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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Paul E. McKenney (paulmck@linux.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
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