linux-kernel - Re: [PATCH v6 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

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Message-ID: <20200713113600.GA43129@hirez.programming.kicks-ass.net>
Date:   Mon, 13 Jul 2020 13:36:00 +0200
From:   peterz@...radead.org
To:     Qais Yousef <qais.yousef@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Doug Anderson <dianders@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v6 1/2] sched/uclamp: Add a new sysctl to control RT
 default boost value

On Mon, Jul 13, 2020 at 01:21:25PM +0200, Peter Zijlstra wrote:
> +	 * copy_process()			sysctl_uclamp
> +	 *					  uclamp_min_rt = X;
> +	 *   write_lock(&tasklist_lock)		  read_lock(&tasklist_lock)
> +	 *   // link thread			  smp_mb__after_spinlock()
> +	 *   write_unlock(&tasklist_lock)	  read_unlock(&tasklist_lock);
> +	 *   sched_post_fork()			  for_each_process_thread()
> +	 *     __uclamp_sync_rt()		    __uclamp_sync_rt()
> +	 *
> +	 * Ensures that either sched_post_fork() will observe the new
> +	 * uclamp_min_rt or for_each_process_thread() will observe the new
> +	 * task.
> +	 */

more specifically this has the cases:


A)

	   copy_process()			sysctl_uclamp
						  uclamp_min_rt = X;
	     write_lock(&tasklist_lock)
	     // link thread
	     write_unlock(&tasklist_lock)
	     sched_post_fork()			  read_lock(&tasklist_lock)
	       __uclamp_sync_rt()		  smp_mb__after_spinlock()
                                                  read_unlock(&tasklist_lock);
                                                  for_each_process_thread()
                                                    __uclamp_sync_rt()


Where write_unlock()'s RELEASE matches read_lock() ACQUIRE and
guarantees for_each_process_thread() must observe the new thread.


B)


	   copy_process()			sysctl_uclamp
						  uclamp_min_rt = X;
						  read_lock(&tasklist_lock)
						  smp_mb__after_spinlock()
						  read_unlock(&tasklist_lock);
	     write_lock(&tasklist_lock)		  for_each_process_thread()
	     // link thread			    __uclamp_sync_rt()
             write_unlock(&tasklist_lock)
             sched_post_fork()
               __uclamp_sync_rt()

Where read_unlock()'s RELEASE matches write_lock()'s ACQUIRE and
sched_post_fork() must observe the uclamp_min_t STORE.

The smp_mb__after_spinlock() might be superfluous, but like said, brain
isn't working.