linux-kernel - Re: [PATCH 4/9] sched: Clean up idle task SMP logic

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Date:	Thu, 23 Jan 2014 15:52:44 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Paul Turner <pjt@...gle.com>,
	Benjamin Segall <bsegall@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 4/9] sched: Clean up idle task SMP logic

On 23 January 2014 12:37, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Jan 21, 2014 at 06:27:11PM +0100, Vincent Guittot wrote:
>> On 21 January 2014 12:17, Peter Zijlstra <peterz@...radead.org> wrote:
>
>> If i have correctly followed the new function path that is introduced
>> by the patchset, idle_enter_fair is called after idle_balance whereas
>> it must be called before in order to update the
>> runnable_avg_sum/period of the rq before evaluating the interest of
>> pulling cfs task
>
> Its idle_exit_fair, that's moved from pre_schedule to put_prev_task and
> thus indeed has crossed idle_balance.

ok, so i probably mix the changes introduced by your patches and a
potential issue that was already present before

idle_enter/exit_fair are used to be sure to account correctly the run
time of a rq in its sched_avg field, so they must be called before
entering/leaving idle. Your patch ensures that they will be called
correctly. Now, the idle_balance is used to check the interest of
pulling task on this newly idle CPU and it could use the sched_avg
field in a near future regarding the discussion around a energy aware
scheduler. as a result, we must update the sched_avg field before
running the idle_balance (that's not the case even before your
patches)

So one solution is probably to move idle_enter_fair into the idle_balance

Regards,
Vincent

>
> Yeah, I can leave that pre_schedule thing in place, however all that has
> be thinking.
>
> Ideally we'd do something like the below; but I must admit to still
> being slightly confused about the idle_{enter,exit}_fair() calls, their
> comment doesn't seem to clarify things.
>
> Steve, I don't think I wrecked rt/deadline by pulling in the
> pre_schedule call into pick_next_task(), but then, who knows :-)
>
> The only thing I really don't like is having that double conditional for
> the direct fair_sched_class call, but I didn't see a way around that,
> other than dropping that entirely. Then again, we cut out a conditional
> and indirect call by getting rid of pre_schedule() -- so it might just
> balance out.
>

[snip]
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