linux-kernel - Re: [RFC PATCH v5] sched/fair: select idle cpu from idle cpumask for task wakeup

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Date:   Mon, 7 Dec 2020 19:05:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Aubrey Li <aubrey.li@...ux.intel.com>, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        mgorman@...hsingularity.net, qais.yousef@....com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        tim.c.chen@...ux.intel.com, linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...e.de>, Jiang Biao <benbjiang@...il.com>
Subject: Re: [RFC PATCH v5] sched/fair: select idle cpu from idle cpumask for
 task wakeup

On Mon, Dec 07, 2020 at 04:52:24PM +0000, Valentin Schneider wrote:
> 
> On 07/12/20 15:48, Peter Zijlstra wrote:
> > On Wed, Nov 18, 2020 at 12:31:13PM +0800, Aubrey Li wrote:
> >> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> >> index f324dc36fc43..6f5947673e66 100644
> >> --- a/kernel/sched/idle.c
> >> +++ b/kernel/sched/idle.c
> >> @@ -163,6 +163,7 @@ static void cpuidle_idle_call(void)
> >>       */
> >>
> >>      if (cpuidle_not_available(drv, dev)) {
> >> +		update_idle_cpumask(this_rq(), true);
> >>              tick_nohz_idle_stop_tick();
> >>
> >>              default_idle_call();
> >> @@ -193,6 +194,7 @@ static void cpuidle_idle_call(void)
> >>                      max_latency_ns = dev->forced_idle_latency_limit_ns;
> >>              }
> >>
> >> +		update_idle_cpumask(this_rq(), true);
> >>              tick_nohz_idle_stop_tick();
> >>
> >>              next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
> >> @@ -205,10 +207,12 @@ static void cpuidle_idle_call(void)
> >>               */
> >>              next_state = cpuidle_select(drv, dev, &stop_tick);
> >>
> >> -		if (stop_tick || tick_nohz_tick_stopped())
> >> +		if (stop_tick || tick_nohz_tick_stopped()) {
> >> +			update_idle_cpumask(this_rq(), true);
> >>                      tick_nohz_idle_stop_tick();
> >
> > We already have a callback in tick_nohz_idle_stop_tick(), namely
> > nohz_balance_enter_idle().
> 
> That's a no-op for !NO_HZ_COMMON though. For similar reasons, Aubrey moved
> the clearing of the cpumask to scheduler_tick().
> 
> Are you saying this mechanism should only be driven for NO_HZ kernels?

IFF it keys off of the tick being stopped, then yes. But as said in the
other email, I think that's a very dubious thing to begin with.