On 11/05/2018 05:14 PM, Linus Torvalds wrote: > On Mon, Nov 5, 2018 at 12:12 PM Vlastimil Babka <vba...@suse.cz> wrote: >> I didn't spot an obvious mistake in the patch itself, so it looks >> like some bad interaction between scheduler and the mmap downgrade? > I'm thinking it's RWSEM_SPIN_ON_OWNER that ends up being confused by > the downgrade. > > It looks like the benchmark used to be basically CPU-bound, at about > 800% CPU, and now it's somewhere in the 200% CPU region: > > will-it-scale.time.percent_of_cpu_this_job_got > > 800 +-+-------------------------------------------------------------------+ > |.+.+.+.+.+.+.+. .+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+..+.+.+.+. .+.+.+.| > 700 +-+ +. + | > | | > 600 +-+ | > | | > 500 +-+ | > | | > 400 +-+ | > | | > 300 +-+ | > | | > 200 O-O O O O O O | > | O O O O O O O O O O O O O O O O O O | > 100 +-+-------------------------------------------------------------------+ > > which sounds like the downgrade really messes with the "spin waiting > for lock" logic. > > I'm thinking it's the "wake up waiter" logic that has some bad > interaction with spinning, and breaks that whole optimization. > > Adding Waiman and Davidlohr to the participants, because they seem to > be the obvious experts in this area. > > Linus
Optimistic spinning on rwsem is done only on writers spinning on a writer-owned rwsem. If a write-lock is downgraded to a read-lock, all the spinning waiters will quit. That may explain the drop in cpu utilization. I do have a old patch that enable a certain amount of reader spinning which may help the situation. I can rebase that and send it out for review if people have interest. Cheers, Longman
