Re: [PATCH v2 2/2] xfs: kick extra large ioends to completion workqueue

[Brian Foster <bfoster@redhat.com>]

On Mon, Oct 05, 2020 at 08:55:37PM -0700, Darrick J. Wong wrote:
> On Mon, Oct 05, 2020 at 11:21:02AM -0400, Brian Foster wrote:
> > We've had reports of soft lockup warnings in the iomap ioend
> > completion path due to very large bios and/or bio chains. Divert any
> > ioends with 256k or more pages to process to the workqueue so
> > completion occurs in non-atomic context and can reschedule to avoid
> > soft lockup warnings.
> 
> Hmmmm... is there any way we can just make end_page_writeback faster?
> 

I'm not sure that would help us. It's not doing much work as it is. The
issue is just that we effectively queue so many of them to a single bio
completion due to either bio chaining or the physical page merging
implemented by multipage bvecs.

> TBH it still strikes me as odd that we'd cap ioends this way just to
> cover for the fact that we have to poke each and every page.
> 

I suppose, but it's not like we don't already account for constructing
bios that must be handed off to a workqueue for completion processing.
Also FWIW this doesn't cap ioend size like my original patch does. It
just updates XFS to send them to the completion workqueue.

> (Also, those 'bool atomic' in the other patch make me kind of nervous --
> how do we make sure (from a QA perspective) that nobody gets that wrong?)
> 

Yeah, that's a bit ugly. If somebody has a better idea on the factoring
I'm interested in hearing about it. My understanding is that in_atomic()
is not reliable and/or generally frowned upon, hence the explicit
context parameter.

Also, I don't have the error handy but my development kernel complains
quite clearly if we make a call that can potentially sleep in atomic
context. I believe this is the purpose of the __might_sleep()
(CONFIG_DEBUG_ATOMIC_SLEEP) annotation.

Brian

> --D
> 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> > 
> > v2:
> > - Fix type in macro.
> > 
> >  fs/xfs/xfs_aops.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> > index 3e061ea99922..c00cc0624986 100644
> > --- a/fs/xfs/xfs_aops.c
> > +++ b/fs/xfs/xfs_aops.c
> > @@ -30,6 +30,13 @@ XFS_WPC(struct iomap_writepage_ctx *ctx)
> >  	return container_of(ctx, struct xfs_writepage_ctx, ctx);
> >  }
> >  
> > +/*
> > + * Kick extra large ioends off to the workqueue. Completion will process a lot
> > + * of pages for a large bio or bio chain and a non-atomic context is required to
> > + * reschedule and avoid soft lockup warnings.
> > + */
> > +#define XFS_LARGE_IOEND	(262144ULL << PAGE_SHIFT)
> > +
> >  /*
> >   * Fast and loose check if this write could update the on-disk inode size.
> >   */
> > @@ -239,7 +246,8 @@ static inline bool xfs_ioend_needs_workqueue(struct iomap_ioend *ioend)
> >  {
> >  	return ioend->io_private ||
> >  		ioend->io_type == IOMAP_UNWRITTEN ||
> > -		(ioend->io_flags & IOMAP_F_SHARED);
> > +		(ioend->io_flags & IOMAP_F_SHARED) ||
> > +		(ioend->io_size >= XFS_LARGE_IOEND);
> >  }
> >  
> >  STATIC void
> > -- 
> > 2.25.4
> > 
>