Again, assuming that RCU is informed of CPUs in the kernel, regardless
> of whether or not the tick is on that that point in time.
> 
Yeah, I see, no problem for a normal idle.

But for a short idle, we want to return to the task ASAP. Even though RCU cost
is not notable, it would still be better for me if we can save some cycles in
idle entry and idle exit.

Do we have any problem if we skip RCU idle enter/exit under a fast idle scenario?
My understanding is, if tick is not stopped, then we don't need inform RCU in
idle path, it can be informed in irq exit.

Thanks,
-Aubrey

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#1691802

On Wed, Jul 19, 2017 at 01:44:06PM +0800, Li, Aubrey wrote:
> On 2017/7/18 23:20, Paul E. McKenney wrote:
> 
> >> 2) for rcu idle enter/exit, I measured the details which Paul provided, and
> >> the result matches with what I have measured before, nothing notable found.
> >> But it still makes more sense if we can make rcu idle enter/exit hooked with
> >> tick off. (it's possible other workloads behave differently)
> > 
> > Again, assuming that RCU is informed of CPUs in the kernel, regardless
> > of whether or not the tick is on that that point in time.
> > 
> Yeah, I see, no problem for a normal idle.
> 
> But for a short idle, we want to return to the task ASAP. Even though RCU cost
> is not notable, it would still be better for me if we can save some cycles in
> idle entry and idle exit.
> 
> Do we have any problem if we skip RCU idle enter/exit under a fast idle scenario?
> My understanding is, if tick is not stopped, then we don't need inform RCU in
> idle path, it can be informed in irq exit.

Indeed, the problem arises when the tick is stopped.

							Thanx, Paul

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#1691826 — Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

On Wed, 19 Jul 2017, Paul E. McKenney wrote:

> > Do we have any problem if we skip RCU idle enter/exit under a fast idle scenario?
> > My understanding is, if tick is not stopped, then we don't need inform RCU in
> > idle path, it can be informed in irq exit.
>
> Indeed, the problem arises when the tick is stopped.

Well is there a boundary when you would want the notification calls? I
would think that even an idle period of a couple of seconds does not
necessarily require a callback to rcu. Had some brokenness here where RCU
calls did not occur for hours or so. At some point the system ran out of
memory but thats far off.

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#1692028

On Wed, Jul 19, 2017 at 10:03:22AM -0500, Christopher Lameter wrote:
> On Wed, 19 Jul 2017, Paul E. McKenney wrote:
> 
> > > Do we have any problem if we skip RCU idle enter/exit under a fast idle scenario?
> > > My understanding is, if tick is not stopped, then we don't need inform RCU in
> > > idle path, it can be informed in irq exit.
> >
> > Indeed, the problem arises when the tick is stopped.
> 
> Well is there a boundary when you would want the notification calls? I
> would think that even an idle period of a couple of seconds does not
> necessarily require a callback to rcu. Had some brokenness here where RCU
> calls did not occur for hours or so. At some point the system ran out of
> memory but thats far off.

Yeah, I should spell this out more completely, shouldn't I?  And get
it into the documentation if it isn't already there...  Where it is
currently at best hinted at.  :-/

1.	If a CPU is either idle or executing in usermode, and RCU believes
	it is non-idle, the scheduling-clock tick had better be running.
	Otherwise, you will get RCU CPU stall warnings.  Or at best,
	very long (11-second) grace periods, with a pointless IPI waking
	the CPU each time.

2.	If a CPU is in a portion of the kernel that executes RCU read-side
	critical sections, and RCU believes this CPU to be idle, you can get
	random memory corruption.  DON'T DO THIS!!!

	This is one reason to test with lockdep, which will complain
	about this sort of thing.

3.	If a CPU is in a portion of the kernel that is absolutely
	positively no-joking guaranteed to never execute any RCU read-side
	critical sections, and RCU believes this CPU to to be idle,
	no problem.  This sort of thing is used by some architectures
	for light-weight exception handlers, which can then avoid the
	overhead of rcu_irq_enter() and rcu_irq_exit().  Some go further
	and avoid the entireties of irq_enter() and irq_exit().

	Just make very sure you are running some of your tests with
	CONFIG_PROVE_RCU=y.

4.	If a CPU is executing in the kernel with the scheduling-clock
	interrupt disabled and RCU believes this CPU to be non-idle,
	and if the CPU goes idle (from an RCU perspective) every few
	jiffies, no problem.  It is usually OK for there to be the
	occasional gap between idle periods of up to a second or so.

	If the gap grows too long, you get RCU CPU stall warnings.

