Re: [PATCH] iomap: Address soft lockup in iomap_finish_ioend()

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Jan 04, 2022 at 08:36:42AM -0500, Brian Foster wrote:
> On Sat, Jan 01, 2022 at 05:39:45PM +0000, Trond Myklebust wrote:
> ...
> > 
> > Fair enough. As long as someone is working on a solution, then I'm
> > happy. Just a couple of things:
> > 
> > Firstly, we've verified that the cond_resched() in the bio loop does
> > suffice to resolve the issue with XFS, which would tend to confirm what
> > you're saying above about the underlying issue being the ioend chain
> > length.
> > 
> > Secondly, note that we've tested this issue with a variety of older
> > kernels, including 4.18.x, 5.1.x and 5.15.x, so please bear in mind
> > that it would be useful for any fix to be backward portable through the
> > stable mechanism.
> > 
> 
> I've sent a couple or so different variants of this in the past. The
> last I believe was here [1], but still never seemed to go anywhere
> (despite having reviews on the first couple patches). That one was
> essentially a sequence of adding a cond_resched() call in the iomap code
> to address the soft lockup warning followed by capping the ioend size
> for latency reasons.

Huh.  I wonder why I didn't ever merge that?  I said I was going to do
that for 5.14 and ... never did.  ISTR Matthew saying something about
wanting to key the decision off of the number of pages/folios we'd have
to touch, and then musing about adding QOS metrics, me getting fussy
about that, trying to figure out if there was a way to make
iomap_finish_page_writeback cheaper, and ...

<checks notes>

...and decided that since the folio merge was imminent (HA!) I would
merge it after all the dust settled.  Add several months of Things I
Still Cannot Talk About and now it's 2022. :(

Ah, ok, I'll go reply elsewhere in the thread since I think my thinking
on all this has evolved somewhat since then.

--D

> 
> Brian
> 
> [1] https://lore.kernel.org/linux-xfs/20210517171722.1266878-1-bfoster@redhat.com/
> 
> > 
> > Thanks, and Happy New Year!
> > 
> >   Trond
> > 
> > -- 
> > Trond Myklebust
> > Linux NFS client maintainer, Hammerspace
> > trond.myklebust@hammerspace.com
> > 
> > 
>