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SubjectRe: [PATCH 1/1] sched/cputime: Mitigate performance regression in times()/clock_gettime()
On Mon, Aug 15, 2016 at 09:49:05AM +0200, Giovanni Gherdovich wrote:
> > 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
>

Is this really equivalent though? It updates one task instead of all
tasks in the group and there is no guarantee that tsk == current.
Glancing at it, it should monotonically increase but it looks like it
would calculate stale data.

--
Mel Gorman
SUSE Labs

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