linux-kernel - Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required RT CPU capacity

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20160831164009.GF10153@twins.programming.kicks-ass.net>
Date:   Wed, 31 Aug 2016 18:40:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Steve Muckle <steve.muckle@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <Juri.Lelli@....com>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH 2/2] sched: cpufreq: use rt_avg as estimate of required
 RT CPU capacity

On Wed, Aug 31, 2016 at 06:28:10PM +0200, Thomas Gleixner wrote:
> > That is the way it's been with cpufreq and many systems (including all
> > mobile devices) rely on that to not destroy power. RT + variable cpufreq
> > is not deterministic.
> > 
> > Given we don't have good constraints on RT tasks I don't think we should
> > try to strengthen the semantics there. Folks should either move to DL if
> > they want determinism *and* not-sucky power, or continue disabling
> > cpufreq if they are able to do so.
> 
> RT deterministic behaviour is all about meeting the deadlines. If your
> deadline is relaxed enough that you can meet it even with the lowest cpu
> frequency then it's perfectly fine to enable cpufreq. The same logic applies
> to C-States.
> 
> There are a lot of RT systems out there which enable both. If cpufreq or
> c-states cause a deadline violation because the constraints of the system are
> tight, then people will disable it and we need a knob for both.
> 
> Realtime is not as fast as possible. It's as fast as specified.

Sure, problem is of course that RR/FIFO doesn't specify anything so the
users are left to prod knobs.

Another problem is that we have many semi related knobs; we have the
global RT runtime limit knob, but that doesn't affect cpufreq (maybe it
should) and cpufreq has knobs to set f_min and f_max, which again are
unaware of RT anything.

So before we go do anything, I'd like input on what is needed and how
things should tie together to make most sense.