On 11/7/18 3:47 PM, Mikulas Patocka wrote: > > > On Wed, 7 Nov 2018, Mike Snitzer wrote: > >> On Tue, Nov 06 2018 at 4:34pm -0500, >> Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: >> >>> Hi >>> >>> These are the device mapper percpu patches. >>> >>> Note that I didn't test request-based device mapper because I don't have >>> hardware for it (the patches don't convert request-base targets to percpu >>> values, but there are a few inevitable changes in dm-rq.c). >> >> Patches 1 - 3 make sense. But the use of percpu inflight counters isn't >> something I can get upstream. Any more scalable counter still needs to >> be wired up to the block stats interfaces (the one you did in patch 5 is >> only for the "inflight" fsffs file, there is also the generic diskstats >> callout to part_in_flight(), etc). Wiring up both part_in_flight() and >> part_in_flight_rw() to optionally callout to a new callback isn't going >> to fly.. especially if that callout is looping up the sum of percpu >> counters. >> >> I checked with Jens and now that in 4.21 all of the old request-based IO >> path is gone (and given that blk-mq bypasses use of ->in_flight[]): the >> only consumer of the existing ->in_flight[] is the bio-based IO path. >> >> Given that now only bio-based is consuming it, and your work was focused >> on making bio-based DM's "pending" IO accounting more scalable, it is >> best to just change block core's ->in_flight[] directly. >> >> But Jens is against switching to using percpu counters because they are >> really slow when summing the counts. And diskstats does that >> frequently. Jens said at least 2 other attempts were made and rejected >> to switch over to percpu counters. > > I'd like to know - which kernel part needs to sum the percpu IO counters > frequently? > > My impression was that the counters need to be summed only when the user > is reading the files in sysfs and that is not frequent at all. part_round_stats() does it on IO completion - only every jiffy, but it's enough that previous attempts at percpu inflight counters only worked for some cases, and were worse for others. -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
