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

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Message-ID: <CA+55aFw-dSLHevtTWtmtR+HRpzRGEHzQxC6nrMnM-oYgmvi7Qg@mail.gmail.com>
Date:	Mon, 14 Sep 2015 16:58:21 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <sasha.levin@...cle.com>, wanpeng.li@...mail.com,
	Peter Anvin <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Huang Ying <ying.huang@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:sched/core] sched: 'Annotate' migrate_tasks()

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.

              Linus
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