Re: [PATCH] psi: annotate refault stalls from IO submission

[Dave Chinner <david@fromorbit.com>]

On Tue, Jul 23, 2019 at 01:34:50PM -0600, Jens Axboe wrote:
> On 7/23/19 1:04 PM, Johannes Weiner wrote:
> > CCing Jens for bio layer stuff
> > 
> > On Tue, Jul 23, 2019 at 10:02:26AM +1000, Dave Chinner wrote:
> >> Even better: If this memstall and "refault" check is needed to
> >> account for bio submission blocking, then page cache iteration is
> >> the wrong place to be doing this check. It should be done entirely
> >> in the bio code when adding pages to the bio because we'll only ever
> >> be doing page cache read IO on page cache misses. i.e. this isn't
> >> dependent on adding a new page to the LRU or not - if we add a new
> >> page then we are going to be doing IO and so this does not require
> >> magic pixie dust at the page cache iteration level
> > 
> > That could work. I had it at the page cache level because that's
> > logically where the refault occurs. But PG_workingset encodes
> > everything we need from the page cache layer and is available where
> > the actual stall occurs, so we should be able to push it down.
> > 
> >> e.g. bio_add_page_memstall() can do the working set check and then
> >> set a flag on the bio to say it contains a memstall page. Then on
> >> submission of the bio the memstall condition can be cleared.
> > 
> > A separate bio_add_page_memstall() would have all the problems you
> > pointed out with the original patch: it's magic, people will get it
> > wrong, and it'll be hard to verify and notice regressions.
> > 
> > How about just doing it in __bio_add_page()? PG_workingset is not
> > overloaded - when we see it set, we can generally and unconditionally
> > flag the bio as containing userspace workingset pages.
> > 
> > At submission time, in conjunction with the IO direction, we can
> > clearly tell whether we are reloading userspace workingset data,
> > i.e. stalling on memory.
> > 
> > This?
> 
> Not vehemently opposed to it, even if it sucks having to test page flags
> in the hot path.

That's kinda why I suggested the bio_add_page_memstall() variant for
the page cache read IO paths where this check would be required.

Not fussed either way, this is much cleaner and easier to maintain
IMO....

-Dave.
-- 
Dave Chinner
david@fromorbit.com