linux-kernel - Re: [PATCH] sched/core: Fix arch_scale_freq

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Date:   Wed, 14 Dec 2022 09:49:45 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yair Podemsky <ypodemsk@...hat.com>, peterz@...radead.org,
        mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, rafael.j.wysocki@...el.com,
        ggherdovich@...e.cz, jlelli@...hat.com, mtosatti@...hat.com,
        nsaenz@...nel.org, linux-kernel@...r.kernel.org
Cc:     ypodemsk@...hat.com
Subject: Re: [PATCH] sched/core: Fix arch_scale_freq_tick() on tickless systems

On 30/11/22 14:51, Yair Podemsky wrote:
> In order for the scheduler to be frequency invariant we measure the
> ratio between the maximum cpu frequency and the actual cpu frequency.
> During long tickless periods of time the calculations that keep track
> of that might overflow, in the function scale_freq_tick():
>
> if (check_shl_overflow(acnt, 2*SCHED_CAPACITY_SHIFT, &acnt))
> »       goto error;
>
> eventually forcing the kernel to disable the feature for all cpus,
> and show the warning message
> "Scheduler frequency invariance went wobbly, disabling!".
> Let's avoid that by limiting the frequency invariant calculations
> to cpus with regular tick.
>
> Fixes: e2b0d619b400 ("x86, sched: check for counters overflow in frequency invariant accounting")
> Signed-off-by: Yair Podemsky <ypodemsk@...hat.com>
> Suggested-by: "Peter Zijlstra (Intel)" <peterz@...radead.org>

Reviewed-by: Valentin Schneider <vschneid@...hat.com>

> ---
> V1 -> V2: solution approach was changed from detecting long tickless periods
> to frequency invariant measurements on housekeeping cpus only.
> Link: ee89073a1e9de11c7bd7726eb5da71a0e8795099.camel@...hat.com
> ---
>  kernel/sched/core.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index daff72f00385..1bb0a840c817 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5469,7 +5469,9 @@ void scheduler_tick(void)
>       unsigned long thermal_pressure;
>       u64 resched_latency;
>
> -	arch_scale_freq_tick();
> +	if (housekeeping_cpu(cpu, HK_TYPE_TICK))
> +		arch_scale_freq_tick();
> +
>       sched_clock_tick();
>
>       rq_lock(rq, &rf);
> --
> 2.31.1