linux-kernel - Re: [Eas-dev] [PATCH V4 1/3] sched: cpufreq: Allow remote cpufreq callbacks

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Date:   Wed, 26 Jul 2017 22:34:58 -0700
From:   "Joel Fernandes (Google)" <joel.opensrc@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-pm@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        smuckle.linux@...il.com, eas-dev@...ts.linaro.org,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [Eas-dev] [PATCH V4 1/3] sched: cpufreq: Allow remote cpufreq callbacks

Hi Viresh,

On Wed, Jul 26, 2017 at 2:22 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> We do not call cpufreq callbacks from scheduler core for remote
> (non-local) CPUs currently. But there are cases where such remote
> callbacks are useful, specially in the case of shared cpufreq policies.
>
> This patch updates the scheduler core to call the cpufreq callbacks for
> remote CPUs as well.
>
> For now, all the registered utilization update callbacks are updated to
> return early if remote callback is detected. That is, this patch just
> moves the decision making down in the hierarchy.
>
> Later patches would enable remote callbacks for shared policies.
>
> Based on initial work from Steve Muckle.
>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
<snip>
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -72,10 +72,15 @@ static DEFINE_PER_CPU(struct sugov_cpu, sugov_cpu);
>
>  /************************ Governor internals ***********************/
>
> -static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> +static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time,
> +                                    int target_cpu)
>  {
>         s64 delta_ns;
>
> +       /* Don't allow remote callbacks */
> +       if (smp_processor_id() != target_cpu)
> +               return false;
> +
>         if (sg_policy->work_in_progress)
>                 return false;
>
> @@ -221,7 +226,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>         sugov_set_iowait_boost(sg_cpu, time, flags);
>         sg_cpu->last_update = time;
>
> -       if (!sugov_should_update_freq(sg_policy, time))
> +       if (!sugov_should_update_freq(sg_policy, time, hook->cpu))
>                 return;

Since with the remote callbacks now possible, isn't it unsafe to
modify sg_cpu and sg_policy structures without a lock in
sugov_update_single?

Unlike sugov_update_shared, we don't acquire any lock in
sugov_update_single before updating these structures. Did I miss
something?


thanks,

-Joel