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Date:   Mon, 13 Jul 2020 13:36:00 +0200
From:   peterz@infradead.org
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Doug Anderson <dianders@chromium.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Quentin Perret <qperret@google.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Patrick Bellasi <patrick.bellasi@matbug.net>,
        Pavan Kondeti <pkondeti@codeaurora.org>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v6 1/2] sched/uclamp: Add a new sysctl to control RT
 default boost value
Message-ID: <20200713113600.GA43129@hirez.programming.kicks-ass.net>
References: <20200706142839.26629-1-qais.yousef@arm.com>
 <20200706142839.26629-2-qais.yousef@arm.com>
 <20200713112125.GG10769@hirez.programming.kicks-ass.net>
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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.