linux-kernel - Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3a)

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Message-ID: <20100111220446.GA14937@Krystal>
Date:	Mon, 11 Jan 2010 17:04:46 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	josh@...htriplett.org, tglx@...utronix.de, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, laijs@...fujitsu.com, dipankar@...ibm.com,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
	barrier (v3a)

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Mon, 2010-01-11 at 15:52 -0500, Mathieu Desnoyers wrote:
> > 
> > So the clear bit can occur far, far away in the future, we don't care.
> > We'll just send extra IPIs when unneeded in this time-frame.
> 
> I think we should try harder not to disturb CPUs, particularly in the
> face of RT tasks and DoS scenarios. Therefore I don't think we should
> just wildly send to mm_cpumask(), but verify (although speculatively)
> that the remote tasks' mm matches ours.
> 

Well, my point of view is that if IPI TLB shootdown does not care about
disturbing CPUs running other processes in the time window of the lazy
removal, why should we ? We're adding an overhead very close to that of
an unrequired IPI shootdown which returns immediately without doing
anything.

The tradeoff here seems to be:
- more overhead within switch_mm() for more precise mm_cpumask.
vs
- lazy removal of the cpumask, which implies that some processors
  running a different process can receive the IPI for nothing.

I really doubt we could create an IPI DoS based on such a small
time window.

Thanks,

Mathieu


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