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

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

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.

--D

> Brian
>