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

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Date:	Sat, 9 Jan 2010 20:12:55 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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

* Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> On Sat, Jan 09, 2010 at 06:16:40PM -0500, Steven Rostedt wrote:
> > On Sat, 2010-01-09 at 18:05 -0500, Steven Rostedt wrote:
> > 
> > > Then we should have O(tasks) for spinlocks taken, and 
> > > O(min(tasks, CPUS)) for IPIs.
> > 
> > And for nr tasks >> CPUS, this may help too:
> > 
> > > cpumask = 0;
> > > foreach task {
> > 
> > 	if (cpumask == online_cpus)
> > 		break;
> > 
> > > 	spin_lock(task_rq(task)->rq->lock);
> > > 	if (task_rq(task)->curr == task)
> > > 		cpu_set(task_cpu(task), cpumask);
> > > 	spin_unlock(task_rq(task)->rq->lock);
> > > }
> > > send_ipi(cpumask);
> 
> Good point, erring on the side of sending too many IPIs is safe.  One
> might even be able to just send the full set if enough of the CPUs were
> running the current process and none of the remainder were running
> real-time threads.  And yes, it would then be necessary to throttle
> calls to sys_membarrier().
> 
> Quickly hiding behind a suitable boulder...  ;-)

:)

One quick counter-argument against IPI-to-all: that will wake up all
CPUs, including those which are asleep. Not really good for
energy-saving.

Mathieu

> 
> 							Thanx, Paul

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