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

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Date:   Wed, 8 Jul 2020 14:08:33 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        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 07/08/20 12:05, Valentin Schneider wrote:
> > AFAIU rcu_read_lock() is light weight. So having the protection applied is more
> > robust against future changes.
> 
> So I think the one thing you win by having this dance with mb's and the
> suggested handling of the task list is that you do not need any
> rcu_synchronize() anymore. Both approaches have merit, it's just that the
> way I understood the suggestion to add sched_post_fork() was to simplify
> the ordering of the update with the aforementioned scheme.

The synchronize_rcu() is not for sched_post_fork(). It is to deal with the
preemption problem.

> 
> >
> >>
> >> sched_post_fork() being preempted out is a bit more annoying, but what
> >> prevents us from making that bit preempt-disabled?
> >
> > preempt_disable() is not friendly to RT and heavy handed approach IMO.
> >
> 
> True, but this is both an infrequent and slow sysctl path, so I don't think
> RT would care much.

There's an easy answer for that. But first I'm not sure what problem are we
discussing here.

What is the problem with rcu? And how is preempt_disable() fixes it or improves
on it?

Thanks

--
Qais Yousef