Re: [RFC PATCH] qspinlock: Improve performance by reducing load instruction rollback

From: Peter Zijlstra
Date: Mon Oct 19 2015 - 05:34:18 EST


On Mon, Oct 19, 2015 at 09:58:23AM +0200, Ingo Molnar wrote:
>
> * ling.ma.program@xxxxxxxxx <ling.ma.program@xxxxxxxxx> wrote:
>
> > From: Ma Ling <ling.ml@xxxxxxxxxxxxxxx>
> >
> > 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.
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