linux-kernel - Re: [PATCH v2 2/5] sched: Teach scheduler to understand ONRQ

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Message-ID: <1406755550.18758.12.camel@localhost.localdomain>
Date:	Thu, 31 Jul 2014 01:25:50 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Kirill Tkhai <ktkhai@...allels.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, nicolas.pitre@...aro.org,
	pjt@...gle.com, rostedt@...dmis.org, umgwanakikbuti@...il.com,
	tim.c.chen@...ux.intel.com, mingo@...nel.org
Subject: Re: [PATCH v2 2/5] sched: Teach scheduler to understand
 ONRQ_MIGRATING state

В Ср, 30/07/2014 в 16:41 +0200, Oleg Nesterov пишет:
> On 07/30, Kirill Tkhai wrote:
> >
> > В Вт, 29/07/2014 в 18:19 +0200, Oleg Nesterov пишет:
> > > On 07/29, Kirill Tkhai wrote:
> > > >
> > > > How about this? Everything is inside task_rq_lock() now. The patch
> > > > became much less.
> > >
> > > And with this change task_migrating() is not possible under
> > > task_rq_lock() or __task_rq_lock(). This means that 1/5 can be simplified
> > > too.
> >
> > It seems to me it won't be useless anyway. In every place we underline
> > that a task is exactly queued or dequeued, so it's not necessary to remember
> > whether it is migrating or not. This is a cleanup, though it's big.
> 
> But, otoh, when you read the code which does "if (task_queued())" it is not
> clear whether this code knows that task_migrating() is not possible, or we
> should treat the task_migrating() state specially.
> 
> But I agree, this is subjective, I leave this to you and Peter.
> 
> > > __migrate_swap_task() is probably the notable exception...
> > >
> > > Off-topic, but it takes 2 ->pi_lock's. This means it can deadlock with
> > > try_to_wake_up_local() (if a 3rd process does ttwu() and waits for
> > > ->on_cpu == 0). But I guess __migrate_swap_task() should not play with
> > > PF_WQ_WORKER threads.
> >
> > Hmm.. I'm surprised, PF_WQ_WORKER threads may be unbound. But it seems
> > we still can't pass them to try_to_wake_up_local.
> 
> Why? See wq_worker_sleeping/try_to_wake_up_local in __schedule().
> 
> But perhaps I misunderstood you, and probably I was not clear. If
> wq_worker_sleeping() returns !NULL then both task should be local, surely
> we do not want to migrate them.

I mean it was surprising for me that PF_WQ_WORKER threads may be unbound...
I hope wq_worker_sleeping() does not return them.

Nothing important from me about this question.

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