linux-kernel - Re: [rfc] lru_add_drain

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Message-ID: <20090907141818.GA8394@redhat.com>
Date:	Mon, 7 Sep 2009 16:18:18 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
	linux-mm <linux-mm@...ck.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] lru_add_drain_all() vs isolation

On 09/07, Peter Zijlstra wrote:
>
> On Mon, 2009-09-07 at 15:35 +0200, Oleg Nesterov wrote:
> >
> > Failed to google the previous discussion. Could you please point me?
> > What is the problem?
>
> Ah, the general problem is that when we carve up the machine into
> partitions using cpusets, we still get machine wide tickles on all cpus
> from workqueue stuff like schedule_on_each_cpu() and flush_workqueue(),
> even if some cpus don't actually used their workqueue.
>
> So the below limits lru_add_drain() activity to cpus that actually have
> pages in their per-cpu lists.

Thanks Peter!

> flush_workqueue() could limit itself to cpus that had work queued since
> the last flush_workqueue() invocation, etc.

But "work queued since the last flush_workqueue() invocation" just means
"has work queued". Please note that flush_cpu_workqueue() does nothing
if there are no works, except it does lock/unlock of cwq->lock.

IIRC, flush_cpu_workqueue() has to lock/unlock to avoid the races with
CPU hotplug, but _perhaps_ flush_workqueue() can do the check lockless.

Afaics, we can add the workqueue_struct->cpu_map_has_works to help
flush_workqueue(), but this means we should complicate insert_work()
and run_workqueue() which should set/clear the bit. But given that
flush_workqueue() should be avoided anyway, I am not sure.

Oleg.

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