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Message-ID: <5df03ae2-6c5d-aa38-4a4d-632c0f484140@bytedance.com>
Date:   Thu, 14 Jul 2022 16:00:31 +0800
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Subject: Re: [PATCH 1/5] sched/fair: ignore SIS_UTIL when has idle core
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To:     Yicong Yang <yangyicong@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Vincent Guittot <vincent.guittot@linaro.org>
Cc:     yangyicong@hisilicon.com, Josh Don <joshdon@google.com>,
        Chen Yu <yu.c.chen@intel.com>, linux-kernel@vger.kernel.org
References: <20220712082036.5130-1-wuyun.abel@bytedance.com>
 <20220712082036.5130-2-wuyun.abel@bytedance.com>
 <8e7d75d4-613e-f35e-e932-323789666fb1@huawei.com>
 <4dde05be-8470-5984-0a30-ba077b9fe6bd@bytedance.com>
 <e8a59a8f-1e0c-2bb6-2d1b-4e76f5c511f5@huawei.com>
From:   Abel Wu <wuyun.abel@bytedance.com>
In-Reply-To: <e8a59a8f-1e0c-2bb6-2d1b-4e76f5c511f5@huawei.com>
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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@bytedance.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.