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[PATCH] sched/pelt: fix false running accounting in PELT

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  [PATCH] sched/pelt: fix false running accounting in PELT Vincent Guittot <vincent.guittot@linaro.org> - 2017-06-30 16:00 +0200
    [PATCH v2] sched/pelt: fix false running accounting Vincent Guittot <vincent.guittot@linaro.org> - 2017-07-01 07:10 +0200
    Re: [PATCH] sched/pelt: fix false running accounting in PELT Vincent Guittot <vincent.guittot@linaro.org> - 2017-07-01 07:20 +0200
    Re: [PATCH] sched/pelt: fix false running accounting in PELT kbuild test robot <lkp@intel.com> - 2017-07-01 18:50 +0200

#1678846 — [PATCH] sched/pelt: fix false running accounting in PELT

The running state is a subset of runnable state which means that running
can't be set if runnable (weight) is cleared. There are corner cases
where the current sched_entity has been already dequeued but cfs_rq->curr
has not been updated yet and still points to the dequeued sched_entity.
If ___update_load_avg is called at that time, weight will be 0 and running
will be set which is not possible.

This case happens during pick_next_task_fair() when a cfs_rq becomes idles.
The current sched_entity has been dequeued so se->on_rq is cleared and
cfs_rq->weight is null. But cfs_rq->curr still points to se (it will be
cleared when picking the idle thread). Because the cfs_rq becomes idle,
idle_balance() is called and ends up to call update_blocked_averages()
with these wrong running and runnable states.

Add a test in ___update_load_avg to correct the running state in this case.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 008c514..5fdcb42 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2968,6 +2968,24 @@ ___update_load_avg(u64 now, int cpu, struct sched_avg *sa,
 	sa->last_update_time += delta << 10;
 
 	/*
+	 * running is a subset of runnable (weight) so running can't be set if
+	 * runnable is clear. But there are some corner cases where the current
+	 * se has been already dequeued but cfs_rq->curr still points to it.
+	 * This means that weight will be 0 but not running for a sched_entity
+	 * but also for a cfs_rq if the latter becomes idle. As an example,
+	 * this happens during idle_balance() which calls
+	 * update_blocked_averages()
+	 */
+	if (weight)
+		running = 1;
+
+	/*
+	 * Scale time to reflect the amount a computation effectively done
+	 * during the time slot at current capacity
+	 */
+	delta = scale_time(delta, cpu, sa, weight, running);
+
+	/*
 	 * Now we know we crossed measurement unit boundaries. The *_avg
 	 * accrues by two steps:
 	 *
-- 
2.7.4

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#1679266 — [PATCH v2] sched/pelt: fix false running accounting

The running state is a subset of runnable state which means that running
can't be set if runnable (weight) is cleared. There are corner cases
where the current sched_entity has been already dequeued but cfs_rq->curr
has not been updated yet and still points to the dequeued sched_entity.
If ___update_load_avg is called at that time, weight will be 0 and running
will be set which is not possible.

This case happens during pick_next_task_fair() when a cfs_rq becomes idles.
The current sched_entity has been dequeued so se->on_rq is cleared and
cfs_rq->weight is null. But cfs_rq->curr still points to se (it will be
cleared when picking the idle thread). Because the cfs_rq becomes idle,
idle_balance() is called and ends up to call update_blocked_averages()
with these wrong running and runnable states.

Add a test in ___update_load_avg to correct the running state in this case.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 008c514..bc36a75 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2968,6 +2968,18 @@ ___update_load_avg(u64 now, int cpu, struct sched_avg *sa,
 	sa->last_update_time += delta << 10;
 
 	/*
+	 * running is a subset of runnable (weight) so running can't be set if
+	 * runnable is clear. But there are some corner cases where the current
+	 * se has been already dequeued but cfs_rq->curr still points to it.
+	 * This means that weight will be 0 but not running for a sched_entity
+	 * but also for a cfs_rq if the latter becomes idle. As an example,
+	 * this happens during idle_balance() which calls
+	 * update_blocked_averages()
+	 */
+	if (!weight)
+		running = 0;
+
+	/*
 	 * Now we know we crossed measurement unit boundaries. The *_avg
 	 * accrues by two steps:
 	 *
-- 
2.7.4

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#1679267

On 30 June 2017 at 15:58, Vincent Guittot <vincent.guittot@linaro.org> wrote:
> The running state is a subset of runnable state which means that running
> can't be set if runnable (weight) is cleared. There are corner cases
> where the current sched_entity has been already dequeued but cfs_rq->curr
> has not been updated yet and still points to the dequeued sched_entity.
> If ___update_load_avg is called at that time, weight will be 0 and running
> will be set which is not possible.
>
> This case happens during pick_next_task_fair() when a cfs_rq becomes idles.
> The current sched_entity has been dequeued so se->on_rq is cleared and
> cfs_rq->weight is null. But cfs_rq->curr still points to se (it will be
> cleared when picking the idle thread). Because the cfs_rq becomes idle,
> idle_balance() is called and ends up to call update_blocked_averages()
> with these wrong running and runnable states.
>
> Add a test in ___update_load_avg to correct the running state in this case.*
>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>

The v1 is just wrong. I have sent a v2 with correct patch

sorry for the disturb
Vincent

> ---
>  kernel/sched/fair.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 008c514..5fdcb42 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2968,6 +2968,24 @@ ___update_load_avg(u64 now, int cpu, struct sched_avg *sa,
>         sa->last_update_time += delta << 10;
>
>         /*
> +        * running is a subset of runnable (weight) so running can't be set if
> +        * runnable is clear. But there are some corner cases where the current
> +        * se has been already dequeued but cfs_rq->curr still points to it.
> +        * This means that weight will be 0 but not running for a sched_entity
> +        * but also for a cfs_rq if the latter becomes idle. As an example,
> +        * this happens during idle_balance() which calls
> +        * update_blocked_averages()
> +        */
> +       if (weight)
> +               running = 1;
> +
> +       /*
> +        * Scale time to reflect the amount a computation effectively done
> +        * during the time slot at current capacity
> +        */
> +       delta = scale_time(delta, cpu, sa, weight, running);
> +
> +       /*
>          * Now we know we crossed measurement unit boundaries. The *_avg
>          * accrues by two steps:
>          *
> --
> 2.7.4
>

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#1679383

[Multipart message — attachments visible in raw view] — view raw

Hi Vincent,

[auto build test ERROR on tip/sched/core]
[also build test ERROR on v4.12-rc7 next-20170630]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Vincent-Guittot/sched-pelt-fix-false-running-accounting-in-PELT/20170702-002907
config: i386-randconfig-x078-07012120 (attached as .config)
compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

All errors (new ones prefixed by >>):

   kernel/sched/fair.c: In function '___update_load_avg':
>> kernel/sched/fair.c:2986:10: error: implicit declaration of function 'scale_time' [-Werror=implicit-function-declaration]
     delta = scale_time(delta, cpu, sa, weight, running);
             ^~~~~~~~~~
   cc1: some warnings being treated as errors

vim +/scale_time +2986 kernel/sched/fair.c

  2980			running = 1;
  2981	
  2982		/*
  2983		 * Scale time to reflect the amount a computation effectively done
  2984		 * during the time slot at current capacity
  2985		 */
> 2986		delta = scale_time(delta, cpu, sa, weight, running);
  2987	
  2988		/*
  2989		 * Now we know we crossed measurement unit boundaries. The *_avg

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

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