From: Valentin Schneider <valentin.schneider@arm.com>
To: Qais Yousef <qais.yousef@arm.com>
Cc: linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Morten Rasmussen <morten.rasmussen@arm.com>,
Quentin Perret <qperret@google.com>,
Pavan Kondeti <pkondeti@codeaurora.org>,
Rik van Riel <riel@surriel.com>
Subject: Re: [PATCH 6/8] sched/fair: Filter out locally-unsolvable misfit imbalances
Date: Wed, 03 Feb 2021 18:43:06 +0000 [thread overview]
Message-ID: <jhjlfc52ds5.mognet@arm.com> (raw)
In-Reply-To: <20210203151619.3sa42rqo7eihlfcz@e107158-lin>
On 03/02/21 15:16, Qais Yousef wrote:
> On 01/28/21 18:31, Valentin Schneider wrote:
>> Consider the following (hypothetical) asymmetric CPU capacity topology,
>> with some amount of capacity pressure (RT | DL | IRQ | thermal):
>>
>> DIE [ ]
>> MC [ ][ ]
>> 0 1 2 3
>>
>> | CPU | capacity_orig | capacity |
>> |-----+---------------+----------|
>> | 0 | 870 | 860 |
>> | 1 | 870 | 600 |
>> | 2 | 1024 | 850 |
>> | 3 | 1024 | 860 |
>>
>> If CPU1 has a misfit task, then CPU0, CPU2 and CPU3 are valid candidates to
>> grant the task an uplift in CPU capacity. Consider CPU0 and CPU3 as
>> sufficiently busy, i.e. don't have enough spare capacity to accommodate
>> CPU1's misfit task. This would then fall on CPU2 to pull the task.
>
> I think this scenario would be hard in practice, but not impossible. Maybe
> gaming could push the system that hard.
>
Actually I wouldn't be surprised if a moderatly busy Android environment
could hit this - slight thermal pressure on the bigs, RT pressure because
we know folks love (ab)using RT, a pinch of IRQs in the mix...
>> @@ -8450,11 +8457,21 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>> continue;
>>
>> /* Check for a misfit task on the cpu */
>> - if (sd_has_asym_cpucapacity(env->sd) &&
>> - sgs->group_misfit_task_load < rq->misfit_task_load) {
>> - sgs->group_misfit_task_load = rq->misfit_task_load;
>> - *sg_status |= SG_OVERLOAD;
>> - }
>> + if (!sd_has_asym_cpucapacity(env->sd) ||
>> + !rq->misfit_task_load)
>> + continue;
>> +
>> + *sg_status |= SG_OVERLOAD;
>> + sgs->group_has_misfit_task = true;
>> +
>> + /*
>> + * Don't attempt to maximize load for misfit tasks that can't be
>> + * granted a CPU capacity uplift.
>> + */
>> + if (cpu_capacity_greater(env->dst_cpu, i))
>> + sgs->group_misfit_task_load = max(
>> + sgs->group_misfit_task_load,
>> + rq->misfit_task_load);
>
> nit: missing curly braces around the if.
>
Ack.
>> @@ -8504,7 +8521,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>> /* Don't try to pull misfit tasks we can't help */
>> if (static_branch_unlikely(&sched_asym_cpucapacity) &&
>> sgs->group_type == group_misfit_task &&
>> - (!capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) ||
>> + (!sgs->group_misfit_task_load ||
>> sds->local_stat.group_type != group_has_spare))
>> return false;
>>
>> @@ -9464,15 +9481,18 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>> case migrate_misfit:
>> /*
>> * For ASYM_CPUCAPACITY domains with misfit tasks we
>> - * simply seek the "biggest" misfit task.
>> + * simply seek the "biggest" misfit task we can
>> + * accommodate.
>> */
>> + if (!cpu_capacity_greater(env->dst_cpu, i))
>> + continue;
>
> Both this hunk and the one above mean we will end up searching harder to pull
> the task into the right cpu taking actual capacity into account. Which is
> a good improvement.
