linux-kernel - Re: [PATCH 2/2] sched: update runqueue clock before migrations away

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Date:	Fri, 13 Dec 2013 09:48:45 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Chris Redpath <chris.redpath@....com>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"alex.shi@...aro.org" <alex.shi@...aro.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"bsegall@...gle.com" <bsegall@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 2/2] sched: update runqueue clock before migrations away

On 12 December 2013 19:24, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Dec 10, 2013 at 03:55:43PM +0000, Chris Redpath wrote:
>> >That's guestimating the last_runnable_update based on decay_count, and
>> >per the previous the decay count can get slightly out of sync.
>>
>> The guesstimation works fine, the issue is only that we can't tell at
>> this point how much time that entity was asleep when the CPU it ran on
>> has no tick and since it is too expensive to synchronize the clocks,
>> there isn't (currently) a way to find out without updating the rq we
>> came from.
>>
>> I can't see anything handy lying around in struct rq, but is there a
>> copy of the jiffies held anywhere per-cpu presuming it would stop being
>> updated when the tick stops? If not, I could store one somewhere as part
>> of turning the tick off and then we could use the difference between
>> that and the current jiffies count to estimate the amount of time in
>> limbo. That would almost certainly be accurate enough for me - a few ms
>> won't hurt but when we lose seconds it does.
>
> Would pre_schedule_idle() -> rq_last_tick_reset() -> rq->last_sched_tick
> be useful?
>
> I suppose we could easily lift that to NO_HZ_COMMON.
>
> Which raises another point; I dislike this idle pre/post business. Why
> can't the post_schedule_idle() calls be done from pick_next_task_idle()
> and pre_schedule_idle() done from put_prev_task_idle() ?

The update of runnable_avg for a cpu that becomes idle, must be done
before the call to idle_balance which will use it; So idle_enter_fair
is called in pre_schedule_idle.
idle_exit_fair has been put in the post_schedule function in order to
be symmetric and coherent but nothing prevent from moving it in
pick_next_task_idle.

Vincent

>
> That would avoid the post_schedule() rq->lock dance.
>
> Vince?
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