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

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Message-ID: <ab909cde-5e1e-a91e-473a-97dd2bd46e63@huawei.com>
Date:   Thu, 14 Jul 2022 16:16:54 +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 16:00, 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.
> .

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