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

Mathieu Desnoyers
2010-01-10T01:13:00Z

* 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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