linux-kernel - Re: [PATCH 1/5] sched/fair: ignore SIS

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Message-ID: <86582457-06eb-b239-37e5-7d46fade8a6f@huawei.com>
Date:   Thu, 14 Jul 2022 16:34:09 +0800
From:   Yicong Yang <yangyicong@...wei.com>
To:     Abel Wu <wuyun.abel@...edance.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
CC:     <yangyicong@...ilicon.com>, Josh Don <joshdon@...gle.com>,
        Chen Yu <yu.c.chen@...el.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] sched/fair: ignore SIS_UTIL when has idle core

On 2022/7/14 16:16, Yicong Yang wrote:
> On 2022/7/14 16:00, Abel Wu wrote:
>>
>> On 7/14/22 3:15 PM, Yicong Yang Wrote:
>>> On 2022/7/14 14:58, Abel Wu wrote:
>>>>
>>>> On 7/14/22 2:19 PM, Yicong Yang Wrote:
>>>>> On 2022/7/12 16:20, Abel Wu wrote:
>>>>>> When SIS_UTIL is enabled, SIS domain scan will be skipped if
>>>>>> the LLC is overloaded. Since the overloaded status is checked
>>>>>> in the load balancing at LLC level, the interval is llc_size
>>>>>> miliseconds. The duration might be long enough to affect the
>>>>>> overall system throughput if idle cores are out of reach in
>>>>>> SIS domain scan.
>>>>>>
>>>>>> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
>>>>>> ---
>>>>>>    kernel/sched/fair.c | 15 +++++++++------
>>>>>>    1 file changed, 9 insertions(+), 6 deletions(-)
>>>>>>
>>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>>> index a78d2e3b9d49..cd758b3616bd 100644
>>>>>> --- a/kernel/sched/fair.c
>>>>>> +++ b/kernel/sched/fair.c
>>>>>> @@ -6392,16 +6392,19 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>>>>>>        struct sched_domain *this_sd;
>>>>>>        u64 time = 0;
>>>>>>    -    this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>>>>>> -    if (!this_sd)
>>>>>> -        return -1;
>>>>>> -
>>>>>>        cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
>>>>>>    -    if (sched_feat(SIS_PROP) && !has_idle_core) {
>>>>>> +    if (has_idle_core)
>>>>>> +        goto scan;
>>>>>> +
>>>>>> +    if (sched_feat(SIS_PROP)) {
>>>>>>            u64 avg_cost, avg_idle, span_avg;
>>>>>>            unsigned long now = jiffies;
>>>>>>    +        this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>>>>>> +        if (!this_sd)
>>>>>> +            return -1;
>>>>>> +
>>>>>
>>>>> I don't follow the change here. True that this_sd is used only in SIS_PROP, but it seems irrelevant with your
>>>>> commit. Does the position of this make any performance difference?
>>>>
>>>> No, this change doesn't make much difference to performance. Are
>>>> you suggesting that I should make this a separate patch?
>>>>
>>>
>>> It just makes me think that dereference is unnecessary if this_cpu and target locates in
>>> the same LLC, since it's already been passed. But since you noticed no difference it may
>>> have little effect. :)
>>>
>>
>> Hmm.. Not exactly. The sched-domains are cpu private

>>, and this_cpu can be in another LLC than target.

This is not the condition I meant to, I would have thought it would make some sense only when they're in
the same LLC. Anyway since no difference for dereference or not, it doesn't matter at all.
Thanks for the explanation.

>> .
> 
> yes. you're right. sorry for get this messed.
> 
> .
>