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    SubjectRe: [PATCH V3 RESEND RFC 1/2] sched: Bail out of yield_to when source and target runqueue has one task
    On 01/25/2013 04:17 PM, Ingo Molnar wrote:
    >
    > * Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> wrote:
    >
    >> * Ingo Molnar <mingo@kernel.org> [2013-01-24 11:32:13]:
    >>
    >>>
    >>> * Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> wrote:
    >>>
    >>>> From: Peter Zijlstra <peterz@infradead.org>
    >>>>
    >>>> In case of undercomitted scenarios, especially in large guests
    >>>> yield_to overhead is significantly high. when run queue length of
    >>>> source and target is one, take an opportunity to bail out and return
    >>>> -ESRCH. This return condition can be further exploited to quickly come
    >>>> out of PLE handler.
    >>>>
    >>>> (History: Raghavendra initially worked on break out of kvm ple handler upon
    >>>> seeing source runqueue length = 1, but it had to export rq length).
    >>>> Peter came up with the elegant idea of return -ESRCH in scheduler core.
    >>>>
    >>>> Signed-off-by: Peter Zijlstra <peterz@infradead.org>
    >>>> Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
    >>>> Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
    >>>> Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
    >>>> Acked-by: Andrew Jones <drjones@redhat.com>
    >>>> Tested-by: Chegu Vinod <chegu_vinod@hp.com>
    >>>> ---
    >>>>
    >>>> 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 2d8927f..fc219a5 100644
    >>>> --- a/kernel/sched/core.c
    >>>> +++ b/kernel/sched/core.c
    >>>> @@ -4289,7 +4289,10 @@ 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)
    >>>> {
    >>>> @@ -4303,6 +4306,15 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
    >>>>
    >>>> again:
    >>>> p_rq = task_rq(p);
    >>>> + /*
    >>>> + * If we're the only runnable task on the rq and target rq also
    >>>> + * has only one task, there's absolutely no point in yielding.
    >>>> + */
    >>>> + if (rq->nr_running == 1 && p_rq->nr_running == 1) {
    >>>> + yielded = -ESRCH;
    >>>> + goto out_irq;
    >>>> + }
    >>>
    >>> Looks good to me in principle.
    >>>
    >>> Would be nice to get more consistent benchmark numbers. Once
    >>> those are unambiguously showing that this is a win:
    >>>
    >>> Acked-by: Ingo Molnar <mingo@kernel.org>
    >>>
    >>
    >> I ran the test with kernbench and sysbench again on 32 core mx3850
    >> machine with 32 vcpu guests. Results shows definite improvements.
    >>
    >> ebizzy and dbench show similar improvement for 1x overcommit
    >> (note that stdev for 1x in dbench is lesser improvemet is now seen at
    >> only 20%)
    >>
    >> [ all the experiments are taken out of 8 run averages ].
    >>
    >> The patches benefit large guest undercommit scenarios, so I believe
    >> with large guest performance improvemnt is even significant. [ Chegu
    >> Vinod results show performance near to no ple cases ]. Unfortunately I
    >> do not have a machine to test larger guest (>32).
    >>
    >> Ingo, Please let me know if this is okay to you.
    >>
    >> base kernel = 3.8.0-rc4
    >>
    >> +-----------+-----------+-----------+------------+-----------+
    >> kernbench (time in sec lower is better)
    >> +-----------+-----------+-----------+------------+-----------+
    >> base stdev patched stdev %improve
    >> +-----------+-----------+-----------+------------+-----------+
    >> 1x 46.6028 1.8672 42.4494 1.1390 8.91234
    >> 2x 99.9074 9.1859 90.4050 2.6131 9.51121
    >> +-----------+-----------+-----------+------------+-----------+
    >> +-----------+-----------+-----------+------------+-----------+
    >> sysbench (time in sec lower is better)
    >> +-----------+-----------+-----------+------------+-----------+
    >> base stdev patched stdev %improve
    >> +-----------+-----------+-----------+------------+-----------+
    >> 1x 18.7402 0.3764 17.7431 0.3589 5.32065
    >> 2x 13.2238 0.1935 13.0096 0.3152 1.61981
    >> +-----------+-----------+-----------+------------+-----------+
    >>
    >> +-----------+-----------+-----------+------------+-----------+
    >> ebizzy (records/sec higher is better)
    >> +-----------+-----------+-----------+------------+-----------+
    >> base stdev patched stdev %improve
    >> +-----------+-----------+-----------+------------+-----------+
    >> 1x 2421.9000 19.1801 5883.1000 112.7243 142.91259
    >> +-----------+-----------+-----------+------------+-----------+
    >>
    >> +-----------+-----------+-----------+------------+-----------+
    >> dbench (throughput MB/sec higher is better)
    >> +-----------+-----------+-----------+------------+-----------+
    >> base stdev patched stdev %improve
    >> +-----------+-----------+-----------+------------+-----------+
    >> 1x 11675.9900 857.4154 14103.5000 215.8425 20.79061
    >> +-----------+-----------+-----------+------------+-----------+
    >
    > The numbers look pretty convincing, thanks. The workloads were
    > CPU bound most of the time, right?

    Yes. CPU bound most of the time. I also used tmpfs to reduce io
    overhead (for dbbench).



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