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

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 01/26/2013 12:19 AM, Ingo Molnar wrote:
>
> * Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> wrote:
>
>> 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).
>
> Ok, cool.
>
> Which tree will this be upstreamed through - the KVM tree? I'd
> suggest the KVM tree because KVM will be the one exposed to the
> effects of this change.

Thanks Ingo.

Marcelo, Could you please take this into kvm tree.. ?