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    SubjectRe: [RFC PATCH 0/4] Gang scheduling in CFS
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    On Mon, 2012-02-20 at 17:32 +0530, Srivatsa Vaddagiri wrote:
    > * Nikunj A Dadhania <nikunj@linux.vnet.ibm.com> [2012-02-20 17:23:16]:
    >
    > > On Mon, 20 Feb 2012 11:51:13 +0100, Peter Zijlstra <peterz@infradead.org> wrote:
    > > > On Mon, 2012-02-20 at 13:38 +0530, Nikunj A Dadhania wrote:
    > > > > +#ifdef CONFIG_PARAVIRT_FLUSH_TLB
    > > > > + while (!cpumask_empty(to_cpumask(f->flush_cpumask)) && --loop)
    > > > > + cpu_relax();
    > > > > + if (!loop && !cpumask_empty(to_cpumask(f->flush_cpumask)))
    > > > > + halt();
    > > >
    > > >
    > > > That's just vile, you don't need to wait for it, all you need to make
    > > > sure is that when that vcpu wakes up it does the flush.
    > > >
    > > Yes, but we are not sure the vcpu will be sleeping or running. In the
    > > case where vcpus are running, it might be beneficial to wait a while.
    >
    > I guess one possibility is for host scheduler to export run/preempt
    > information to guest OS, as was discussed in the context of this thread
    > as well:
    >
    > http://lkml.org/lkml/2010/4/6/223

    Doesn't need to be the host scheduler, KVM itself can do that just fine
    on guest entry/exit.

    > Essentially guest OS will know (using such exported information) which of its
    > vcpus are currently running and thus do a busy-wait for them.

    Right, do something like:

    again:
    for_each_cpu_in_mask(cpu, flush_cpumask) {
    if !vcpu-running {
    set_flush-on-enter(cpu)
    if !vcpu-running
    cpumask_clear(flush_cpumask, cpu); // vm-enter will do
    }
    wait-a-while-for-mask-to-clear
    if (!cpumask_empty)
    goto again;

    with the appropriate memory barriers and atomic instructions, that way
    you can skip waiting for vcpus that are not in guest mode and vm-enter
    will fixup.


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