linux-kernel - Re: [PATCH v2 2/6] sched/cpuset: Bring back cpuset

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Message-ID: <20230504061842.GC1734100@hirez.programming.kicks-ass.net>
Date:   Thu, 4 May 2023 08:18:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Qais Yousef <qyousef@...alina.io>,
        Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Hao Luo <haoluo@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
        claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
        bristot@...hat.com, mathieu.poirier@...aro.org,
        cgroups@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Wei Wang <wvw@...gle.com>, Rick Yiu <rickyiu@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH v2 2/6] sched/cpuset: Bring back cpuset_mutex

On Wed, May 03, 2023 at 09:22:24AM +0200, Juri Lelli wrote:

>  /*
> - * There are two global locks guarding cpuset structures - cpuset_rwsem and
> + * There are two global locks guarding cpuset structures - cpuset_mutex and
>   * callback_lock. We also require taking task_lock() when dereferencing a
>   * task's cpuset pointer. See "The task_lock() exception", at the end of this
> - * comment.  The cpuset code uses only cpuset_rwsem write lock.  Other
> - * kernel subsystems can use cpuset_read_lock()/cpuset_read_unlock() to
> - * prevent change to cpuset structures.
> + * comment.  The cpuset code uses only cpuset_mutex. Other kernel subsystems
> + * can use cpuset_lock()/cpuset_unlock() to prevent change to cpuset
> + * structures.
>   *
>   * A task must hold both locks to modify cpusets.  If a task holds
> - * cpuset_rwsem, it blocks others wanting that rwsem, ensuring that it
> - * is the only task able to also acquire callback_lock and be able to
> - * modify cpusets.  It can perform various checks on the cpuset structure
> - * first, knowing nothing will change.  It can also allocate memory while
> - * just holding cpuset_rwsem.  While it is performing these checks, various
> - * callback routines can briefly acquire callback_lock to query cpusets.
> - * Once it is ready to make the changes, it takes callback_lock, blocking
> - * everyone else.
> + * cpuset_mutex, it blocks others, ensuring that it is the only task able to
> + * also acquire callback_lock and be able to modify cpusets.  It can perform
> + * various checks on the cpuset structure first, knowing nothing will change.
> + * It can also allocate memory while just holding cpuset_mutex.  While it is
> + * performing these checks, various callback routines can briefly acquire
> + * callback_lock to query cpusets.  Once it is ready to make the changes, it
> + * takes callback_lock, blocking everyone else.
>   *
>   * Calls to the kernel memory allocator can not be made while holding
>   * callback_lock, as that would risk double tripping on callback_lock
> @@ -403,16 +402,16 @@ static struct cpuset top_cpuset = {
>   * guidelines for accessing subsystem state in kernel/cgroup.c
>   */
>  
> -DEFINE_STATIC_PERCPU_RWSEM(cpuset_rwsem);
> +static DEFINE_MUTEX(cpuset_mutex);

Perhaps extend the comment to state you explicitly want a mutex for PI
etc.. ?