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Date: Fri, 14 Jul 2023 16:22:56 +0200
From: Tobias Huschle <huschle@...ux.ibm.com>
To: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Ricardo Neri <ricardo.neri@...el.com>,
"Ravi V . Shankar" <ravi.v.shankar@...el.com>,
Ben Segall <bsegall@...gle.com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
Steven Rostedt <rostedt@...dmis.org>,
Valentin Schneider <vschneid@...hat.com>,
Ionela Voinescu <ionela.voinescu@....com>, x86@...nel.org,
linux-kernel@...r.kernel.org,
Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
naveen.n.rao@...ux.vnet.ibm.com,
Yicong Yang <yangyicong@...ilicon.com>,
Barry Song <v-songbaohua@...o.com>,
Chen Yu <yu.c.chen@...el.com>, Hillf Danton <hdanton@...a.com>
Subject: Re: [Patch v3 3/6] sched/fair: Implement prefer sibling imbalance
calculation between asymmetric groups
On 2023-07-14 15:14, Shrikanth Hegde wrote:
> On 7/8/23 4:27 AM, Tim Chen wrote:
>> From: Tim C Chen <tim.c.chen@...ux.intel.com>
>>
>> In the current prefer sibling load balancing code, there is an
>> implicit
>> assumption that the busiest sched group and local sched group are
>> equivalent, hence the tasks to be moved is simply the difference in
>> number of tasks between the two groups (i.e. imbalance) divided by
>> two.
>>
>> However, we may have different number of cores between the cluster
>> groups,
>> say when we take CPU offline or we have hybrid groups. In that case,
>> we should balance between the two groups such that #tasks/#cores ratio
>> is the same between the same between both groups. Hence the imbalance
>
> nit: type here. the same between is repeated.
>
>> computed will need to reflect this.
>>
>> Adjust the sibling imbalance computation to take into account of the
>> above considerations.
>>
>> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
>> ---
>> kernel/sched/fair.c | 41 +++++++++++++++++++++++++++++++++++++----
>> 1 file changed, 37 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index f636d6c09dc6..f491b94908bf 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -9372,6 +9372,41 @@ static inline bool smt_balance(struct lb_env
>> *env, struct sg_lb_stats *sgs,
>> return false;
>> }
>>
>> +static inline long sibling_imbalance(struct lb_env *env,
>> + struct sd_lb_stats *sds,
>> + struct sg_lb_stats *busiest,
>> + struct sg_lb_stats *local)
>> +{
>> + int ncores_busiest, ncores_local;
>> + long imbalance;
>
> can imbalance be unsigned int or unsigned long? as sum_nr_running is
> unsigned int.
>
>> +
>> + if (env->idle == CPU_NOT_IDLE || !busiest->sum_nr_running)
>> + return 0;
>> +
>> + ncores_busiest = sds->busiest->cores;
>> + ncores_local = sds->local->cores;
>> +
>> + if (ncores_busiest == ncores_local) {
>> + imbalance = busiest->sum_nr_running;
>> + lsub_positive(&imbalance, local->sum_nr_running);
>> + return imbalance;
>> + }
>> +
>> + /* Balance such that nr_running/ncores ratio are same on both groups
>> */
>> + imbalance = ncores_local * busiest->sum_nr_running;
>> + lsub_positive(&imbalance, ncores_busiest * local->sum_nr_running);
>> + /* Normalize imbalance and do rounding on normalization */
>> + imbalance = 2 * imbalance + ncores_local + ncores_busiest;
>> + imbalance /= ncores_local + ncores_busiest;
>> +
>
> Could this work for case where number of CPU/cores would differ
> between two sched groups in a sched domain? Such as problem pointed
> by tobias on S390. It would be nice if this patch can work for that
> case
> as well. Ran numbers for a few cases. It looks to work.
> https://lore.kernel.org/lkml/20230704134024.GV4253@hirez.programming.kicks-ass.net/T/#rb0a7dcd28532cafc24101e1d0aed79e6342e3901
>
Just stumbled upon this patch series as well. In this version it looks
similar to the prototypes I played around with, but more complete.
So I'm happy that my understanding of the load balancer was kinda
correct :)
From a functional perspective this appears to address the issues we saw
on s390.
>
>
>> + /* Take advantage of resource in an empty sched group */
>> + if (imbalance == 0 && local->sum_nr_running == 0 &&
>> + busiest->sum_nr_running > 1)
>> + imbalance = 2;
>> +
>
> I don't see how this case would be true. When there are unequal number
> of cores and local->sum_nr_ruuning
> is 0, and busiest->sum_nr_running is atleast 2, imbalance will be
> atleast 1.
>
>
> Reviewed-by: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
>
>> + return imbalance;
>> +}
>> +
>> static inline bool
>> sched_reduced_capacity(struct rq *rq, struct sched_domain *sd)
>> {
>> @@ -10230,14 +10265,12 @@ static inline void
>> calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>> }
>>
>> if (busiest->group_weight == 1 || sds->prefer_sibling) {
>> - unsigned int nr_diff = busiest->sum_nr_running;
>> /*
>> * When prefer sibling, evenly spread running tasks on
>> * groups.
>> */
>> env->migration_type = migrate_task;
>> - lsub_positive(&nr_diff, local->sum_nr_running);
>> - env->imbalance = nr_diff;
>> + env->imbalance = sibling_imbalance(env, sds, busiest, local);
>> } else {
>>
>> /*
>> @@ -10424,7 +10457,7 @@ static struct sched_group
>> *find_busiest_group(struct lb_env *env)
>> * group's child domain.
>> */
>> if (sds.prefer_sibling && local->group_type == group_has_spare &&
>> - busiest->sum_nr_running > local->sum_nr_running + 1)
>> + sibling_imbalance(env, &sds, busiest, local) > 1)
>> goto force_balance;
>>
>> if (busiest->group_type != group_overloaded) {
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