linux-kernel - Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention

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Date:	Fri, 17 Dec 2010 19:41:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Frank Rowand <frank.rowand@...sony.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention

On Fri, 2010-12-17 at 19:24 +0100, Peter Zijlstra wrote:
> > Even simpler. This can race with, say, __migrate_task() which does
> > deactivate_task + activate_task and temporary clears on_rq. Although
> > this is simple to fix, I think.
> 
> Yes, another hole.. 

__migrate_task() is a tad special in that it should only migrate
runnable tasks, but of course we can have it preempted with ->state !=
TASK_RUNNING, which makes life interesting, but we can fix that by
including a PREEMPT_ACTIVE test in ttwu.

Other such callers are more straight fwd though:

  rt_mutex_setprio()
  set_user_nice()
  sched_move_task()
  __sched_setscheduler()
  normalize_task()

none of those are fast-path operations, so it is quite doable to also
take the ->pi_lock there and fully serialize them.
  
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