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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Tue, 8 Oct 2019 20:45:11 +0200
Message-ID: <CAKfTPtB+UKLzOtt9dujGJtZBXB9jr-fm-C5xxkDCCDtGVwj78Q@mail.gmail.com>
Subject: Re: [PATCH v3 04/10] sched/fair: rework load_balance
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Phil Auld <pauld@redhat.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Quentin Perret <quentin.perret@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <Morten.Rasmussen@arm.com>,
        Hillf Danton <hdanton@sina.com>
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On Tue, 8 Oct 2019 at 19:39, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Oct 08, 2019 at 05:30:02PM +0200, Vincent Guittot wrote:
>
> > This is how I plan to get ride of the problem:
> > +             if (busiest->group_weight == 1 || sds->prefer_sibling) {
> > +                     unsigned int nr_diff = busiest->sum_h_nr_running;
> > +                     /*
> > +                      * When prefer sibling, evenly spread running tasks on
> > +                      * groups.
> > +                      */
> > +                     env->migration_type = migrate_task;
> > +                     lsub_positive(&nr_diff, local->sum_h_nr_running);
> > +                     env->imbalance = nr_diff >> 1;
> > +                     return;
> > +             }
>
> I'm thinking the max_t(long, 0, ...); variant reads a lot simpler and
> really _should_ work given that -fno-strict-overflow / -fwrapv mandates
> 2s complement.

Another point that I have overlooked is that sum_h_nr_running is
unsigned int whereas imbalance is long

In fact, (long) (unsigned long A - unsigned long B) >> 1 works correctly
but
(long) (unsigned int A - unsigned int B) >> 1 doesn't