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

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Date:	Thu, 7 Jan 2010 08:39:23 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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, rostedt@...dmis.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, Jan 07, 2010 at 09:32:16AM +0100, Peter Zijlstra wrote:
> On Wed, 2010-01-06 at 21:02 -0800, Paul E. McKenney wrote:
> > 
> > Beats the heck out of user-mode signal handlers!!!  And it is hard
> > to imagine groveling through runqueues ever being a win, even on very
> > large systems.  The only reasonable optimization I can imagine is to
> > turn this into a no-op for a single-threaded process, but there are
> > other ways to do that optimization.
> > 
> > Reviewed-by: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> Then imagine someone doing:
> 
>  while (1)
>   sys_membarrier();
> 
> on your multi node machine, see how happy you are then.

I guess in that situation, I would be feeling no pain.  Or anything else
for that matter.  :-/

So, good point!!!  I stand un-Reviewed-By.

I could imagine throttling the requests, as well as batching them.  If
any CPU does a sys_membarrier() after this CPU's sys_membarrier has
entered the kernel, then this CPU can simply return.  A token-bucket
approach could throttle things nicely, but at some point it becomes
better to just do POSIX signals.

							Thanx, Paul
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