>
Note that those extra checks are to make sure we *don't* downmigrate tasks
(as stated somewhere above, this change lets find_busiest_queue() iterate
over CPUs bigger than the local CPU's, which wasn't the case before). A "big"
CPU will still get the chance to pull a "medium" task, even if a "medium"
CPU would have been a "better" choice.
next prev parent reply other threads:[~2021-02-03 18:44 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-28 18:31 [PATCH 0/8] sched/fair: misfit task load-balance tweaks Valentin Schneider
2021-01-28 18:31 ` [PATCH 1/8] sched/fair: Clean up active balance nr_balance_failed trickery Valentin Schneider
2021-02-03 15:14 ` Qais Yousef
2021-02-03 18:42 ` Valentin Schneider
2021-02-04 15:05 ` Qais Yousef
2021-02-05 13:51 ` Vincent Guittot
2021-02-05 14:05 ` Valentin Schneider
2021-02-05 14:34 ` Vincent Guittot
2021-01-28 18:31 ` [PATCH 2/8] sched/fair: Add more sched_asym_cpucapacity static branch checks Valentin Schneider
2021-02-03 15:14 ` Qais Yousef
2021-02-09 8:42 ` Vincent Guittot
2021-01-28 18:31 ` [PATCH 3/8] sched/fair: Tweak misfit-related capacity checks Valentin Schneider
2021-02-03 15:15 ` Qais Yousef
2021-02-03 18:42 ` Valentin Schneider
2021-02-05 14:31 ` Vincent Guittot
2021-02-05 16:59 ` Valentin Schneider
2021-02-05 17:17 ` Vincent Guittot
2021-02-05 20:07 ` Valentin Schneider
2021-02-08 15:29 ` Vincent Guittot
2021-02-08 17:49 ` Valentin Schneider
2021-01-28 18:31 ` [PATCH 4/8] sched/fair: Use dst_cpu's capacity rather than group {min, max} capacity Valentin Schneider
2021-02-03 15:15 ` Qais Yousef
2021-01-28 18:31 ` [PATCH 5/8] sched/fair: Make check_misfit_status() only compare dynamic capacities Valentin Schneider
2021-02-03 15:15 ` Qais Yousef
2021-02-04 10:49 ` Dietmar Eggemann
2021-02-04 11:34 ` Valentin Schneider
2021-02-04 14:57 ` Dietmar Eggemann
2021-01-28 18:31 ` [PATCH 6/8] sched/fair: Filter out locally-unsolvable misfit imbalances Valentin Schneider
2021-02-03 15:16 ` Qais Yousef
2021-02-03 18:43 ` Valentin Schneider [this message]
2021-01-28 18:31 ` [PATCH 7/8] sched/fair: Attempt misfit active balance when migration_type != migrate_misfit Valentin Schneider
2021-02-03 15:16 ` Qais Yousef
2021-02-03 18:43 ` Valentin Schneider
2021-02-04 11:44 ` Dietmar Eggemann
2021-02-04 12:22 ` Valentin Schneider
2021-02-09 8:58 ` Vincent Guittot
2021-02-09 18:19 ` Valentin Schneider
2021-01-28 18:31 ` [PATCH 8/8] sched/fair: Relax task_hot() for misfit tasks Valentin Schneider
2021-02-03 15:17 ` Qais Yousef
2021-02-08 16:21 ` Vincent Guittot
2021-02-08 18:24 ` Valentin Schneider
2021-02-09 8:56 ` Vincent Guittot
2021-02-03 15:14 ` [PATCH 0/8] sched/fair: misfit task load-balance tweaks Qais Yousef
2021-02-03 18:43 ` Valentin Schneider
2021-02-04 12:03 ` Dietmar Eggemann
2021-02-04 12:36 ` Valentin Schneider
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