linux-kernel - Re: [PATCH] cros_ec_spi: Even though we're RT priority, don't bump cpu freq

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Message-ID: <20200624160109.GK4781@hirez.programming.kicks-ass.net>
Date:   Wed, 24 Jun 2020 18:01:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Doug Anderson <dianders@...omium.org>,
        Benson Leung <bleung@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        hsinyi@...omium.org, Joel Fernandes <joelaf@...gle.com>,
        Nicolas Boichat <drinkcat@...omium.org>,
        Gwendal Grignou <gwendal@...omium.org>,
        Quentin Perret <qperret@...gle.com>, ctheegal@...eaurora.org,
        Guenter Roeck <groeck@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cros_ec_spi: Even though we're RT priority, don't bump
 cpu freq

On Tue, Jun 23, 2020 at 05:40:21PM +0100, Qais Yousef wrote:
> On 06/22/20 11:21, Doug Anderson wrote:
> 
> [...]
> 
> > > If you propose something that will help the discussion. I think based on the
> > > same approach Peter has taken to prevent random RT priorities. In uclamp case
> > > I think we just want to allow driver to opt RT tasks out of the default
> > > boosting behavior.
> > >
> > > I'm a bit wary that this extra layer of tuning might create a confusion, but
> > > I can't reason about why is it bad for a driver to say I don't want my RT task
> > > to be boosted too.
> > 
> > Right.  I was basically just trying to say "turn my boosting off".
> > 
> > ...so I guess you're saying that doing a v2 of my patch with the
> > proper #ifdef protection wouldn't be a good way to go and I'd need to
> > propose some sort of API for this?
> 
> It's up to Peter really.
> 
> It concerns me in general to start having in-kernel users of uclamp that might
> end up setting random values (like we ended having random RT priorities), that
> really don't mean a lot outside the context of the specific system it was
> tested on. Given the kernel could run anywhere, it's hard to rationalize what's
> okay or not.
> 
> Opting out of default RT boost for a specific task in the kernel, could make
> sense though it still concerns me for the same reasons. Is this okay for all
> possible systems this can run on?
> 
> It feels better for userspace to turn RT boosting off for all tasks if you know
> your system is powerful, or use the per task API to switch off boosting for the
> tasks you know they don't need it.
> 
> But if we want to allow in-kernel users, IMO it needs to be done in
> a controlled way, in a similar manner Peter changed how RT priority can be set
> in the kernel.
> 
> It would be good hear what Peter thinks.

Hurmph.. I think I understand the problem, but I'm not sure what to do
about it :-(

Esp. given the uclamp optimization patches now under consideration. That
is, if random drivers are going to install uclamps, then that will
automagically enable the static_key and make the scheduler slower, even
if that driver isn't particularly interesting to the user.