linux-kernel - Re: [PATCH 4/4] sched: add hooks for workqueue

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Date:	Mon, 31 May 2010 12:05:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 4/4] sched: add hooks for workqueue

On Mon, 2010-05-31 at 11:58 +0200, Tejun Heo wrote:
> Hello, Peter.
> 
> On 05/31/2010 10:01 AM, Peter Zijlstra wrote:
> > On Thu, 2010-05-13 at 12:48 +0200, Tejun Heo wrote:
> >> Concurrency managed workqueue needs to know when workers are going to
> >> sleep and waking up, and, when a worker goes to sleep, be able to wake
> >> up another worker to maintain adequate concurrency.  This patch
> >> introduces PF_WQ_WORKER to identify workqueue workers and adds the
> >> following two hooks.
> >>
> >> * wq_worker_waking_up(): called when a worker is woken up.
> >>
> >> * wq_worker_sleeping(): called when a worker is going to sleep and may
> >>   return a pointer to a local task which should be woken up.  The
> >>   returned task is woken up using try_to_wake_up_local() which is
> >>   simplified ttwu which is called under rq lock and can only wake up
> >>   local tasks.
> > 
> > This changelog seems to lack explanation for why you need the wakeup
> > callback.
> 
> Because cmwq "needs to know when workers are going to sleep and waking
> up, and, when a worker goes to sleep, be able to wake up another
> worker to maintain adequate concurrency".  

That again only explains what you use the sleep hook for, not what you
want to use the wakeup hook for (putting a worker to sleep when there
are now 2 runnable seems like a good use).
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