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

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Message-ID: <1262874458.28171.3728.camel@gandalf.stny.rr.com>
Date:	Thu, 07 Jan 2010 09:27:38 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com, laijs@...fujitsu.com,
	dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
 barrier

On Thu, 2010-01-07 at 01:19 -0500, Mathieu Desnoyers wrote:

> I see your point.

Actually you are missing the point ;-)

> 
> This would probably be good for machines with very large number of cpus
> and without IPI broadcast support, running processes with only few
> threads. I really start to think that we should have some way to compare
> the number of threads belonging to a process and choose between the
> broadcast IPI and the per-cpu IPI depending if we are over or under an
> arbitrary threshold.


This has nothing to do with performance. It has to do with a thread
should not interfere with a thread belonging to another process. We
really don't care how long the sys_membarrier() takes (it's the slow
path anyway). We do care that a critical RT task is being interrupted by
some java thread sending thousands of IPIs.

-- Steve


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