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Date: Wed, 28 Nov 2012 12:33:20 +0530
From: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To: habanero@...ux.vnet.ibm.com
CC: Andrew Jones <drjones@...hat.com>,
Marcelo Tosatti <mtosatti@...hat.com>,
Gleb Natapov <gleb@...hat.com>,
Chegu Vinod <chegu_vinod@...com>,
Peter Zijlstra <peterz@...radead.org>,
"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
Avi Kivity <avi@...hat.com>, Rik van Riel <riel@...hat.com>,
Srikar <srikar@...ux.vnet.ibm.com>,
"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>,
LKML <linux-kernel@...r.kernel.org>,
Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>
Subject: Re: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source and
target runqueue has one task
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@...radead.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@...radead.org>
>>>> Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
>>>> Reviewed-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
>>>> Signed-off-by: Raghavendra K T <raghavendra.kt@...ux.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@...hat.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.
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