linux-kernel - Re: [PATCH v3 1/3] sched: introduce a new migration flag to task

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Message-ID: <20151015183414.GN3816@twins.programming.kicks-ass.net>
Date:	Thu, 15 Oct 2015 20:34:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Byungchul Park <byungchul.park@....com>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	yuyang.du@...el.com, pjt@...gle.com, efault@....de,
	tglx@...utronix.de
Subject: Re: [PATCH v3 1/3] sched: introduce a new migration flag to
 task_struct

On Thu, Oct 15, 2015 at 09:26:44PM +0900, Byungchul Park wrote:
> On Thu, Oct 15, 2015 at 01:18:13PM +0200, Peter Zijlstra wrote:
> > On Thu, Oct 15, 2015 at 06:01:14PM +0900, byungchul.park@....com wrote:
> > > From: Byungchul Park <byungchul.park@....com>
> > > 
> > > This patch removes a weird coupling between se->avg.last_update_time and
> > > the condition checking for migration, and introduce a new migration flag.
> > > Now, scheduler can use the flag instead of se->avg.last_update_time to
> > > check if migration already happened or not.
> > 
> > Was there a problem with that coupling? This does not explain.
> 
> The reason why i introduce the new flag is that 3/3 patch makes
> se->avg.last_update_time non-zero consistently, so we cannot use the
> condition "se->avg.last_update_time == 0" to check if migration has
> happened.

It would have been ever so good to have had that in the Changelog.

> > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > > index af6f252..66d0552 100644
> > > --- a/kernel/sched/sched.h
> > > +++ b/kernel/sched/sched.h
> > > @@ -1158,6 +1158,7 @@ static const u32 prio_to_wmult[40] = {
> > >  #define ENQUEUE_WAKING		0
> > >  #endif
> > >  #define ENQUEUE_REPLENISH	8
> > > +#define ENQUEUE_MIGRATED	16
> > 
> > Won't actually apply that..
> 
> Okay, I got your concern, let me think more..

Ah, so what I means was that that hunk will not apply. I see that what I
typed did not express that as clearly as I might have wanted.

Current code looks like:

#define ENQUEUE_REPLENISH       0x08
#define ENQUEUE_RESTORE		0x10

> By the way, what do you think about the approach of 3/3 patch?

Again a wee bit of worry. I like it for 64bits as there its nearly free
and does the right thing, but on 32bit we add 2 smp_rmb() invocations
(which are not free on ARM etc..). [Note that one could rewrite that
function to require but a single rmb].
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