linux-kernel - Re: workqueue cpu affinity

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Date:	Wed, 11 Jun 2008 20:08:15 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Max Krasnyansky <maxk@...lcomm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...e.hu,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Paul Jackson <pj@....com>, menage@...gle.com,
	linux-kernel@...r.kernel.org, Mark Hounschell <dmarkh@....rr.com>
Subject: Re: workqueue cpu affinity

On 06/10, Max Krasnyansky wrote:
>
> Here is some backgound on this. Full cpu isolation requires some tweaks to the
> workqueue handling. Either the workqueue threads need to be moved (which is my
> current approach), or work needs to be redirected when it's submitted.

_IF_ we have to do this, I think it is much better to move cwq->thread.

> Peter Zijlstra wrote:
> > The advantage of creating a more flexible or fine-grained flush is that
> > large machine also profit from it.
> I agree, our current workqueue flush scheme is expensive because it has to
> schedule on each online cpu. So yes improving flush makes sense in general.

Yes, it is easy to implement flush_work(struct work_struct *work) which
only waits for that work, so it can't hang unless it was enqueued on the
isolated cpu.

But in most cases it is enough to just do

	if (cancel_work_sync(work))
		work->func(work);

Or we can add flush_workqueue_cpus(struct workqueue_struct *wq, cpumask_t *cpu_map).

But I don't think we should change the behaviour of flush_workqueue().

> This will require a bit of surgery across the entire tree. There is a lot of
> code that calls flush_scheduled_work()

Almost all of them should be changed to use cancel_work_sync().

Oleg.

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