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 18:52:02 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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 12-Sep 19:42, Peter Zijlstra wrote:
> 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.

Just remembered a past experience where we had unaligned access traps
on some machine because... don't you see any potentially issue on
using bitfleds like you suggest above ?

I'm thinking to:

   commit 317d359df95d ("sched/core: Force proper alignment of 'struct util_est'")

-- 
#include <best/regards.h>

Patrick Bellasi