5.	If a CPU is either idle or executing in usermode, and RCU believes
	it to be idle, of course no problem.

6.	If a CPU is executing in the kernel, the kernel code
	path is passing through quiescent states at a reasonable
	frequency (preferably about once per few jiffies, but the
	occasional excursion to a second or so is usually OK) and the
	scheduling-clock interrupt is enabled, of course no problem.

	If the gap between a successive pair of quiescent states grows
	too long, you get RCU CPU stall warnings.

For purposes of comparison, the default time until RCU CPU stall warning
in mainline is 21 seconds.  A number of distros set this to 60 seconds.
Back in the 90s, the analogous timeout for DYNIX/ptx was 1.5 seconds.  :-/

Hence "a second or so" instead of your "a couple of seconds".  ;-)

Please see below for the corresponding patch to RCU's requirements.
Thoughts?

							Thanx, Paul

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

commit 693c9bfa43f92570dd362d8834440b418bbb994a
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Wed Jul 19 09:52:58 2017 -0700

    doc: Set down RCU's scheduling-clock-interrupt needs
    
    This commit documents the situations in which RCU needs the
    scheduling-clock interrupt to be enabled, along with the consequences
    of failing to meet RCU's needs in this area.
    
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 95b30fa25d56..7980bee5607f 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -2080,6 +2080,8 @@ Some of the relevant points of interest are as follows:
 <li>	<a href="#Scheduler and RCU">Scheduler and RCU</a>.
 <li>	<a href="#Tracing and RCU">Tracing and RCU</a>.
 <li>	<a href="#Energy Efficiency">Energy Efficiency</a>.
+<li>	<a href="#Scheduling-Clock Interrupts and RCU">
+	Scheduling-Clock Interrupts and RCU</a>.
 <li>	<a href="#Memory Efficiency">Memory Efficiency</a>.
 <li>	<a href="#Performance, Scalability, Response Time, and Reliability">
 	Performance, Scalability, Response Time, and Reliability</a>.
@@ -2532,6 +2534,113 @@ I learned of many of these requirements via angry phone calls:
 Flaming me on the Linux-kernel mailing list was apparently not
 sufficient to fully vent their ire at RCU's energy-efficiency bugs!
 
+<h3><a name="Scheduling-Clock Interrupts and RCU">
+Scheduling-Clock Interrupts and RCU</a></h3>
+
+<p>
+The kernel transitions between in-kernel non-idle execution, userspace
+execution, and the idle loop.
+Depending on kernel configuration, RCU handles these states differently:
+
+<table border=3>
+<tr><th><tt>HZ</tt> Kconfig</th>
+	<th>In-Kernel</th>
+		<th>Usermode</th>
+			<th>Idle</th></tr>
+<tr><th align="left"><tt>HZ_PERIODIC</tt></th>
+	<td>Can rely on scheduling-clock interrupt.</td>
+		<td>Can rely on scheduling-clock interrupt and its
+		    detection of interrupt from usermode.</td>
+			<td>Can rely on RCU's dyntick-idle detection.</td></tr>
+<tr><th align="left"><tt>NO_HZ_IDLE</tt></th>
+	<td>Can rely on scheduling-clock interrupt.</td>
+		<td>Can rely on scheduling-clock interrupt and its
+		    detection of interrupt from usermode.</td>
+			<td>Can rely on RCU's dyntick-idle detection.</td></tr>
+<tr><th align="left"><tt>NO_HZ_FULL</tt></th>
+	<td>Can only sometimes rely on scheduling-clock interrupt.
+	    In other cases, it is necessary to bound kernel execution
+	    times and/or use IPIs.</td>
+		<td>Can rely on RCU's dyntick-idle detection.</td>
+			<td>Can rely on RCU's dyntick-idle detection.</td></tr>
+</table>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why can't <tt>NO_HZ_FULL</tt> in-kernel execution rely on the
+	scheduling-clock interrupt, just like <tt>HZ_PERIODIC</tt>
+	and <tt>NO_HZ_IDLE</tt> do?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because, as a performance optimization, <tt>NO_HZ_FULL</tt>
+	does not necessarily re-enable the scheduling-clock interrupt
+	on entry to each and every system call.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+However, RCU must be reliably informed as to whether any given
+CPU is currently in the idle loop, and, for <tt>NO_HZ_FULL</tt>,
+also whether that CPU is executing in usermode, as discussed
+<a href="#Energy Efficiency">earlier</a>.
+It also requires that the scheduling-clock interrupt be enabled when
+RCU needs it to be:
+
+<ol>
+<li>	If a CPU is either idle or executing in usermode, and RCU believes
+	it is non-idle, the scheduling-clock tick had better be running.
+	Otherwise, you will get RCU CPU stall warnings.  Or at best,
+	very long (11-second) grace periods, with a pointless IPI waking
+	the CPU from time to time.
+<li>	If a CPU is in a portion of the kernel that executes RCU read-side
+	critical sections, and RCU believes this CPU to be idle, you will get
+	random memory corruption.  <b>DON'T DO THIS!!!</b>
+
+	<br>This is one reason to test with lockdep, which will complain
+	about this sort of thing.
+<li>	If a CPU is in a portion of the kernel that is absolutely
+	positively no-joking guaranteed to never execute any RCU read-side
+	critical sections, and RCU believes this CPU to to be idle,
+	no problem.  This sort of thing is used by some architectures
+	for light-weight exception handlers, which can then avoid the
+	overhead of <tt>rcu_irq_enter()</tt> and <tt>rcu_irq_exit()</tt>
+	at exception entry and exit, respectively.
+	Some go further and avoid the entireties of <tt>irq_enter()</tt>
+	and <tt>irq_exit()</tt>.
+
+	<br>Just make very sure you are running some of your tests with
+	<tt>CONFIG_PROVE_RCU=y</tt>, just in case one of your code paths
+	was in fact joking about not doing RCU read-side critical sections.
+<li>	If a CPU is executing in the kernel with the scheduling-clock
+	interrupt disabled and RCU believes this CPU to be non-idle,
+	and if the CPU goes idle (from an RCU perspective) every few
+	jiffies, no problem.  It is usually OK for there to be the
+	occasional gap between idle periods of up to a second or so.
+
+	<br>If the gap grows too long, you get RCU CPU stall warnings.
+<li>	If a CPU is either idle or executing in usermode, and RCU believes
+	it to be idle, of course no problem.
+<li>	If a CPU is executing in the kernel, the kernel code
+	path is passing through quiescent states at a reasonable
+	frequency (preferably about once per few jiffies, but the
+	occasional excursion to a second or so is usually OK) and the
+	scheduling-clock interrupt is enabled, of course no problem.
+
+	<br>If the gap between a successive pair of quiescent states grows
+	too long, you get RCU CPU stall warnings.
+</ol>
+
+<p>
+But as long as RCU is properly informed of kernel state transitions between
+in-kernel execution, usermode execution, and idle, and as long as the
+scheduling-clock interrupt is enabled when RCU needs it to be, you
+can rest assured that the bugs you encounter will be in some other
+part of RCU or some other part of the kernel!
+
 <h3><a name="Memory Efficiency">Memory Efficiency</a></h3>
 
