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SubjectRe: [PATCH 1/1] sched/cputime: Mitigate performance regression in times()/clock_gettime()
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Hello Stanislaw,

On Fri, 2016-08-12 at 14:10 +0200, Stanislaw Gruszka wrote:
>
> I measured (partial) revert performance on 4.7 using mmtest instructions
> from Giovanni and also tested some other possible fix (draft version):
>
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 75f98c5..54fdf6d 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -294,6 +294,8 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
> unsigned int seq, nextseq;
> unsigned long flags;
>
> + (void) task_sched_runtime(tsk);
> +
> rcu_read_lock();
> /* Attempt a lockless read on the first round. */
> nextseq = 0;
> @@ -308,7 +310,7 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
> task_cputime(t, &utime, &stime);
> times->utime += utime;
> times->stime += stime;
> - times->sum_exec_runtime += task_sched_runtime(t);
> + times->sum_exec_runtime += t->se.sum_exec_runtime;
> }
> /* If lockless access failed, take the lock. */
> nextseq = 1;
> ---
> mmtest benchmark results are below (full compare-kernels.sh output is in attachment):
>
> vanila-4.7 revert prefetch patch
> 4.74 ( 0.00%) 3.04 ( 35.93%) 4.09 ( 13.81%) 1.30 ( 72.59%)
> 5.49 ( 0.00%) 5.00 ( 8.97%) 5.34 ( 2.72%) 1.03 ( 81.16%)
> 6.12 ( 0.00%) 4.91 ( 19.73%) 5.97 ( 2.40%) 0.90 ( 85.27%)
> 6.68 ( 0.00%) 4.90 ( 26.66%) 6.02 ( 9.75%) 0.88 ( 86.89%)
> 7.21 ( 0.00%) 5.13 ( 28.85%) 6.70 ( 7.09%) 0.87 ( 87.91%)
> 7.66 ( 0.00%) 5.22 ( 31.80%) 7.17 ( 6.39%) 0.92 ( 88.01%)
> 7.91 ( 0.00%) 5.36 ( 32.22%) 7.30 ( 7.72%) 0.95 ( 87.97%)
> 7.95 ( 0.00%) 5.35 ( 32.73%) 7.34 ( 7.66%) 1.06 ( 86.66%)
> 8.00 ( 0.00%) 5.33 ( 33.31%) 7.38 ( 7.73%) 1.13 ( 85.82%)
> 5.61 ( 0.00%) 3.55 ( 36.76%) 4.53 ( 19.23%) 2.29 ( 59.28%)
> 5.66 ( 0.00%) 4.32 ( 23.79%) 4.75 ( 16.18%) 3.65 ( 35.46%)
> 5.98 ( 0.00%) 4.97 ( 16.87%) 5.96 ( 0.35%) 3.62 ( 39.40%)
> 6.58 ( 0.00%) 4.94 ( 24.93%) 6.04 ( 8.32%) 3.63 ( 44.89%)
> 7.19 ( 0.00%) 5.18 ( 28.01%) 6.68 ( 7.13%) 3.65 ( 49.22%)
> 7.67 ( 0.00%) 5.27 ( 31.29%) 7.16 ( 6.63%) 3.62 ( 52.76%)
> 7.88 ( 0.00%) 5.36 ( 31.98%) 7.28 ( 7.58%) 3.65 ( 53.71%)
> 7.99 ( 0.00%) 5.39 ( 32.52%) 7.40 ( 7.42%) 3.65 ( 54.25%)
>
> Patch works because we we update sum_exec_runtime on current thread
> what assure we see proper sum_exec_runtime value on different CPUs. I
> tested it with reproducers from commits 6e998916dfe32 and d670ec13178d0,
> patch did not break them. I'm going to run some other test.
>
> Patch is draft version for early review, task_sched_runtime() will be
> simplified (since it's called only current thread) and possibly split
> into two functions: one that call update_curr() and other that return
> sum_exec_runtime (assure it's consistent on 32 bit arches).
>
> Stanislaw

Thank you for having a look at this.
Your patch performs very well, even better than the pre-6e998916dfe3
numbers I was aiming for. I confirm your results on my test machine
(Sandy Bridge, 32 cores, 2 NUMA nodes).
I didn't apply on the very latest 4.8-rc but used what I had handy for
comparison (i.e. 4.7-rc7 and the parent of 6e998916dfe3).
As I said, my measurements match yours (my tables follow); looks like
your diff cures the problem while mine cures the symptoms.

clock_gettime():

threads 4.7-rc7 3.18-rc3 4.7-rc7 + prefetch 4.7-rc7 + Stanislaw
(pre-6e998916dfe3)
2 3.48 2.23 ( 35.68%) 3.06 ( 11.83%) 1.08 ( 68.81%)
5 3.33 2.83 ( 14.84%) 3.25 ( 2.40%) 0.71 ( 78.55%)
8 3.37 2.84 ( 15.80%) 3.26 ( 3.30%) 0.56 ( 83.49%)
12 3.32 3.09 ( 6.69%) 3.37 ( -1.60%) 0.42 ( 87.28%)
21 4.01 3.14 ( 21.70%) 3.90 ( 2.74%) 0.35 ( 91.35%)
30 3.63 3.28 ( 9.75%) 3.36 ( 7.41%) 0.28 ( 92.23%)
48 3.71 3.02 ( 18.69%) 3.11 ( 16.27%) 0.39 ( 89.39%)
79 3.75 2.88 ( 23.23%) 3.16 ( 15.74%) 0.46 ( 87.76%)
110 3.81 2.95 ( 22.62%) 3.25 ( 14.80%) 0.56 ( 85.41%)
128 3.88 3.05 ( 21.28%) 3.31 ( 14.76%) 0.62 ( 84.10%)

times():

threads 4.7-rc7 3.18-rc3 4.7-rc7 + prefetch 4.7-rc7 + Stanislaw
(pre-6e998916dfe3)
2 3.65 2.27 ( 37.94%) 3.25 ( 11.03%) 1.62 ( 55.71%)
5 3.45 2.78 ( 19.34%) 3.17 ( 7.92%) 2.33 ( 32.28%)
8 3.52 2.79 ( 20.66%) 3.22 ( 8.69%) 2.06 ( 41.44%)
12 3.29 3.02 ( 8.33%) 3.36 ( -2.04%) 2.00 ( 39.18%)
21 4.07 3.10 ( 23.86%) 3.92 ( 3.78%) 2.07 ( 49.18%)
30 3.87 3.33 ( 13.80%) 3.40 ( 12.17%) 1.89 ( 51.12%)
48 3.79 2.96 ( 21.94%) 3.16 ( 16.61%) 1.69 ( 55.46%)
79 3.88 2.88 ( 25.82%) 3.28 ( 15.42%) 1.60 ( 58.81%)
110 3.90 2.98 ( 23.73%) 3.38 ( 13.35%) 1.73 ( 55.61%)
128 4.00 3.10 ( 22.40%) 3.38 ( 15.45%) 1.66 ( 58.52%)


Regards,
Giovanni

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