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    SubjectRe: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler
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    On Mon, 2012-09-24 at 17:22 +0530, Raghavendra K T wrote:
    > On 09/24/2012 05:04 PM, Peter Zijlstra wrote:
    > > On Fri, 2012-09-21 at 17:29 +0530, Raghavendra K T wrote:
    > >> In some special scenarios like #vcpu<= #pcpu, PLE handler may
    > >> prove very costly, because there is no need to iterate over vcpus
    > >> and do unsuccessful yield_to burning CPU.
    > >
    > > What's the costly thing? The vm-exit, the yield (which should be a nop
    > > if its the only task there) or something else entirely?
    > >
    > Both vmexit and yield_to() actually,
    >
    > because unsuccessful yield_to() overall is costly in PLE handler.
    >
    > This is because when we have large guests, say 32/16 vcpus, and one
    > vcpu is holding lock, rest of the vcpus waiting for the lock, when they
    > do PL-exit, each of the vcpu try to iterate over rest of vcpu list in
    > the VM and try to do directed yield (unsuccessful). (O(n^2) tries).
    >
    > this results is fairly high amount of cpu burning and double run queue
    > lock contention.
    >
    > (if they were spinning probably lock progress would have been faster).
    > As Avi/Chegu Vinod had felt it is better to avoid vmexit itself, which
    > seems little complex to achieve currently.

    OK, so the vmexit stays and we need to improve yield_to.

    How about something like the below, that would allow breaking out of the
    for-each-vcpu loop and simply going back into the vm, right?

    ---
    kernel/sched/core.c | 25 +++++++++++++++++++------
    1 file changed, 19 insertions(+), 6 deletions(-)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index b38f00e..5d5b355 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -4272,7 +4272,10 @@ EXPORT_SYMBOL(yield);
    * It's the caller's job to ensure that the target task struct
    * can't go away on us before we can do any checks.
    *
    - * Returns true if we indeed boosted the target task.
    + * Returns:
    + * true (>0) if we indeed boosted the target task.
    + * false (0) if we failed to boost the target.
    + * -ESRCH if there's no task to yield to.
    */
    bool __sched yield_to(struct task_struct *p, bool preempt)
    {
    @@ -4284,6 +4287,15 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
    local_irq_save(flags);
    rq = this_rq();

    + /*
    + * If we're the only runnable task on the rq, there's absolutely no
    + * point in yielding.
    + */
    + if (rq->nr_running == 1) {
    + yielded = -ESRCH;
    + goto out_irq;
    + }
    +
    again:
    p_rq = task_rq(p);
    double_rq_lock(rq, p_rq);
    @@ -4293,13 +4305,13 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
    }

    if (!curr->sched_class->yield_to_task)
    - goto out;
    + goto out_unlock;

    if (curr->sched_class != p->sched_class)
    - goto out;
    + goto out_unlock;

    if (task_running(p_rq, p) || p->state)
    - goto out;
    + goto out_unlock;

    yielded = curr->sched_class->yield_to_task(rq, p, preempt);
    if (yielded) {
    @@ -4312,11 +4324,12 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
    resched_task(p_rq->curr);
    }

    -out:
    +out_unlock:
    double_rq_unlock(rq, p_rq);
    +out_irq:
    local_irq_restore(flags);

    - if (yielded)
    + if (yielded > 0)
    schedule();

    return yielded;


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