 <p>

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#1692338

On 2017/7/19 22:48, Paul E. McKenney wrote:
> On Wed, Jul 19, 2017 at 01:44:06PM +0800, Li, Aubrey wrote:
>> On 2017/7/18 23:20, Paul E. McKenney wrote:
>>
>>>> 2) for rcu idle enter/exit, I measured the details which Paul provided, and
>>>> the result matches with what I have measured before, nothing notable found.
>>>> But it still makes more sense if we can make rcu idle enter/exit hooked with
>>>> tick off. (it's possible other workloads behave differently)
>>>
>>> Again, assuming that RCU is informed of CPUs in the kernel, regardless
>>> of whether or not the tick is on that that point in time.
>>>
>> Yeah, I see, no problem for a normal idle.
>>
>> But for a short idle, we want to return to the task ASAP. Even though RCU cost
>> is not notable, it would still be better for me if we can save some cycles in
>> idle entry and idle exit.
>>
>> Do we have any problem if we skip RCU idle enter/exit under a fast idle scenario?
>> My understanding is, if tick is not stopped, then we don't need inform RCU in
>> idle path, it can be informed in irq exit.
> 
> Indeed, the problem arises when the tick is stopped.

My question is, does problem arise when the tick is *not* stopped (skipping nohz idle)?

instead of 

static void cpuidle_idle_call()
{
	rcu_idle_enter()
	......
	rcu_idle_exit()
}

I want

static void cpuidle_idle_call()
{
	if (tick stopped)
		rcu_idle_enter()
	......
	if (tick stopped)
		rcu_idle_exit()
}

