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Message-ID: <67d55d76-a619-7b05-0e6c-1a097f702bbb@shopee.com>
Date: Fri, 29 Sep 2023 19:56:30 +0800
From: Haifeng Xu <haifeng.xu@...pee.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
juri.lelli@...hat.com, dietmar.eggemann@....com,
rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
bristot@...hat.com, vschneid@...hat.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: Filter root_task_group at the beginning
On 2023/9/29 05:03, Ingo Molnar wrote:
>
> * Haifeng Xu <haifeng.xu@...pee.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@...pee.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
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