linux-kernel - Re: [tip:sched/core] sched: 'Annotate' migrate

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Message-ID: <20150915075741.GV16853@twins.programming.kicks-ass.net>
Date:	Tue, 15 Sep 2015 09:57:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Sasha Levin <sasha.levin@...cle.com>, wanpeng.li@...mail.com,
	Peter Anvin <hpa@...or.com>, Huang Ying <ying.huang@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:sched/core] sched: 'Annotate' migrate_tasks()

On Mon, Sep 14, 2015 at 04:58:21PM -0700, Linus Torvalds wrote:
> On Sun, Sep 13, 2015 at 3:56 AM, tip-bot for Wanpeng Li
> <tipbot@...or.com> wrote:
> > +               lockdep_unpin_lock(&rq->lock);
> > +               raw_spin_unlock(&rq->lock);
> > +               raw_spin_lock(&next->pi_lock);
> > +               raw_spin_lock(&rq->lock);
> 
> So _if_ this is ever likely to be a performance-critical piece of
> code, it might be worth doing
> 
>     if (!raw_spin_trylock(&next->pi_lock)) {
>         .. do the above unlock and relock in the right order sequence ..
>     }
> 
> to avoid doing that whole "unlock just to relock in the right order" thing.
> 
> That's particularly true in cases where dropping one of the locks can
> necessitate re-doing some checks.

Correct, but this is the hot-unplug, migrate the few tasks that are now
on a dead CPU away path, so meh ;-)
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