linux-kernel - Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish

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Date:	Thu, 17 Sep 2015 16:08:19 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Chris Mason <clm@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Josef Bacik <jbacik@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Neil Brown <neilb@...e.de>, Christoph Hellwig <hch@....de>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()

On Thu, Sep 17, 2015 at 3:42 PM, Chris Mason <clm@...com> wrote:
>
> Playing around with the plug a little, most of the unplugs are coming
> from the cond_resched_lock().  Not really sure why we are doing the
> cond_resched() there, we should be doing it before we retake the lock
> instead.
>
> This patch takes my box (with dirty thresholds at 1.5GB/3GB) from 195K
> files/sec up to 213K.  Average IO size is the same as 4.3-rc1.

Ok, so at least for you, part of the problem really ends up being that
there's a mix of the "synchronous" unplugging (by the actual explicit
"blk_finish_plug(&plug);") and the writeback that is handed off to
kblockd_workqueue.

I'm not seeing why that should be an issue. Sure, there's some CPU
overhead to context switching, but I don't see that it should be that
big of a deal.

I wonder if there is something more serious wrong with the kblockd_workqueue.

                    Linus
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