linux-kernel - Re: [RFC patch] introduce sys_membarrier(): process-wide memory barrier (v9)

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Date:	Thu, 25 Feb 2010 12:25:26 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Nick Piggin <npiggin@...e.de>, Chris Friesen <cfriesen@...tel.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nicholas Miell <nmiell@...cast.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>, mingo@...e.hu,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com
Subject: Re: [RFC patch] introduce sys_membarrier(): process-wide memory
 barrier (v9)

On Thu, 2010-02-25 at 11:53 -0500, Mathieu Desnoyers wrote:

> > It would be very trivial compared to the process-private case. Just IPI
> > all CPUs. It would allow older kernels to work with newer process based
> > apps as they get implemented. But... not a really big deal I suppose.
> 
> This is actually what I did in v1 of the patch, but this implementation met
> resistance from the RT people, who were concerned about the impact on RT tasks
> of a lower priority process doing lots of sys_membarrier() calls. So if we want
> to do other-process-aware sys_membarrier(), we would have to iterate on all
> cpus, for every running process shared memory maps and see if there is something
> shared with all shm of the current process. This is clearly not as trivial as
> just broadcasting the IPI to all cpus.

Right, it may require another syscall or parameter to let the tasks
register a shared page. Then have some mechanism to find a way to
quickly check if a CPU is running a process with that page.

-- Steve


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