linux-kernel - Re: [patch 2/3] scheduler: add full memory barriers upon task switch at runqueue lock/unlock

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Message-ID: <1265039052.29013.28.camel@gandalf.stny.rr.com>
Date:	Mon, 01 Feb 2010 10:44:12 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nicholas Miell <nmiell@...cast.net>, laijs@...fujitsu.com,
	dipankar@...ibm.com, josh@...htriplett.org, dvhltc@...ibm.com,
	niv@...ibm.com, tglx@...utronix.de, Valdis.Kletnieks@...edu,
	dhowells@...hat.com
Subject: Re: [patch 2/3] scheduler: add full memory barriers upon task
 switch at runqueue lock/unlock

On Mon, 2010-02-01 at 10:23 -0500, Steven Rostedt wrote:
> On Tue, 2010-02-02 at 01:58 +1100, Nick Piggin wrote:
> > On Mon, Feb 01, 2010 at 09:47:59AM -0500, Mathieu Desnoyers wrote:
> > > * Nick Piggin (npiggin@...e.de) wrote:
> > > > Well I just mean that it's something for -rt to work out. Apps can
> > > > still work if the call is unsupported completely.
> > > 
> > > OK, so we seem to be settling for the spinlock-based sys_membarrier()
> > > this time, which is much less intrusive in terms of scheduler
> > > fast path modification, but adds more system overhead each time
> > > sys_membarrier() is called. This trade-off makes sense to me, as we
> > > expect the scheduler to execute _much_ more often than sys_membarrier().
> > > 
> > > When I get confirmation that's the route to follow from both of you,
> > > I'll go back to the spinlock-based scheme for v9.
> > 
> > I think locking or cacheline bouncing DoS is just something we can't
> > realistically worry too much about in the standard kernel. No further
> > than just generally good practice of good scalability, avoiding
> > starvations and long lock hold times etc.
> > 
> > So I would prefer the simpler version that doesn't add overhead to
> > ctxsw, at least for the first implementation.
> 
> Acked-by: Steven Rostedt <rostedt@...dmis.org>

I'm in agreement with Nick on this issue. Peter, I think you are being a
bit paranoid here. But that's a good thing. Anyone working in the
scheduler must be paranoid. That's what brings out issues that people
normally do no think about. :-)

-- Steve


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