Re: [tip:sched/core] sched: 'Annotate' migrate_tasks()

[Peter Zijlstra <peterz@infradead.org>]

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@zytor.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 ;-)