linux-kernel - Re: [PATCH] cgroup: Simplify code in css_set_move

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Message-ID: <adb7418c-39a2-6202-970a-a039ad8201dd@shopee.com>
Date:   Fri, 28 Oct 2022 19:29:57 +0800
From:   Haifeng Xu <haifeng.xu@...pee.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     tj@...nel.org, lizefan.x@...edance.com, hannes@...xchg.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup: Simplify code in css_set_move_task

On 2022/10/27 16:05, Michal Koutný wrote:
> Hello.
> 
> On Thu, Oct 20, 2022 at 07:47:01AM +0000, "haifeng.xu" <haifeng.xu@...pee.com> wrote:
>> -	lockdep_assert_held(&css_set_lock);
>> +	if (!cset || cset is either getting the first task or losing the last(cset))
>> +		return;
> 
> 1) the guard should be css_set_populated() ^ populated (when unsetting
> the populated trait)
> 
> 2) I'd keep the lockdep_assert_held() after it anyway.
> 
> Also the commit message should explain what's the reason to move
> css_set_populated() after css_set_move_task().
> 
> 
> Thanks,
> Michal
Hi, Michal.

1) If calls 'css_set_update_populated' , the cset is either getting the
first task or losing the last. There is a need to update the populated
state of the cset only when 'css_set_populated' returns false.
In other words, the last has been deleted from from_cset and the first
hasn't been added to to_cset yet.

2) Thanks for your suggestion. I'll keep 'lockdep_assert_held'.

3) In order to update the populated state of to_cset the same way
from_cset does, 'css_set_update_populated' is also invoked during the
process of moving a task to to_cset.

Thanks,
Haifeng