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

[Nicholas Piggin]

On Tue, 1 Aug 2017 13:00:23 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Tue, Aug 01, 2017 at 08:39:28PM +1000, Nicholas Piggin wrote:

> > Right, I just don't see what real problem this opens up that you don't
> > already have when you are not hard partitioned, therefore it doesn't
> > make sense to add a slowdown to the context switch fastpath to close
> > one hole in the sieve.
> > 
> > Completely recognizing that other architectures can do it without
> > taking rq lock at all and will not be forced to do so.  
> 
> If we can limit this to hard partitioned, that would be good indeed.
> 
> I'm just trying to avoid having two implementation of this thing. At the
> same time I very much understand your reluctance to add this barrier.

Well I think we could have some kind of
for_each_cpu_where_this_process_is_running macro that is needed to
abstract the arch details.

Presumably we're already going to get two implementations of that one --
I can't imagine x86 would be happy with doing a for_all_cpus iteration
just because arm does not have the cpumask. powerpc will only make that
3 :)

> 
> In any case, supposing we can do that intent thing. How horrible would
> something like:
> 
> 
>   context_switch()
>     if (unlikely(mm->needs_barrier))
>       smp_mb__after_unlock_lock();
> 
> 
> be? We only need the extra barrier when we switch _into_ mm's that care
> about sys_membarrier() in the first place. At which point they pay the
> price.

Not beautiful :) and it would also have to have an arch speicific bit on
the other side. Although yes it gives a different way to reduce cost without
rq.

So Paul and googling filled me in on the importance of this syscall. Also
I do appreciate the concern about taking rq lock. I just think maybe we
(powerpc) pay a few more cycles in the new syscall rather than context
switch. It will take a little while to get a good idea of performance and
behaviour on bigger systems where this will matter most.