linux-kernel - Re: [PATCH -next] cgroup/cpuset: Reduce the lock protecting CS_SCHED_LOAD

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Message-ID: <ZlOBAmStNZZU6Z-N@slm.duckdns.org>
Date: Sun, 26 May 2024 08:35:46 -1000
From: Tejun Heo <tj@...nel.org>
To: Xiu Jianfeng <xiujianfeng@...wei.com>
Cc: longman@...hat.com, lizefan.x@...edance.com, hannes@...xchg.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] cgroup/cpuset: Reduce the lock protecting
 CS_SCHED_LOAD_BALANCE

On Sat, May 25, 2024 at 09:46:48AM +0000, Xiu Jianfeng wrote:
> In the cpuset_css_online(), clearing the CS_SCHED_LOAD_BALANCE bit
> of cs->flags is guarded by callback_lock and cpuset_mutex. There is
> no problem with itself, because it is consistent with the description
> of there two global lock at the beginning of this file. However, since
> the operation of checking, setting and clearing the flag bit is atomic,
> protection of callback_lock is unnecessary here, see CS_SPREAD_*. so
> to make it more consistent with the other code, move the operation
> outside the critical section of callback_lock.
> 
> No functional changes intended.
> 
> Signed-off-by: Xiu Jianfeng <xiujianfeng@...wei.com>
> ---
>  kernel/cgroup/cpuset.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index f9d2a3487645..315f8cbd6d35 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -4038,6 +4038,12 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
>  		set_bit(CS_SPREAD_PAGE, &cs->flags);
>  	if (is_spread_slab(parent))
>  		set_bit(CS_SPREAD_SLAB, &cs->flags);
> +	/*
> +	 * For v2, clear CS_SCHED_LOAD_BALANCE if parent is isolated
> +	 */
> +	if (cgroup_subsys_on_dfl(cpuset_cgrp_subsys) &&
> +	    !is_sched_load_balance(parent))
> +		clear_bit(CS_SCHED_LOAD_BALANCE, &cs->flags);

The code looks weird to me. It's doing the same thing under the
is_in_v2_mode() block and the cgroup_subsys_on_dfl() block and the former is
also run when the latter condition is true. Looks like we can get rid of the
latter block? Waiman?

Thanks.

-- 
tejun