Re: [PATCH] cpufreq: schedutil: Don't ignore limit changes when util is unchanged

[Rafael J. Wysocki]

On Thu, Apr 10, 2025 at 4:45 AM Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx> wrote:
>
> From: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
>
> When utilization is unchanged, a policy limits update is ignored unless
> CPUFREQ_NEED_UPDATE_LIMITS is set. This occurs because limits_changed
> depends on the old broken behavior of need_freq_update to trigger a call
> into cpufreq_driver_resolve_freq() to evaluate the changed policy limits.
>
> After fixing need_freq_update, limit changes are ignored without
> CPUFREQ_NEED_UPDATE_LIMITS, at least until utilization changes enough to
> make map_util_freq() return something different.
>
> Fix the ignored limit changes by preserving the value of limits_changed
> until get_next_freq() is called, so limits_changed can trigger a call to
> cpufreq_driver_resolve_freq().
>
> Reported-and-tested-by: Stephan Gerhold <stephan.gerhold@xxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/Z_Tlc6Qs-tYpxWYb@xxxxxxxxxx
> Fixes: 8e461a1cb43d6 ("cpufreq: schedutil: Fix superfluous updates caused by need_freq_update")
> Signed-off-by: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
> ---
>  kernel/sched/cpufreq_schedutil.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 1a19d69b91ed3..f37b999854d52 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -82,7 +82,6 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
>                 return false;
>
>         if (unlikely(sg_policy->limits_changed)) {
> -               sg_policy->limits_changed = false;
>                 sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
>                 return true;
>         }
> @@ -171,9 +170,11 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
>         freq = get_capacity_ref_freq(policy);
>         freq = map_util_freq(util, freq, max);
>
> -       if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
> +       if (freq == sg_policy->cached_raw_freq && !sg_policy->limits_changed &&
> +           !sg_policy->need_freq_update)
>                 return sg_policy->next_freq;
>
> +       sg_policy->limits_changed = false;

AFAICS, after this code modification, a limit change may be missed due
to a possible race with sugov_limits() which cannot happen if
sg_policy->limits_changed is only cleared when it is set before
updating sg_policy->need_freq_update.

>         sg_policy->cached_raw_freq = freq;
>         return cpufreq_driver_resolve_freq(policy, freq);
>  }
> --
> 2.49.0
>