linux-kernel - Re: [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups

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Date:   Wed, 12 Sep 2018 19:42:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values
 into CPU's clamp groups

On Wed, Sep 12, 2018 at 06:35:15PM +0100, Patrick Bellasi wrote:
> On 12-Sep 18:12, Peter Zijlstra wrote:

> > No idea; but if you want to go all fancy you can replace he whole
> > uclamp_map thing with something like:
> > 
> > struct uclamp_map {
> > 	union {
> > 		struct {
> > 			unsigned long v : 10;
> > 			unsigned long c : BITS_PER_LONG - 10;
> > 		};
> > 		atomic_long_t s;
> > 	};
> > };
> 
> That sounds really cool and scary at the same time :)
> 
> The v:10 requires that we never set SCHED_CAPACITY_SCALE>1024
> or that we use it to track a percentage value (i.e. [0..100]).

Or we pick 11 bits, it seems unlikely that capacity be larger than 2k.

> One of the last patches introduces percentage values to userspace.
> But, I was considering that in kernel space we should always track
> full scale utilization values.
> 
> The c:(BITS_PER_LONG-10) restricts the range of concurrently active
> SE refcounting the same clamp value. Which, for some 32bit systems is
> only 4 milions among tasks and cgroups... maybe still reasonable...

Yeah, on 32bit having 4M tasks seems 'unlikely'.