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:   Thu, 13 Sep 2018 21:14:22 +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 06:52:02PM +0100, Patrick Bellasi wrote:
> 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'")

There should not be (and I'm still confused by that particular commit
you reference). If we access everything through the uclamp_map::s, and
only use the bitfields to interpret the results, it all 'works'.

The tricky thing we did earlier was trying to use u64 accesses for 2
u32 variables. And somehow ia64 didn't get the alignment right.