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

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Message-ID: <20160421123341.GJ3408@twins.programming.kicks-ass.net>
Date:	Thu, 21 Apr 2016 14:33:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	"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 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.

So your Changelog is actively wrong, the tasks _are_ still running,
albeit not for very much longer.