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

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Date:	Thu, 7 Jan 2010 11:16:57 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
	barrier

On Thu, Jan 07, 2010 at 01:59:42PM -0500, Steven Rostedt wrote:
> On Thu, 2010-01-07 at 10:39 -0800, Paul E. McKenney wrote:
> 
> > > sys_membarrier() should "insert" mb() on behalf of B "instead"
> > > of barrier(), right? But, if we send IPI, B enters kernel mode
> > > and returns to user-mode. Should this imply mb() in any case?
> > 
> > Hello, Oleg,
> > 
> > The issue is with some suggested optimizations that would avoid sending
> > the IPI to CPUs that are not running threads in the same process as the
> > thread executing the sys_membarrier().  Some forms of these optimizations
> > sample ->mm without locking, and the question is whether this is safe.
> 
> Note, we are not suggesting optimizations. It has nothing to do with
> performance of the syscall. We just can't allow one process to be DoSing
> another process on another cpu by it sending out millions of IPIs.
> Mathieu already showed that you could cause a 2x slowdown to the
> unrelated tasks.

I would have said that we are trying to optimize our way out of a DoS
situation, but point taken.  Whatever we choose to call it, the discussion
is on the suggested modifications, not strictly on the original patch.  ;-)

						Thanx, Paul
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