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

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Date:	Thu, 07 Jan 2010 09:44:15 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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

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.



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