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

Raghavendra K T
2013-01-25T15:58:21Z

On 01/25/2013 04:35 PM, Andrew Jones wrote:
> On Fri, Jan 25, 2013 at 04:10:25PM +0530, Raghavendra K T 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 ].
>
> The last results you posted for dbench for the patched 1x case were
> showing much better throughput than the no-ple 1x case, which is what
> was strange. Is that still happening? You don't have the no-ple 1x
> data here this time. The percent errors look a lot better.

I re-ran the experiment and almost got 4% (13500 vs 14100) less
throughput compared to patched for no-ple case. ( I believe this
variation may be due to having 4 guest with 3 idle.. as no-ple is very
sensitive after 1x).