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Message-ID: <c98baaab-aee6-6983-bc29-836488f73d8d@huawei.com>
Date: Tue, 13 Jun 2023 15:44:50 +0800
From: Yicong Yang <yangyicong@...wei.com>
To: Chen Yu <yu.c.chen@...el.com>,
"Gautham R. Shenoy" <gautham.shenoy@....com>
CC: <yangyicong@...ilicon.com>, <peterz@...radead.org>,
<mingo@...hat.com>, <juri.lelli@...hat.com>,
<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
<tim.c.chen@...ux.intel.com>, <mgorman@...e.de>,
<vschneid@...hat.com>, <linux-kernel@...r.kernel.org>,
<linux-arm-kernel@...ts.infradead.org>, <rostedt@...dmis.org>,
<bsegall@...gle.com>, <bristot@...hat.com>,
<prime.zeng@...wei.com>, <jonathan.cameron@...wei.com>,
<ego@...ux.vnet.ibm.com>, <srikar@...ux.vnet.ibm.com>,
<linuxarm@...wei.com>, <21cnbao@...il.com>,
<kprateek.nayak@....com>, <wuyun.abel@...edance.com>,
Barry Song <song.bao.hua@...ilicon.com>
Subject: Re: [PATCH v8 2/2] sched/fair: Scan cluster before scanning LLC in
wake-up path
On 2023/6/12 13:22, Chen Yu wrote:
> On 2023-06-12 at 10:31:39 +0530, Gautham R. Shenoy wrote:
>> Hello Yicong,
>>
>>
>> On Tue, May 30, 2023 at 03:02:53PM +0800, Yicong Yang wrote:
>>> From: Barry Song <song.bao.hua@...ilicon.com>
>> [..snip..]
>>
>>> @@ -7103,7 +7127,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>>> bool has_idle_core = false;
>>> struct sched_domain *sd;
>>> unsigned long task_util, util_min, util_max;
>>> - int i, recent_used_cpu;
>>> + int i, recent_used_cpu, prev_aff = -1;
>>>
>>> /*
>>> * On asymmetric system, update task utilization because we will check
>>> @@ -7130,8 +7154,11 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>>> */
>>> if (prev != target && cpus_share_cache(prev, target) &&
>>> (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
>>> - asym_fits_cpu(task_util, util_min, util_max, prev))
>>> - return prev;
>>> + asym_fits_cpu(task_util, util_min, util_max, prev)) {
>>> + if (cpus_share_lowest_cache(prev, target))
>>
>> For platforms without the cluster domain, the cpus_share_lowest_cache
>> check is a repetition of the cpus_share_cache(prev, target) check. Can
>> we avoid this using a static branch check for cluster ?
>>
>>
> Sounds good.
>>> + return prev;
>>> + prev_aff = prev;
>>> + }
>>>
>>> /*
>>> * Allow a per-cpu kthread to stack with the wakee if the
>>> @@ -7158,7 +7185,10 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>>> (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) &&
>>> cpumask_test_cpu(p->recent_used_cpu, p->cpus_ptr) &&
>>> asym_fits_cpu(task_util, util_min, util_max, recent_used_cpu)) {
>>> - return recent_used_cpu;
>>> + if (cpus_share_lowest_cache(recent_used_cpu, target))
>>
>> Same here.
>>
>>> + return recent_used_cpu;
>>> + } else {
>>> + recent_used_cpu = -1;
>>> }
>>>
>>> /*
>>> @@ -7199,6 +7229,17 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>>> if ((unsigned)i < nr_cpumask_bits)
>>> return i;
>>>
>>> + /*
>>> + * For cluster machines which have lower sharing cache like L2 or
>>> + * LLC Tag, we tend to find an idle CPU in the target's cluster
>>> + * first. But prev_cpu or recent_used_cpu may also be a good candidate,
>>> + * use them if possible when no idle CPU found in select_idle_cpu().
>>> + */
>>> + if ((unsigned int)prev_aff < nr_cpumask_bits)
>>> + return prev_aff;
>>
>> Shouldn't we check if prev_aff (and the recent_used_cpu below) is
>> still idle ?
>>
>>
> When we reach here, the target is non-idle, and the prev_aff is idle.
> Although there is a race condition that prev_aff becomes non-idle
> and target becomes idle after select_idle_cpu(), this window might be
> small IMO.
>
Yes. Races here but adding a check won't cost much, so it's ok for me
to check it or not.
Thanks.
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