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@xxxxxxxxxxxxx
