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Date: Sat, 9 Jan 2010 21:19:43 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> 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 On Sat, Jan 09, 2010 at 08:12:55PM -0500, Mathieu Desnoyers wrote: > * 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. Good point. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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