Re: [PATCH V3 RESEND RFC 1/2] sched: Bail out of yield_to when source and target runqueue has one task

Raghavendra K T
2013-01-25T15:54:50Z

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).