Re: [PATCH] sched/fair: check for idle core

[Vincent Guittot]

Hi Julia,

On Tue, 20 Oct 2020 at 19:21, Julia Lawall <Julia.Lawall@xxxxxxxx> wrote:
>
> On a thread wakeup, the change [1] from runnable load average to load
> average for comparing candidate cores means that recent short-running
> daemons on the core where a thread ran previously can be considered to
> have a higher load than the core performing the wakeup, even when the
> core where the thread ran previously is currently idle.  This can
> cause a thread to migrate, taking the place of some other thread that
> is about to wake up, and so on.  To avoid unnecessary migrations,
> extend wake_affine_idle to check whether the core where the thread
> previously ran is currently idle, and if so return that core as the
> target.
>
> [1] commit 11f10e5420f6ce ("sched/fair: Use load instead of runnable
> load in wakeup path")
>
> This particularly has an impact when using passive (intel_cpufreq)
> power management, where kworkers run every 0.004 seconds on all cores,
> increasing the likelihood that an idle core will be considered to have
> a load.
>
> The following numbers were obtained with the benchmarking tool
> hyperfine (https://github.com/sharkdp/hyperfine) on the NAS parallel
> benchmarks (https://www.nas.nasa.gov/publications/npb.html).  The
> tests were run on an 80-core Intel(R) Xeon(R) CPU E7-8870 v4 @
> 2.10GHz.  Active (intel_pstate) and passive (intel_cpufreq) power
> management were used.  Times are in seconds.  All experiments use all
> 160 hardware threads.
>
>         v5.9/active             v5.9+patch/active
> bt.C.c  24.725724+-0.962340     23.349608+-1.607214
> lu.C.x  29.105952+-4.804203     25.249052+-5.561617
> sp.C.x  31.220696+-1.831335     30.227760+-2.429792
> ua.C.x  26.606118+-1.767384     25.778367+-1.263850
>
>         v5.9/passive            v5.9+patch/passive
> bt.C.c  25.330360+-1.028316     23.544036+-1.020189
> lu.C.x  35.872659+-4.872090     23.719295+-3.883848
> sp.C.x  32.141310+-2.289541     29.125363+-0.872300
> ua.C.x  29.024597+-1.667049     25.728888+-1.539772
>
> On the smaller data sets (A and B) and on the other NAS benchmarks
> there is no impact on performance.
>
> Signed-off-by: Julia Lawall <Julia.Lawall@xxxxxxxx>

Reviewed-by Vincent Guittot <vincent.guittot@xxxxxxxxxx>

>
> ---
>  kernel/sched/fair.c |    3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index aa4c6227cd6d..9b23dad883ee 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5804,6 +5804,9 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync)
>         if (sync && cpu_rq(this_cpu)->nr_running == 1)
>                 return this_cpu;
>
> +       if (available_idle_cpu(prev_cpu))
> +               return prev_cpu;
> +
>         return nr_cpumask_bits;
>  }
>
>