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Date:   Fri, 29 Sep 2023 19:56:30 +0800
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Subject: Re: [PATCH] sched: Filter root_task_group at the beginning
To:     Ingo Molnar <mingo@kernel.org>
Cc:     peterz@infradead.org, mingo@redhat.com, vincent.guittot@linaro.org,
        juri.lelli@redhat.com, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, vschneid@redhat.com,
        linux-kernel@vger.kernel.org
References: <20230922094336.394865-1-haifeng.xu@shopee.com>
 <ZRXqFScg/vORHoqw@gmail.com>
From:   Haifeng Xu <haifeng.xu@shopee.com>
In-Reply-To: <ZRXqFScg/vORHoqw@gmail.com>
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On 2023/9/29 05:03, Ingo Molnar wrote:
> 
> * Haifeng Xu <haifeng.xu@shopee.com> wrote:
> 
>> We can't change the weight of the root cgroup. Let's handle
>> root_task_group before doing any real work including acquiring
>> the shares_mutex.
>>
>> Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
>> ---
>>  kernel/sched/fair.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index a80a73909dc2..1ac2df87e070 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -12594,6 +12594,9 @@ int sched_group_set_shares(struct task_group *tg, unsigned long shares)
>>  {
>>  	int ret;
>>  
>> +	if (tg == &root_task_group)
>> +		return -EINVAL;
>> +
>>  	mutex_lock(&shares_mutex);
>>  	if (tg_is_idle(tg))
>>  		ret = -EINVAL;
> 
> So what's the motivation, how common is this case? 

It's not common.

The users of __sched_group_set_shares() are sched_group_set_idle() and sched_group_set_shares().
So I want to follow the way in sched_group_set_idle(). If so, we can remove the redundant checks in
__sched_group_set_shares() because all users have filtered the root_task_group.

> 
> Normally this should be a -EINVAL error code path, which sane user-space
> presumably never conscisously tries to call in that fashion, right?

Yes.

> 
> It's not worth optimizing pathological cases, especially
> since we check for the root CG inside __sched_group_set_shares()
> already:
> 
>         /*
>          * We can't change the weight of the root cgroup.
>          */
>         if (!tg->se[0])
>                 return -EINVAL;
> 
> 
> Thanks,
> 
> 	Ingo