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Date:   Fri, 14 Sep 2018 09:51:15 +0100
From:   Patrick Bellasi <patrick.bellasi@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
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: <20180914085115.GM1413@e110439-lin>
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On 13-Sep 21:14, Peter Zijlstra wrote:
> 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'.

Yes, the problem above was different... still I was wondering if there
could be bitfields related alignment issue lurking somewhere.
But, as you say, if we always R/W atomically via uclamp_map::s there
should be none.

> 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.

Right, np... sorry for the noise.

-- 
#include <best/regards.h>

Patrick Bellasi