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Date: Wed, 7 Mar 2018 13:26:07 +0100
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>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
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        Paul Turner <pjt@google.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
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        Juri Lelli <juri.lelli@redhat.com>, Todd Kjos <tkjos@android.com>,
        Joel Fernandes <joelaf@google.com>, Steve Muckle <smuckle@google.com>
Subject: Re: [PATCH v5 1/4] sched/fair: add util_est on top of PELT
Message-ID: <20180307122607.GN25181@hirez.programming.kicks-ass.net>
References: <20180222170153.673-1-patrick.bellasi@arm.com>
 <20180222170153.673-2-patrick.bellasi@arm.com>
 <20180306190241.GH25201@hirez.programming.kicks-ass.net>
 <20180307114711.GB2211@e110439-lin>
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On Wed, Mar 07, 2018 at 11:47:11AM +0000, Patrick Bellasi wrote:
> On 06-Mar 20:02, Peter Zijlstra wrote:
> > On Thu, Feb 22, 2018 at 05:01:50PM +0000, Patrick Bellasi wrote:
> > > +struct util_est {
> > > +	unsigned int			enqueued;
> > > +	unsigned int			ewma;
> > > +#define UTIL_EST_WEIGHT_SHIFT		2
> > > +};
> > 
> > > +	ue = READ_ONCE(p->se.avg.util_est);
> > 
> > > +	WRITE_ONCE(p->se.avg.util_est, ue);
> > 
> > That is actually quite dodgy... and relies on the fact that we have the
> > 8 byte case in __write_once_size() and __read_once_size()
> > unconditionally. It then further relies on the compiler DTRT for 32bit
> > platforms, which is generating 2 32bit loads/stores.
> >
> > The advantage is of course that it will use single u64 loads/stores
> > where available.
> 
> Yes, that's mainly an "optimization" for 64bit targets... but perhaps
> the benefits are negligible.
> 
> Do you prefer to keep more "under control" the generated code by using
> two {READ,WRITE}_ONCEs?
> 
> IMO here we can also go with just the WRITE_ONCEs. I don't see a case
> for the compiler to mangle load/store. While the WRITE_ONCE are still
> required to sync with non rq-lock serialized code.
> But... maybe I'm missing something... ?

I'm not sure we rely on READ/WRITE_ONCE() of 64bit variables on 32bit
targets to be sane anywhere else (we could be, I just dont know).

I suspect it all works as expected... but its a tad tricky.