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From: Brian Foster <bfoster@redhat.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>,
	Dave Chinner <david@fromorbit.com>,
	Ritesh Harjani <riteshh@linux.ibm.com>,
	Anju T Sudhakar <anju@linux.vnet.ibm.com>,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, willy@infradead.org,
	minlei@redhat.com
Subject: Re: [PATCH] iomap: Fix the write_count in iomap_add_to_ioend().
Date: Thu, 17 Sep 2020 06:42:19 -0400	[thread overview]
Message-ID: <20200917104219.GA1811187@bfoster> (raw)
In-Reply-To: <20200917080455.GY26262@infradead.org>

On Thu, Sep 17, 2020 at 09:04:55AM +0100, Christoph Hellwig wrote:
> On Wed, Sep 16, 2020 at 09:07:14AM -0400, Brian Foster wrote:
> > Dave described the main purpose earlier in this thread [1]. The initial
> > motivation is that we've had downstream reports of soft lockup problems
> > in writeback bio completion down in the bio -> bvec loop of
> > iomap_finish_ioend() that has to finish writeback on each individual
> > page of insanely large bios and/or chains. We've also had an upstream
> > reports of a similar problem on linux-xfs [2].
> > 
> > The magic number itself was just pulled out of a hat. I picked it
> > because it seemed conservative enough to still allow large contiguous
> > bios (1GB w/ 4k pages) while hopefully preventing I/O completion
> > problems, but was hoping for some feedback on that bit if the general
> > approach was acceptable. I was also waiting for some feedback on either
> > of the two users who reported the problem but I don't think I've heard
> > back on that yet...
> 
> I think the saner answer is to always run large completions in the
> workqueue, and add a bunch of cond_resched() calls, rather than
> arbitrarily breaking up the I/O size.
> 

That wouldn't address the latency concern Dave brought up. That said, I
have no issue with this as a targeted solution for the softlockup issue.
iomap_finish_ioend[s]() is common code for both the workqueue and
->bi_end_io() contexts so that would require either some kind of context
detection (and my understanding is in_atomic() is unreliable/frowned
upon) or a new "atomic" parameter through iomap_finish_ioend[s]() to
indicate whether it's safe to reschedule. Preference?

Brian


  reply	other threads:[~2020-09-17 10:49 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-19 10:28 Anju T Sudhakar
2020-08-20 23:11 ` Dave Chinner
2020-08-21  4:45   ` Ritesh Harjani
2020-08-21  6:00     ` Christoph Hellwig
2020-08-21  9:09       ` Ritesh Harjani
2020-08-21 21:53     ` Dave Chinner
2020-08-22 13:13       ` Christoph Hellwig
2020-08-24 14:28         ` Brian Foster
2020-08-24 15:04           ` Christoph Hellwig
2020-08-24 15:48             ` Brian Foster
2020-08-25  0:42               ` Dave Chinner
2020-08-25 14:49                 ` Brian Foster
2020-08-31  4:01                   ` Ming Lei
2020-08-31 14:35                     ` Brian Foster
2020-09-16  0:12                   ` Darrick J. Wong
2020-09-16  8:45                     ` Christoph Hellwig
2020-09-16 13:07                       ` Brian Foster
2020-09-17  8:04                         ` Christoph Hellwig
2020-09-17 10:42                           ` Brian Foster [this message]
2020-09-17 14:48                             ` Christoph Hellwig
2020-09-17 21:33                               ` Darrick J. Wong
2020-09-17 23:13                           ` Ming Lei
2020-08-21  6:01   ` Christoph Hellwig
2020-08-21  6:07 ` Christoph Hellwig
2020-08-21  8:53   ` Ritesh Harjani
2020-08-21 14:49   ` Jens Axboe
2020-08-21 13:31 ` Matthew Wilcox

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