From: Abel Wu <wuyun.abel@bytedance.com>
To: Tim Chen <tim.c.chen@linux.intel.com>,
Peter Zijlstra <peterz@infradead.org>,
Mel Gorman <mgorman@suse.de>,
Vincent Guittot <vincent.guittot@linaro.org>
Cc: Josh Don <joshdon@google.com>, Chen Yu <yu.c.chen@intel.com>,
K Prateek Nayak <kprateek.nayak@amd.com>,
"Gautham R . Shenoy" <gautham.shenoy@amd.com>,
linux-kernel@vger.kernel.org,
Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [PATCH v5 2/5] sched/fair: Limited scan for idle cores when overloaded
Date: Thu, 15 Sep 2022 11:08:43 +0800 [thread overview]
Message-ID: <055b833b-df0f-057e-9a1b-c70af094cf32@bytedance.com> (raw)
In-Reply-To: <199f559267169850f2bcbca9a5df89df30aa168e.camel@linux.intel.com>
Hi Tim, thanks for your reviewing!
On 9/15/22 6:25 AM, Tim Chen wrote:
> On Fri, 2022-09-09 at 13:53 +0800, Abel Wu wrote:
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 5af9bf246274..7abe188a1533 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6437,26 +6437,42 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>> time = cpu_clock(this);
>> }
>>
>> - if (sched_feat(SIS_UTIL) && !has_idle_core) {
>> + if (sched_feat(SIS_UTIL)) {
>> sd_share = rcu_dereference(per_cpu(sd_llc_shared, target));
>> if (sd_share) {
>> /* because !--nr is the condition to stop scan */
>> nr = READ_ONCE(sd_share->nr_idle_scan) + 1;
>> - /* overloaded LLC is unlikely to have idle cpu/core */
>> - if (nr == 1)
>> +
>> + /*
>> + * Overloaded LLC is unlikely to have idle cpus.
>> + * But if has_idle_core hint is true, a limited
>> + * speculative scan might help without incurring
>> + * much overhead.
>> + */
>> + if (has_idle_core)
>> + nr = nr > 1 ? INT_MAX : 3;
>
> The choice of nr is a very abrupt function of utilization when has_idle_core==true,
> it is either feast or famine. Why is such choice better than a smoother
> reduction of nr vs utilization? I agree that we want to scan more aggressively than
> !has_idle_core, but it is not obvious why the above work better, versus something
> like nr = nr*2+1.
This has been discussed with Mel, and he suggested do simple things
first before scaling the depth.
https://lore.kernel.org/all/20220906095717.maao4qtel4fhbmfq@techsingularity.net/
Thanks and BR,
Abel
next prev parent reply other threads:[~2022-09-15 3:09 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-09 5:52 [PATCH v5 0/5] sched/fair: Improve scan efficiency of SIS Abel Wu
2022-09-09 5:53 ` [PATCH v5 1/5] sched/fair: Ignore SIS_UTIL when has idle core Abel Wu
2022-09-14 21:58 ` Tim Chen
2022-09-09 5:53 ` [PATCH v5 2/5] sched/fair: Limited scan for idle cores when overloaded Abel Wu
2022-09-09 9:29 ` Chen Yu
2022-09-09 10:11 ` Abel Wu
2022-09-14 22:25 ` Tim Chen
2022-09-15 3:08 ` Abel Wu [this message]
2022-09-09 5:53 ` [PATCH v5 3/5] sched/fair: Skip core update if task pending Abel Wu
2022-09-09 10:09 ` Chen Yu
2022-09-09 10:13 ` Abel Wu
2022-09-14 22:37 ` Tim Chen
2022-09-09 5:53 ` [PATCH v5 4/5] sched/fair: Skip SIS domain scan if fully busy Abel Wu
2022-09-14 6:21 ` Yicong Yang
2022-09-14 7:43 ` Abel Wu
2022-09-15 0:22 ` Tim Chen
2022-09-15 3:11 ` Abel Wu
2022-09-09 5:53 ` [PATCH v5 5/5] sched/fair: Introduce SIS_FILTER Abel Wu
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