Re: [PATCH v3 1/3] iomap: resched ioend completion when in non-atomic context

[Brian Foster <bfoster@redhat.com>]

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.

Brian