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Date: Fri, 14 Jul 2023 18:44:58 +0530
From: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
To: Tim Chen <tim.c.chen@...ux.intel.com>,
Peter Zijlstra <peterz@...radead.org>
Cc: 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>,
shrikanth hegde <sshegde@...ux.vnet.ibm.com>,
Tobias Huschle <huschle@...ux.ibm.com>
Subject: Re: [Patch v3 3/6] sched/fair: Implement prefer sibling imbalance
calculation between asymmetric groups
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
> + /* 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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