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
>
