Re: [PATCH V3 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 11/27/2012 07:34 PM, Andrew Theurer wrote:
> On Tue, 2012-11-27 at 16:00 +0530, Raghavendra K T wrote:
>> On 11/26/2012 07:05 PM, Andrew Jones wrote:
>>> On Mon, Nov 26, 2012 at 05:37:54PM +0530, Raghavendra K T 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>
>>>> ---
>>>>
>>>>    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;
>>>> +	}
>>>> +
>>>>    	double_rq_lock(rq, p_rq);
>>>>    	while (task_rq(p) != p_rq) {
>>>>    		double_rq_unlock(rq, p_rq);
>>>> @@ -4310,13 +4322,13 @@ again:
>>>>    	}
>>>>
>>>>    	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) {
>>>> @@ -4329,11 +4341,12 @@ again:
>>>>    			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;
>>>>
>>>
>>> Acked-by: Andrew Jones <drjones@redhat.com>
>>>
>>
>> Thank you Drew.
>>
>> Marcelo Gleb.. Please let me know if you have comments / concerns on the
>> patches..
>>
>> Andrew, Vinod, IMO, the patch set looks good for undercommit scenarios
>> especially for large guests where we do have overhead of vcpu iteration
>> of ple handler..
>
> I agree, looks fine for undercommit scenarios.  I do wonder what happens
> with 1.5x overcommit, where we might see 1/2 the host cpus with runqueue
> of 2 and 1/2 of the host cpus with a runqueue of 1.  Even with this
> change that scenario still might be fine, but it would be nice to see a
> comparison.
>

Hi Andrew, yes thanks for pointing out 1.5x case which should have
theoretical  worst case..
I tried with 2 24 vcpu guests and the same 32 core machine.. Here is
the result..

Ebizzy (rec/sec higher is better)
x base
+ patched
     N       Avg        Stddev
x  10     2688.6     347.55917
+  10     2707.6     260.93728

improvement 0.706%

dbench (Throughput MB/sec higher is better)
x base
+ patched
     N         Avg        Stddev
x  10    3164.712     140.24468
+  10    3244.021     185.92434

Improvement 2.5%

So there is no significant improvement / degradation seen in
1.5x.