linux-kernel - Re: [PATCH] sched/cpufreq: don't trigger cpufreq update w/o real rt/deadline tasks running

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Date:	Thu, 21 Apr 2016 19:17:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	Wanpeng Li <kernellwp@...il.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Steve Muckle <steve.muckle@...aro.org>
Subject: Re: [PATCH] sched/cpufreq: don't trigger cpufreq update w/o real
 rt/deadline tasks running

On Thu, Apr 21, 2016 at 07:07:51PM +0200, Rafael J. Wysocki wrote:
> On Thu, Apr 21, 2016 at 2:33 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Thu, Apr 21, 2016 at 09:09:43AM +0800, Wanpeng Li wrote:
> >> >> Sometimes .update_curr hook is called w/o tasks actually running, it is
> >> >> captured by:
> >> >>
> >> >>          u64 delta_exec = rq_clock_task(rq) - curr->se.exec_start;
> >> >>
> >> >> We should not trigger cpufreq update in this case for rt/deadline
> >> >> classes, and this patch fix it.
> >
> >> I add a print to print when delta_exec is zero for rt class, something
> >
> > So its zero, so what?
> >
> >> like below:
> >
> >>       watchdog/5-48    [005] d...   568.449105: update_curr_rt: rt
> >> delta_exec is zero
> >>       watchdog/5-48    [005] d...   568.449111: <stack trace>
> >>  => put_prev_task_rt
> >>  => pick_next_task_idle
> >
> > So we'll go idle, but as of this point we're still running the rt task.
> 
> Skipping the update in that case might be the right thing to do, though.

It is; the patch looks fine, but the Changelog is entirely
misleading/wrong.

Its not because the task isn't running; it is. Its because we end up
calling update_curr() multiple times and bailing when nothing changed is
indeed the right thing.