From: "Darrick J. Wong" <djwong@kernel.org>
To: Brian Foster <bfoster@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>,
linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v3 1/3] iomap: resched ioend completion when in non-atomic context
Date: Tue, 25 May 2021 18:19:06 -0700 [thread overview]
Message-ID: <20210526011906.GE202078@locust> (raw)
In-Reply-To: <20210524165305.GA202078@locust>
On Mon, May 24, 2021 at 09:53:05AM -0700, Darrick J. Wong wrote:
> On Mon, May 24, 2021 at 07:57:31AM -0400, Brian Foster wrote:
> > On Thu, May 20, 2021 at 02:58:58PM -0700, Darrick J. Wong wrote:
> > > On Tue, May 18, 2021 at 07:38:01AM -0400, Brian Foster wrote:
> > > > On Mon, May 17, 2021 at 06:54:34PM +0100, Matthew Wilcox wrote:
> > > > > On Mon, May 17, 2021 at 01:17:20PM -0400, Brian Foster wrote:
> > > > > > @@ -1084,9 +1084,12 @@ iomap_finish_ioend(struct iomap_ioend *ioend, int error)
> > > > > > next = bio->bi_private;
> > > > > >
> > > > > > /* walk each page on bio, ending page IO on them */
> > > > > > - bio_for_each_segment_all(bv, bio, iter_all)
> > > > > > + bio_for_each_segment_all(bv, bio, iter_all) {
> > > > > > iomap_finish_page_writeback(inode, bv->bv_page, error,
> > > > > > bv->bv_len);
> > > > > > + if (!atomic)
> > > > > > + cond_resched();
> > > > > > + }
> > > > >
> > > > > I don't know that it makes sense to check after _every_ page. I might
> > > > > go for every segment. Some users check after every thousand pages.
> > > > >
> > > >
> > > > The handful of examples I come across on a brief scan (including the
> > > > other iomap usage) have a similar pattern as used here. I don't doubt
> > > > there are others, but I think I'd prefer to have more reasoning behind
> > > > adding more code than might be necessary (i.e. do we expect additional
> > > > overhead to be measurable here?). As it is, the intent isn't so much to
> > > > check on every page as much as this just happens to be the common point
> > > > of the function to cover both long bio chains and single vector bios
> > > > with large numbers of pages.
> > >
> > > It's been a while since I waded through the macro hell to find out what
> > > cond_resched actually does, but iirc it can do some fairly heavyweight
> > > things (disable preemption, call the scheduler, rcu stuff) which is why
> > > we're supposed to be a little judicious about amortizing each call over
> > > a few thousand pages.
> > >
> >
> > It looks to me it just checks some state bit and only does any work if
> > actually necessary. I suppose not doing that less often is cheaper than
> > doing it more, but it's not clear to me it's enough that it really
> > matters and/or warrants more code to filter out calls..
> >
> > What exactly did you have in mind for logic? I suppose we could always
> > stuff a 'if (!(count++ % 1024)) cond_resched();' or some such in the
> > inner loop, but that might have less of an effect on larger chains
> > constructed of bios with fewer pages (depending on whether that might
> > still be possible).
>
> I /was/ thinking about a function level page counter until I noticed
> that iomap_{write,unshare}_actor call cond_resched for every page it
> touches. I withdraw the comment. :)
Oh, also:
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
--D
>
> --D
>
> >
> > Brian
> >
> > > --D
> > >
> > > > Brian
> > > >
> > >
> >
next prev parent reply other threads:[~2021-05-26 1:19 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-17 17:17 [PATCH v3 0/3] iomap: avoid soft lockup warnings on large ioends Brian Foster
2021-05-17 17:17 ` [PATCH v3 1/3] iomap: resched ioend completion when in non-atomic context Brian Foster
2021-05-17 17:54 ` Matthew Wilcox
2021-05-18 11:38 ` Brian Foster
2021-05-20 21:58 ` Darrick J. Wong
2021-05-24 11:57 ` Brian Foster
2021-05-24 16:53 ` Darrick J. Wong
2021-05-26 1:19 ` Darrick J. Wong [this message]
2021-05-22 7:45 ` Ming Lei
2021-05-24 11:57 ` Brian Foster
2021-05-24 14:11 ` Ming Lei
2021-05-17 17:17 ` [PATCH v3 2/3] xfs: kick large ioends to completion workqueue Brian Foster
2021-05-26 1:20 ` Darrick J. Wong
2021-05-17 17:17 ` [PATCH RFC v3 3/3] iomap: bound ioend size to 4096 pages Brian Foster
2021-05-19 13:28 ` Christoph Hellwig
2021-05-19 14:52 ` Brian Foster
2021-05-20 23:27 ` Darrick J. Wong
2021-05-24 12:02 ` Brian Foster
2021-05-25 4:20 ` Darrick J. Wong
2021-05-25 4:29 ` Damien Le Moal
2021-05-25 7:13 ` Dave Chinner
2021-05-25 9:07 ` Andreas Gruenbacher
2021-05-26 2:12 ` Matthew Wilcox
2021-05-26 3:32 ` Darrick J. Wong
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