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Date:   Mon, 13 Jul 2020 18:54:49 +0200
From:   Peter Zijlstra <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
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On Mon, Jul 13, 2020 at 03:27:55PM +0100, Qais Yousef wrote:
> On 07/13/20 15:35, Peter Zijlstra wrote:
> > > I protect this with rcu_read_lock() which as far as I know synchronize_rcu()
> > > will ensure if we do the update during this section; we'll wait for it to
> > > finish. New forkees entering the rcu_read_lock() section will be okay because
> > > they should see the new value.
> > > 
> > > spinlocks() and mutexes seemed inferior to this approach.
> > 
> > Well, didn't we just write in another patch that p->uclamp_* was
> > protected by both rq->lock and p->pi_lock?
> 
> __setscheduler_uclamp() path is holding these locks, not sure by design or it
> just happened this path holds the lock. I can't see the lock in the
> uclamp_fork() path. But it's hard sometimes to unfold the layers of callers,
> especially not all call sites are annotated for which lock is assumed to be
> held.
> 
> Is it safe to hold the locks in uclamp_fork() while the task is still being
> created? My new code doesn't hold it of course.
> 
> We can enforce this rule if you like. Though rcu critical section seems lighter
> weight to me.
> 
> If all of this does indeed start looking messy we can put the update in
> a delayed worker and schedule that instead of doing synchronous setup.

sched_fork() doesn't need the locks, because at that point the task
isn't visible yet. HOWEVER, sched_post_fork() is after pid-hash (per
design) and thus the task is visible, so we can race against
sched_setattr(), so we'd better hold those locks anyway.