linux-kernel - Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

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Message-ID: <20100107150758.GA14259@Krystal>
Date:	Thu, 7 Jan 2010 10:07:58 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	tglx@...utronix.de, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	laijs@...fujitsu.com, dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
	barrier

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Wed, 2010-01-06 at 22:35 -0800, Josh Triplett wrote:
> > 
> > The number of threads doesn't matter nearly as much as the number of
> > threads typically running at a time compared to the number of
> > processors.  Of course, we can't measure that as easily, but I don't
> > know that your proposed heuristic would approximate it well.
> 
> Quite agreed, and not disturbing RT tasks is even more important.
> 
> A simple:
> 
>   for_each_cpu(cpu, current->mm->cpu_vm_mask) {
>      if (cpu_curr(cpu)->mm == current->mm)
>         smp_call_function_single(cpu, func, NULL, 1);
>   }
> 
> seems far preferable over anything else, if you really want you can use
> a cpumask to copy cpu_vm_mask in and unset bits and use the mask with
> smp_call_function_any(), but that includes having to allocate the
> cpumask, which might or might not be too expensive for Mathieu.
> 

I like this ! :)

Following some testing, I think I'll go with your scheme, with 2
smp_call_function_single (one function call for the local thread, one
IPI). If we need more than that, then we allocate a cpumask and call
smp_call_function_many() for the other cpus. I provide benchmarks in my
reply to Josh justifying this choice.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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