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From: Brian Foster <bfoster@redhat.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>,
	Ritesh Harjani <riteshh@linux.ibm.com>,
	Anju T Sudhakar <anju@linux.vnet.ibm.com>,
	darrick.wong@oracle.com, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	willy@infradead.org
Subject: Re: [PATCH] iomap: Fix the write_count in iomap_add_to_ioend().
Date: Mon, 24 Aug 2020 11:48:41 -0400	[thread overview]
Message-ID: <20200824154841.GB295033@bfoster> (raw)
In-Reply-To: <20200824150417.GA12258@infradead.org>

On Mon, Aug 24, 2020 at 04:04:17PM +0100, Christoph Hellwig wrote:
> On Mon, Aug 24, 2020 at 10:28:23AM -0400, Brian Foster wrote:
> > Do I understand the current code (__bio_try_merge_page() ->
> > page_is_mergeable()) correctly in that we're checking for physical page
> > contiguity and not necessarily requiring a new bio_vec per physical
> > page?
> 
> 
> Yes.
> 

Ok. I also realize now that this occurs on a kernel without commit
07173c3ec276 ("block: enable multipage bvecs"). That is probably a
contributing factor, but it's not clear to me whether it's feasible to
backport whatever supporting infrastructure is required for that
mechanism to work (I suspect not).

> > With regard to Dave's earlier point around seeing excessively sized bio
> > chains.. If I set up a large memory box with high dirty mem ratios and
> > do contiguous buffered overwrites over a 32GB range followed by fsync, I
> > can see upwards of 1GB per bio and thus chains on the order of 32+ bios
> > for the entire write. If I play games with how the buffered overwrite is
> > submitted (i.e., in reverse) however, then I can occasionally reproduce
> > a ~32GB chain of ~32k bios, which I think is what leads to problems in
> > I/O completion on some systems. Granted, I don't reproduce soft lockup
> > issues on my system with that behavior, so perhaps there's more to that
> > particular issue.
> > 
> > Regardless, it seems reasonable to me to at least have a conservative
> > limit on the length of an ioend bio chain. Would anybody object to
> > iomap_ioend growing a chain counter and perhaps forcing into a new ioend
> > if we chain something like more than 1k bios at once?
> 
> So what exactly is the problem of processing a long chain in the
> workqueue vs multiple small chains?  Maybe we need a cond_resched()
> here and there, but I don't see how we'd substantially change behavior.
> 

The immediate problem is a watchdog lockup detection in bio completion:

  NMI watchdog: Watchdog detected hard LOCKUP on cpu 25

This effectively lands at the following segment of iomap_finish_ioend():

		...
               /* walk each page on bio, ending page IO on them */
                bio_for_each_segment_all(bv, bio, iter_all)
                        iomap_finish_page_writeback(inode, bv->bv_page, error);

I suppose we could add a cond_resched(), but is that safe directly
inside of a ->bi_end_io() handler? Another option could be to dump large
chains into the completion workqueue, but we may still need to track the
length to do that. Thoughts?

Brian


  reply	other threads:[~2020-08-24 15:49 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 [this message]
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
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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