From: Valentin Schneider <valentin.schneider@arm.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
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>
Subject: Re: [PATCH v3 04/10] sched/fair: rework load_balance
Date: Wed, 2 Oct 2019 11:47:59 +0100 [thread overview]
Message-ID: <31cac0c1-98e4-c70e-e156-51a70813beff@arm.com> (raw)
In-Reply-To: <CAKfTPtBQNJfNmBqpuaefsLzsTrGxJ=2bTs+tRdbOAa9J3eKuVw@mail.gmail.com>
On 02/10/2019 09:30, Vincent Guittot wrote:
>> Isn't that one somewhat risky?
>>
>> Say both groups are classified group_has_spare and we do prefer_sibling.
>> We'd select busiest as the one with the maximum number of busy CPUs, but it
>> could be so that busiest.sum_h_nr_running < local.sum_h_nr_running (because
>> pinned tasks or wakeup failed to properly spread stuff).
>>
>> The thing should be unsigned so at least we save ourselves from right
>> shifting a negative value, but we still end up with a gygornous imbalance
>> (which we then store into env.imbalance which *is* signed... Urgh).
>
> so it's not clear what happen with a right shift on negative signed
> value and this seems to be compiler dependent so even
> max_t(long, 0, (local->idle_cpus - busiest->idle_cpus) >> 1) might be wrong
>
Yeah, right shift on signed negative values are implementation defined. This
is what I was worried about initially, but I think the expression resulting
from the subtraction is unsigned (both terms are unsigned) so this would
just wrap when busiest < local - but that is still a problem.
((local->idle_cpus - busiest->idle_cpus) >> 1) should be fine because we do
have this check in find_busiest_group() before heading off to
calculate_imbalance():
if (busiest->group_type != group_overloaded &&
(env->idle == CPU_NOT_IDLE ||
local->idle_cpus <= (busiest->idle_cpus + 1)))
/* ... */
goto out_balanced;
which ensures the subtraction will be at least 2. We're missing something
equivalent for the sum_h_nr_running case.
> I'm going to update it
>
>
>>
>> [...]
next prev parent reply other threads:[~2019-10-02 10:50 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-19 7:33 [PATCH v3 0/8] sched/fair: rework the CFS load balance Vincent Guittot
2019-09-19 7:33 ` [PATCH v3 01/10] sched/fair: clean up asym packing Vincent Guittot
2019-09-27 23:57 ` Rik van Riel
2019-09-19 7:33 ` [PATCH v3 02/10] sched/fair: rename sum_nr_running to sum_h_nr_running Vincent Guittot
2019-09-27 23:59 ` Rik van Riel
2019-10-01 17:11 ` Valentin Schneider
2019-09-19 7:33 ` [PATCH v3 03/10] sched/fair: remove meaningless imbalance calculation Vincent Guittot
2019-09-28 0:05 ` Rik van Riel
2019-10-01 17:12 ` Valentin Schneider
2019-10-02 6:28 ` Vincent Guittot
2019-09-19 7:33 ` [PATCH v3 04/10] sched/fair: rework load_balance Vincent Guittot
2019-09-30 1:12 ` Rik van Riel
2019-09-30 7:44 ` Vincent Guittot
2019-09-30 16:24 ` Dietmar Eggemann
2019-10-01 8:14 ` Vincent Guittot
2019-10-01 16:52 ` Dietmar Eggemann
2019-10-02 6:44 ` Vincent Guittot
2019-10-02 9:21 ` Dietmar Eggemann
2019-10-08 13:02 ` Peter Zijlstra
2019-10-02 8:23 ` Vincent Guittot
2019-10-02 9:24 ` Dietmar Eggemann
2019-10-01 8:15 ` Dietmar Eggemann
2019-10-01 9:14 ` Vincent Guittot
2019-10-01 16:57 ` Dietmar Eggemann
2019-10-01 17:47 ` Valentin Schneider
2019-10-02 8:30 ` Vincent Guittot
2019-10-02 10:47 ` Valentin Schneider [this message]
2019-10-08 14:16 ` Peter Zijlstra
2019-10-08 14:34 ` Valentin Schneider
2019-10-08 15:30 ` Vincent Guittot
2019-10-08 15:48 ` Valentin Schneider
2019-10-08 17:39 ` Peter Zijlstra
2019-10-08 18:45 ` Vincent Guittot
2019-10-08 16:33 ` Peter Zijlstra
2019-10-08 16:39 ` Valentin Schneider
2019-10-08 17:36 ` Valentin Schneider
2019-10-08 17:55 ` Peter Zijlstra
2019-10-08 18:47 ` Vincent Guittot
2019-10-16 7:21 ` Parth Shah
2019-10-16 11:56 ` Vincent Guittot
2019-10-18 5:34 ` Parth Shah
2019-09-19 7:33 ` [PATCH v3 05/10] sched/fair: use rq->nr_running when balancing load Vincent Guittot
2019-09-19 7:33 ` [PATCH v3 06/10] sched/fair: use load instead of runnable load in load_balance Vincent Guittot
2019-09-19 7:33 ` [PATCH v3 07/10] sched/fair: evenly spread tasks when not overloaded Vincent Guittot
2019-09-19 7:33 ` [PATCH v3 08/10] sched/fair: use utilization to select misfit task Vincent Guittot
2019-10-01 17:12 ` Valentin Schneider
2019-09-19 7:33 ` [PATCH v3 09/10] sched/fair: use load instead of runnable load in wakeup path Vincent Guittot
2019-10-07 15:14 ` Rik van Riel
2019-10-07 15:27 ` Vincent Guittot
2019-10-07 18:06 ` Rik van Riel
2019-09-19 7:33 ` [PATCH v3 10/10] sched/fair: optimize find_idlest_group Vincent Guittot
2019-10-08 14:32 ` [PATCH v3 0/8] sched/fair: rework the CFS load balance Phil Auld
2019-10-08 15:53 ` Vincent Guittot
2019-10-09 19:33 ` Phil Auld
2019-10-10 8:20 ` Vincent Guittot
2019-10-16 7:21 ` Parth Shah
2019-10-16 11:51 ` Vincent Guittot
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