Re: rcu: performance regression

[Peter Zijlstra]

On Tue, 2011-06-14 at 15:14 +0200, Peter Zijlstra wrote:
> On Tue, 2011-06-14 at 06:02 -0700, Paul E. McKenney wrote:
> > > This commit also cause hackbench process mode performance dropping, and
> > > Shaohua's patch do recovered this. But in hackbench testing, the vmstat
> > > show context switch have some reduce. And perf tool show
> > > root_domain->cpupri->prio_to_cpu[]->lock has contention with the commit.
> > 
> > Steven, Peter, would any of the recent fixes address this lock contention? 
> 
> No. People occasionally run into that thing, but given the constraints
> on SCHED_FIFO SMP (run the n highest priority tasks on m cpus; n<=m)
> there isn't really much we can do.
> 
> IIRC Chris Mason once proposed a lockless version, but that opened up
> race windows, Steve might know the details better.

Note that even a lockless version will die of cacheline contention.
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