linux-kernel - Re: workqueue thing

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Date:	Mon, 21 Dec 2009 16:19:46 +0100
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Jens Axboe <jens.axboe@...cle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	torvalds@...ux-foundation.org, awalls@...ix.net,
	linux-kernel@...r.kernel.org, jeff@...zik.org, mingo@...e.hu,
	akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com, avi@...hat.com,
	johannes@...solutions.net
Subject: Re: workqueue thing

On 12/21/2009 15:19, Tejun Heo wrote:
>
> Ah... okay, there are two aspects cmwq invovles the scheduler.
>
> A. Concurrency management.  This is achieved by the scheduler
>     callbacks which watches how many workers are working.
>
> B. Deadlock avoidance.  This requires migrating rescuers to CPUs under
>     allocation distress.  The problem here is that
>     set_cpus_allowed_ptr() doesn't allow migrating tasks to CPUs which
>     are online but !active (CPU_DOWN_PREPARE).

why would you get involved in what-runs-where at all? Wouldn't the scheduler
load balancer be a useful thing ? ;-)


>
> For A, it's far more efficient and robust with scheduler callbacks.
> It's conceptually pretty simple too.

Assuming you don't get tasks that do lots of cpu intense work and that need
to get preempted etc...like raid5/6 sync but also the btrfs checksumming etc etc.
That to me is the weak spot of the scheduler callback thing. We already have
a whole thing that knows how to load balance and preempt tasks... it's called "scheduler".
Let the scheduler do its job and just give it insight in the work (by having a runnable thread)
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