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    SubjectRe: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier
    * 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
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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