On 12/27/2018 08:31 PM, Wang, Kemi wrote: > Hi, Waiman > Did you post that patch? Let's see if it helps.
I did post the patch a while ago. I will need to rebase it to a new baseline. Will do that in a week or 2. -Longman > > -----Original Message----- > From: LKP [mailto:[email protected]] On Behalf Of Waiman Long > Sent: Tuesday, November 6, 2018 6:40 AM > To: Linus Torvalds <[email protected]>; [email protected]; Davidlohr > Bueso <[email protected]> > Cc: [email protected]; Linux Kernel Mailing List > <[email protected]>; Matthew Wilcox <[email protected]>; > [email protected]; Colin King <[email protected]>; Andrew Morton > <[email protected]>; [email protected]; [email protected]; > [email protected] > Subject: Re: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread_ops -64.1% > regression > > On 11/05/2018 05:14 PM, Linus Torvalds wrote: >> On Mon, Nov 5, 2018 at 12:12 PM Vlastimil Babka <[email protected]> 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 > > > _______________________________________________ > LKP mailing list > [email protected] > https://lists.01.org/mailman/listinfo/lkp
