linux-kernel - Re: regression in page writeback

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Message-Id: <20090922190915.a472fb7c.akpm@linux-foundation.org>
Date:	Tue, 22 Sep 2009 19:09:15 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"richard@....demon.co.uk" <richard@....demon.co.uk>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>
Subject: Re: regression in page writeback

On Wed, 23 Sep 2009 10:01:04 +0800 Wu Fengguang <fengguang.wu@...el.com> wrote:

> > If there's still outstanding dirty data for any of those queues, both
> > wb_kupdate() and background_writeout() will take a teeny sleep and then
> > will re-poll the queues.
> > 
> > Did that logic get broken?
> 
> No, but the "teeny sleep" is normally much smaller. When io queue is
> not congested, every io completion event will wakeup the congestion
> waiters. Also A's event could wake up B's waiters.
> 
> __freed_request() always calls blk_clear_queue_congested() if under
> congestion threshold which in turn wakes up congestion waiters:
> 
>         if (rl->count[sync] < queue_congestion_off_threshold(q))
>                 blk_clear_queue_congested(q, sync);
> 

Yes.  Have any problems been demonstrated due to that?

And what's _sufficiently_ wrong with that to justify adding potentially
thousands of kernel threads?  It was always a design objective to avoid
doing that.

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