Or checking tick stop can be put into rcu_idle_enter/exit

Thanks,
-Aubrey

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#1692899

On Thu, Jul 20, 2017 at 09:40:49AM +0800, Li, Aubrey wrote:
> On 2017/7/19 22:48, Paul E. McKenney wrote:
> > On Wed, Jul 19, 2017 at 01:44:06PM +0800, Li, Aubrey wrote:
> >> On 2017/7/18 23:20, Paul E. McKenney wrote:
> >>
> >>>> 2) for rcu idle enter/exit, I measured the details which Paul provided, and
> >>>> the result matches with what I have measured before, nothing notable found.
> >>>> But it still makes more sense if we can make rcu idle enter/exit hooked with
> >>>> tick off. (it's possible other workloads behave differently)
> >>>
> >>> Again, assuming that RCU is informed of CPUs in the kernel, regardless
> >>> of whether or not the tick is on that that point in time.
> >>>
> >> Yeah, I see, no problem for a normal idle.
> >>
> >> But for a short idle, we want to return to the task ASAP. Even though RCU cost
> >> is not notable, it would still be better for me if we can save some cycles in
> >> idle entry and idle exit.
> >>
> >> Do we have any problem if we skip RCU idle enter/exit under a fast idle scenario?
> >> My understanding is, if tick is not stopped, then we don't need inform RCU in
> >> idle path, it can be informed in irq exit.
> > 
> > Indeed, the problem arises when the tick is stopped.
> 
> My question is, does problem arise when the tick is *not* stopped (skipping nohz idle)?
> 
> instead of 
> 
> static void cpuidle_idle_call()
> {
> 	rcu_idle_enter()
> 	......
> 	rcu_idle_exit()
> }
> 
> I want
> 
> static void cpuidle_idle_call()
> {
> 	if (tick stopped)
> 		rcu_idle_enter()
> 	......
> 	if (tick stopped)
> 		rcu_idle_exit()
> }
> 
> Or checking tick stop can be put into rcu_idle_enter/exit

The answer is the traditional "it depends".

If the above change was all that you did, that would be a bug in the
case where the predicted short idle time turned out to in reality be an
indefinite idle time.  RCU would indefinitely believe that the CPU was
non-idle, and would wait for it to report a quiescent state, which it
would not, even given the scheduling-clock interrupt (it is idle, so it
knows RCU already knows that it is idle, you see).  RCU would eventually
send it a resched IPI, which would probably get things going again,
but the best you could hope for was extra resched IPIs and excessively
long grace periods.

To make this work reasonably, you would also need some way to check for
the case where the prediction idle time is short but the real idle time
is very long.

							Thanx, Paul

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#1692947

On 7/20/2017 5:50 AM, Paul E. McKenney wrote:
> To make this work reasonably, you would also need some way to check for
> the case where the prediction idle time is short but the real idle time
> is very long.

so the case where you predict very short but is actually "indefinite", the real
solution likely is that we set a timer some time in the future
(say 100msec, or some other value that is long but not indefinite)
where we wake up the system and make a new prediction,
since clearly we were insanely wrong in the prediction and should try
again.

that or we turn the prediction from a single value into a range of
(expected, upper bound)

where upper bound is likely the next timer or other going-to-happen events.

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#1692978

On Thu, Jul 20, 2017 at 05:50:54AM -0700, Paul E. McKenney wrote:
> > 
> > static void cpuidle_idle_call()
> > {
> > 	rcu_idle_enter()
> > 	......
> > 	rcu_idle_exit()
> > }
> > 
> > I want
> > 
> > static void cpuidle_idle_call()
> > {
> > 	if (tick stopped)
> > 		rcu_idle_enter()
> > 	......
> > 	if (tick stopped)
> > 		rcu_idle_exit()
> > }
> >
> > Or checking tick stop can be put into rcu_idle_enter/exit
> 
> The answer is the traditional "it depends".
> 
> If the above change was all that you did, that would be a bug in the
> case where the predicted short idle time turned out to in reality be an
> indefinite idle time. 

Can't be, you didn't disable the tick after all, so you're guaranteed to
get interrupted by the tick and try again.

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#1693074

On Thu, Jul 20, 2017 at 04:19:40PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 20, 2017 at 05:50:54AM -0700, Paul E. McKenney wrote:
> > > 
> > > static void cpuidle_idle_call()
> > > {
> > > 	rcu_idle_enter()
> > > 	......
> > > 	rcu_idle_exit()
> > > }
> > > 
> > > I want
> > > 
> > > static void cpuidle_idle_call()
> > > {
> > > 	if (tick stopped)
> > > 		rcu_idle_enter()
> > > 	......
> > > 	if (tick stopped)
> > > 		rcu_idle_exit()
> > > }
> > >
> > > Or checking tick stop can be put into rcu_idle_enter/exit
> > 
> > The answer is the traditional "it depends".
> > 
> > If the above change was all that you did, that would be a bug in the
> > case where the predicted short idle time turned out to in reality be an
> > indefinite idle time. 
> 
> Can't be, you didn't disable the tick after all, so you're guaranteed to
> get interrupted by the tick and try again.

I will reserve judgment on that until I see the patch.  But to your point,
I would indeed hope that it works that way.  ;-)

							Thanx, Paul

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