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    SubjectRe: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier
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    On Sat, 2010-01-09 at 16:03 -0800, Paul E. McKenney 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().
    >

    If you need to throttle calls to sys_membarrier(), than why bother
    optimizing it? Again, this is like calling synchronize_sched() in the
    kernel, which is a very heavy operation, and should only be called by
    those that are not performance critical.

    Why are we struggling so much with optimizing the slow path?

    Here's how I take it. This method is much better that sending signals to
    all threads. The advantage the sys_membarrier gives us, is also a way to
    keep user rcu_read_locks barrier free, which means that rcu_read_locks
    are quick and scale well.

    So what if we have a linear decrease in performance with the number of
    threads on the write side?

    -- Steve




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