Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 12 Sep 2018 19:42:36 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Patrick Bellasi <patrick.bellasi@arm.com>
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>, Tejun Heo <tj@kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Paul Turner <pjt@google.com>,
        Quentin Perret <quentin.perret@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Todd Kjos <tkjos@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Steve Muckle <smuckle@google.com>,
        Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values
 into CPU's clamp groups
Message-ID: <20180912174236.GB24106@hirez.programming.kicks-ass.net>
References: <20180828135324.21976-1-patrick.bellasi@arm.com>
 <20180828135324.21976-3-patrick.bellasi@arm.com>
 <20180912134945.GZ24106@hirez.programming.kicks-ass.net>
 <20180912155619.GG1413@e110439-lin>
 <20180912161218.GW24082@hirez.programming.kicks-ass.net>
 <20180912173515.GH1413@e110439-lin>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180912173515.GH1413@e110439-lin>
User-Agent: Mutt/1.10.0 (2018-05-17)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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'.