Messages in this thread | | | Date | Mon, 19 Oct 2015 13:24:27 -0400 | From | Waiman Long <> | Subject | Re: [RFC PATCH] qspinlock: Improve performance by reducing load instruction rollback |
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On 10/19/2015 07:24 AM, Ingo Molnar wrote: > * Peter Zijlstra<peterz@infradead.org> wrote: > >> On Mon, Oct 19, 2015 at 09:58:23AM +0200, Ingo Molnar wrote: >>> * ling.ma.program@gmail.com<ling.ma.program@gmail.com> wrote: >>> >>>> From: Ma Ling<ling.ml@alibaba-inc.com> >>>> >>>> All load instructions can run speculatively but they have to follow >>>> memory order rule in multiple cores as below: >>>> _x = _y = 0 >>>> >>>> Processor 0 Processor 1 >>>> >>>> mov r1, [ _y] //M1 mov [ _x], 1 //M3 >>>> mov r2, [ _x] //M2 mov [ _y], 1 //M4 >>>> >>>> If r1 = 1, r2 must be 1 >>>> >>>> In order to guarantee above rule, although Processor 0 execute >>>> M1 and M2 instruction out of order, they are kept in ROB, >>>> when load buffer for _x in Processor 0 received the update >>>> message from Processor 1, Processor 0 need to roll back >>>> from M2 instruction, which will flush the whole pipeline, >>>> the latency is over the penalty from branch prediction miss. >>>> >>>> In this patch we use lock cmpxchg instruction to force load >>>> instructions to be serialization, the destination operand >>>> receives a write cycle without regard to the result of >>>> the comparison, which can help us to reduce the penalty >>>> from load instruction roll back. >>>> >>>> Our experiment indicates the performance can be improved by 10%~15% >>>> for 2 and 3 threads cases, the conflicts from lock cache line >>>> spend them most of the time. >>> So it would be nice to create a new user-space spinlock testing facility, via a >>> new 'perf bench spinlock' feature or so. That way others can test and validate >>> your results on different hardware as well. >> So its trivial to lift this code into userspace -- in fact, I have that >> somewhere. >> >> The trouble is going to keep them in sync. > So we can just try this optimistically, and if it keeps breaking, we can use the > technique perf uses to sync up the rbtree implementation: we copy the kernel > version into tooling, but run diff against the kernel version and warn at tool > build time that there's divergence. > > I.e. a non-build-fatal force that keeps things in sync. > > Thanks, > > Ingo >
It is on my to-do list. I just want to wrap up my latest PV qspinlock patch before embarking on this adventure.
Cheers, Longman
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