Re: [RFC PATCH v2] membarrier: expedited private command

[Peter Zijlstra]

On Sat, Jul 29, 2017 at 11:58:40AM +1000, Nicholas Piggin wrote:
> I haven't had time to read the thread and understand exactly why you need
> this extra barrier, I'll do it next week. Thanks for cc'ing us on it.

Bottom of here:

https://lkml.kernel.org/r/20170727135610.jwjfvyuacqzj5e4u@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

is probably the fastest way towards understanding the need for a barrier
after rq->curr assignment.

Any barrier after that assignment is good for us, but so far it looks
like PPC doesn't (and PPC only afaict) provide any smp_mb() after that
point.

> A smp_mb is pretty expensive on powerpc CPUs. Removing the sync from
> switch_to increased thread switch performance by 2-3%. Putting it in
> switch_mm may be a little less painful, but still we have to weigh it
> against the benefit of this new functionality. Would that be a net win
> for the average end-user? Seems unlikely.
> 
> But we also don't want to lose sys_membarrier completely. Would it be too
> painful to make  MEMBARRIER_CMD_PRIVATE_EXPEDITED return error, or make it
> fall back to a slower case if we decide not to implement it?

One ugly thing we've thought of is tagging each mm that has used
sys_membarrier() and only issue the smp_mb() for those. That way only
those tasks that actually rely on the syscall get to pay the price.