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

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

On Mon, May 24, 2021 at 07:57:31AM -0400, Brian Foster wrote:
> On Thu, May 20, 2021 at 02:58:58PM -0700, Darrick J. Wong wrote:
> > 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.
> > 
> 
> It looks to me it just checks some state bit and only does any work if
> actually necessary. I suppose not doing that less often is cheaper than
> doing it more, but it's not clear to me it's enough that it really
> matters and/or warrants more code to filter out calls..
> 
> What exactly did you have in mind for logic? I suppose we could always
> stuff a 'if (!(count++ % 1024)) cond_resched();' or some such in the
> inner loop, but that might have less of an effect on larger chains
> constructed of bios with fewer pages (depending on whether that might
> still be possible).

I /was/ thinking about a function level page counter until I noticed
that iomap_{write,unshare}_actor call cond_resched for every page it
touches.  I withdraw the comment. :)

--D

> 
> Brian
> 
> > --D
> > 
> > > Brian
> > > 
> > 
>