linux-kernel - Re: [RFC PATCH 2/2] sched/fair: skip the cache hot CPU in select_idle

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Date:   Tue, 12 Sep 2023 10:06:27 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Tim Chen <tim.c.chen@...el.com>, Aaron Lu <aaron.lu@...el.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] sched/fair: skip the cache hot CPU in
 select_idle_cpu()

On 9/12/23 07:53, Chen Yu wrote:
> Hi Mathieu,
> 
> thanks for the review,
> 
> On 2023-09-11 at 11:43:27 -0400, Mathieu Desnoyers wrote:
>> On 9/11/23 11:26, Mathieu Desnoyers wrote:
>>> On 9/10/23 22:50, Chen Yu wrote:
>> [...]
>>>> ---
>>>>    kernel/sched/fair.c     | 30 +++++++++++++++++++++++++++---
>>>>    kernel/sched/features.h |  1 +
>>>>    kernel/sched/sched.h    |  1 +
>>>>    3 files changed, 29 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index e20f50726ab8..fe3b760c9654 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -6629,6 +6629,21 @@ static void dequeue_task_fair(struct rq *rq,
>>>> struct task_struct *p, int flags)
>>>>        hrtick_update(rq);
>>>>        now = sched_clock_cpu(cpu_of(rq));
>>>>        p->se.prev_sleep_time = task_sleep ? now : 0;
>>>> +#ifdef CONFIG_SMP
>>>> +    /*
>>>> +     * If this rq will become idle, and dequeued task is
>>>> +     * a short sleeping one, check if we can reserve
>>>> +     * this idle CPU for that task for a short while.
>>>> +     * During this reservation period, other wakees will
>>>> +     * skip this 'idle' CPU in select_idle_cpu(), and this
>>>> +     * short sleeping task can pick its previous CPU in
>>>> +     * select_idle_sibling(), which brings better cache
>>>> +     * locality.
>>>> +     */
>>>> +    if (sched_feat(SIS_CACHE) && task_sleep && !rq->nr_running &&
>>>> +        p->se.sleep_avg && p->se.sleep_avg <
>>>> sysctl_sched_migration_cost)
>>>> +        rq->cache_hot_timeout = now + p->se.sleep_avg;
>>>
>>> This is really cool!
>>>
>>> There is one scenario that worries me with this approach: workloads
>>> that sleep for a long time and then have short blocked periods.
>>> Those bursts will likely bring the average to values too high
>>> to stay below sysctl_sched_migration_cost.
>>>
>>> I wonder if changing the code above for the following would help ?
>>>
>>> if (sched_feat(SIS_CACHE) && task_sleep && !rq->nr_running &&
>>> p->se.sleep_avg)
>>>       rq->cache_hot_timeout = now + min(sysctl_sched_migration_cost,
>>> p->se.sleep_avg);
>>>
>>> For tasks that have a large sleep_avg, it would activate this rq
>>> "appear as not idle for rq selection" scheme for a window of
>>> sysctl_sched_migration_cost. If the sleep ends up being a long one,
>>> preventing other tasks from being migrated to this rq for a tiny
>>> window should not matter performance-wise. I would expect that it
>>> could help workloads that come in bursts.
>>
>> There is perhaps a better way to handle bursts:
>>
>> When calculating the sleep_avg, we actually only really care about
>> the sleep time for short bursts, so we could use the sysctl_sched_migration_cost
>> to select which of the sleeps should be taken into account in the avg.
>>
>> We could rename the field "sleep_avg" to "burst_sleep_avg", and have:
>>
>> u64 now = sched_clock_cpu(task_cpu(p));
>>
>> if ((flags & ENQUEUE_WAKEUP) && last_sleep && cpu_online(task_cpu(p)) &&
>>      now > last_sleep && now - last_sleep < sysctl_sched_migration_cost)
>> 	update_avg(&p->se.burst_sleep_avg, now - last_sleep);
>>
>> Then we can have this code is dequeue_task_fair:
>>
>> if (sched_feat(SIS_CACHE) && task_sleep && !rq->nr_running && p->se.busrt_sleep_avg)
>> 	rq->cache_hot_timeout = now + p->se.burst_sleep_avg;
>>
>> Thoughts ?
>>
> 
> This looks reasonable, it would be fine grained to only monitor the short sleep duration
> of that task. Let me try this approach to see if there is any difference.
> 

One more tweak: given that more than one task can update the cache_hot_timeout forward
one after another, and given that some tasks have larger burst_sleep_avg values than
others, I suspect we want to keep the forward movement monotonic with something like:

if (sched_feat(SIS_CACHE) && task_sleep && !rq->nr_running && p->se.burst_sleep_avg &&
     rq->cache_hot_timeout < now + p->se.burst_sleep_avg)
	rq->cache_hot_timeout = now + p->se.burst_sleep_avg;

Thanks,

Mathieu


> thanks,
> Chenyu
> 
>> Thanks,
>>
>> Mathieu
>>
>> -- 
>> Mathieu Desnoyers
>> EfficiOS Inc.
>> https://www.efficios.com
>>

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com