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

[Peter Zijlstra]

On Tue, Oct 20, 2015 at 11:24:02AM +0800, Ling Ma wrote:
> 2015-10-19 17:46 GMT+08:00 Peter Zijlstra <peterz@xxxxxxxxxxxxx>:
> > On Mon, Oct 19, 2015 at 10:27:22AM +0800, 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
> >
> > "lock cmpxchg" makes me think you're working on x86.
> >
> >> instructions to be serialization,
> >
> > smp_rmb() does that, and that's 'free' on x86. Because x86 doesn't do
> > read reordering.
> >
> >> 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.
> >
> > And that makes me think I'm not understanding what you're getting at. If
> > you need to force memory order, a "fence" (or smp_mb()) would still be
> > cheaper than endlessly pulling the line into exclusive state for no
> > reason, right?
> 
> Peter,
> 
> we tested instruction lfence, but we hard to see any benefit, lfence
> only force load instruction ,
> but load instruction still will rollback ,actually cmpxchg behavior is
> more like write operation,
> so we choose it.

But why? I'm just not getting this.

Also LOCK CMPXCHG is 24 cycles when hot, that's almost as bad as
a pipeline flush, and it can be many times worse when it needs to
actually fetch memory from further than L1.
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