Date: Wed, 20 Dec 2017 15:33:44 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Juri Lelli <juri.lelli@redhat.com>, Todd Kjos <tkjos@android.com>,
        Joel Fernandes <joelaf@google.com>
Subject: Re: [PATCH v3 1/6] cpufreq: schedutil: reset sg_cpus's flags at IDLE
 enter
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 <20171130114723.29210-2-patrick.bellasi@arm.com>
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On Thu, Nov 30, 2017 at 11:47:18AM +0000, Patrick Bellasi wrote:
> Currently, sg_cpu's flags are set to the value defined by the last call
> of the cpufreq_update_util(); for RT/DL classes this corresponds to the
> SCHED_CPUFREQ_{RT/DL} flags always being set.
> 
> When multiple CPUs share the same frequency domain it might happen that
> a CPU which executed an RT task, right before entering IDLE, has one of
> the SCHED_CPUFREQ_RT_DL flags set, permanently, until it exits IDLE.
> 
> Although such an idle CPU is _going to be_ ignored by the
> sugov_next_freq_shared():
>   1. this kind of "useless RT requests" are ignored only if more then
>      TICK_NSEC have elapsed since the last update
>   2. we can still potentially trigger an already too late switch to
>      MAX, which starts also a new throttling interval
>   3. the internal state machine is not consistent with what the
>      scheduler knows, i.e. the CPU is now actually idle

So I _really_ hate having to clutter the idle path for this shared case
:/

1, can obviously be fixed by short-circuiting the timeout when idle.

2. not sure how if you do 1; anybody doing a switch will go through
   sugov_next_freq_shared() which will poll all relevant CPUs and per 1
   will see its idle, no?

Not sure what that leaves for 3.

> diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
> index d518664cce4f..6e8ae2aa7a13 100644
> --- a/kernel/sched/idle_task.c
> +++ b/kernel/sched/idle_task.c
> @@ -30,6 +30,10 @@ pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
>  	put_prev_task(rq, prev);
>  	update_idle_core(rq);
>  	schedstat_inc(rq->sched_goidle);
> +
> +	/* kick cpufreq (see the comment in kernel/sched/sched.h). */
> +	cpufreq_update_util(rq, SCHED_CPUFREQ_IDLE);
> +
>  	return rq->idle;
>  }
>  
> -- 
> 2.14.1
>