Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH] sched/fair: Remove the duplicate check from
 group_has_capacity()
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        linux-kernel@vger.kernel.org
References: <20200810010009.92758-1-arch0.zheng@gmail.com>
 <jhjwo26gxlb.mognet@arm.com> <9425382c-2a42-57ca-512d-c93c589dc701@gmail.com>
 <jhjv9hph3h7.mognet@arm.com>
From:   Qi Zheng <arch0.zheng@gmail.com>
Message-ID: <01fe6a9b-fd3a-9b36-b2fa-6cea58415670@gmail.com>
Date:   Tue, 11 Aug 2020 19:44:34 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
In-Reply-To: <jhjv9hph3h7.mognet@arm.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 2020/8/11 下午6:38, Valentin Schneider wrote:
> 
> On 11/08/20 04:39, Qi Zheng wrote:
>> On 2020/8/11 上午2:33, Valentin Schneider wrote:
>>>
>>> On 10/08/20 02:00, Qi Zheng wrote:
>>>> 1. The group_has_capacity() function is only called in
>>>>      group_classify().
>>>> 2. The following inequality has already been checked in
>>>>      group_is_overloaded() which was also called in
>>>>      group_classify().
>>>>
>>>>         (sgs->group_capacity * imbalance_pct) <
>>>>                           (sgs->group_runnable * 100)
>>>>
>>>
>>> Consider group_is_overloaded() returns false because of the first
>>> condition:
>>>
>>>           if (sgs->sum_nr_running <= sgs->group_weight)
>>>                   return false;
>>>
>>> then group_has_capacity() would be the first place where the group_runnable
>>> vs group_capacity comparison would be done.
>>>
>>> Now in that specific case we'll actually only check it if
>>>
>>>     sgs->sum_nr_running == sgs->group_weight
>>>
>>> and the only case where the runnable vs capacity check can fail here is if
>>> there's significant capacity pressure going on. TBH this capacity pressure
>>> could be happening even when there are fewer tasks than CPUs, so I'm not
>>> sure how intentional that corner case is.
>>
>> Maybe some cpus in sg->cpumask are no longer active at the == case,
>> which causes the significant capacity pressure?
>>
> 
> That can only happen in that short window between deactivating a CPU and
> not having rebuilt the sched_domains yet, which sounds quite elusive.
> 

In fact, at the beginning, I added unlikely() here to hint the compiler:

-	if ((sgs->group_capacity * imbalance_pct) <
-			(sgs->group_runnable * 100))
+	if (unlikely((sgs->group_capacity * imbalance_pct) <
+			(sgs->group_runnable * 100)))

The corresponding patch is as follows:

	[PATCH]sched/core: add unlikely in group_has_capacity()

Do you think it is necessary?