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

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Date:   Thu, 14 Jul 2022 16:00:31 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Yicong Yang <yangyicong@...wei.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 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.