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Date: Fri, 14 Jul 2023 16:05:53 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
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>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [Patch v3 1/6] sched/fair: Determine active load balance for
SMT sched groups
On Fri, 2023-07-14 at 18:36 +0530, Shrikanth Hegde wrote:
>
>
> If we consider symmetric platforms which have SMT4 such as power10.
> we have a topology like below. multiple such MC will form DIE(PKG)
>
>
> [0 2 4 6][1 3 5 7][8 10 12 14][9 11 13 15]
> [--SMT--][--SMT--][----SMT---][---SMT----]
> [--sg1--][--sg1--][---sg1----][---sg1----]
> [--------------MC------------------------]
>
> In case of SMT4, if there is any group which has 2 or more tasks, that
> group will be marked as group_smt_balance. previously, if that group had 2
> or 3 tasks, it would have been marked as group_has_spare. Since all the groups have
> SMT that means behavior would be same fully busy right? That can cause some
> corner cases. No?
You raised a good point. I was looking from SMT2
perspective so group_smt_balance implies group_fully_busy.
That is no longer true for SMT4.
I am thinking of the following fix on the current patch
to take care of SMT4. Do you think this addresses
concerns from you and Tobias?
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 294a662c9410..3fc8d3a3bd22 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9588,6 +9588,17 @@ static bool update_sd_pick_busiest(struct lb_env *env,
break;
case group_smt_balance:
+ /* no idle cpus on both groups handled by group_fully_busy below */
+ if (sgs->idle_cpus != 0 || busiest->idle_cpus != 0) {
+ if (sgs->idle_cpus > busiest->idle_cpus)
+ return false;
+ if (sgs->idle_cpus < busiest->idle_cpus)
+ return true;
+ if (sgs->sum_nr_running <= busiest_sum_nr_running)
+ return false;
+ else
+ return true;
+ }
I will be on vacation next three weeks so my response will be slow.
Tim
>
> One example is Lets say sg1 has 4 tasks. and sg2 has 0 tasks and is trying to do
> load balance. Previously imbalance would have been 2, instead now imbalance would be 1.
> But in subsequent lb it would be balanced.
>
>
>
> > + return false;
> > +}
> > +
> > static inline bool
> > sched_reduced_capacity(struct rq *rq, struct sched_domain *sd)
> > {
> > @@ -9425,6 +9464,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> > sgs->group_asym_packing = 1;
> > }
> >
> > + /* Check for loaded SMT group to be balanced to dst CPU */
> > + if (!local_group && smt_balance(env, sgs, group))
> > + sgs->group_smt_balance = 1;
> > +
> > sgs->group_type = group_classify(env->sd->imbalance_pct, group, sgs);
> >
> > /* Computing avg_load makes sense only when group is overloaded */
> > @@ -9509,6 +9552,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> > return false;
> > break;
> >
> > + case group_smt_balance:
> > case group_fully_busy:
> > /*
> > * Select the fully busy group with highest avg_load. In
> > @@ -9537,6 +9581,18 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> > break;
> >
> > case group_has_spare:
> > + /*
> > + * Do not pick sg with SMT CPUs over sg with pure CPUs,
> > + * as we do not want to pull task off SMT core with one task
> > + * and make the core idle.
> > + */
> > + if (smt_vs_nonsmt_groups(sds->busiest, sg)) {
> > + if (sg->flags & SD_SHARE_CPUCAPACITY && sgs->sum_h_nr_running <= 1)
> > + return false;
> > + else
> > + return true;> + }
> > +
> > /*
> > * Select not overloaded group with lowest number of idle cpus
> > * and highest number of running tasks. We could also compare
> > @@ -9733,6 +9789,7 @@ static bool update_pick_idlest(struct sched_group *idlest,
> >
> > case group_imbalanced:
> > case group_asym_packing:
> > + case group_smt_balance:
> > /* Those types are not used in the slow wakeup path */
> > return false;
> >
> > @@ -9864,6 +9921,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
> >
> > case group_imbalanced:
> > case group_asym_packing:
> > + case group_smt_balance:
> > /* Those type are not used in the slow wakeup path */
> > return NULL;
> >
> > @@ -10118,6 +10176,13 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> > return;
> > }
> >
> > + if (busiest->group_type == group_smt_balance) {
> > + /* Reduce number of tasks sharing CPU capacity */
> > + env->migration_type = migrate_task;
> > + env->imbalance = 1;
> > + return;
> > + }
> > +
> > if (busiest->group_type == group_imbalanced) {
> > /*
> > * In the group_imb case we cannot rely on group-wide averages
> > @@ -10363,16 +10428,23 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
> > goto force_balance;
> >
> > if (busiest->group_type != group_overloaded) {
> > - if (env->idle == CPU_NOT_IDLE)
> > + if (env->idle == CPU_NOT_IDLE) {
> > /*
> > * If the busiest group is not overloaded (and as a
> > * result the local one too) but this CPU is already
> > * busy, let another idle CPU try to pull task.
> > */
> > goto out_balanced;
> > + }
> > +
> > + if (busiest->group_type == group_smt_balance &&
> > + smt_vs_nonsmt_groups(sds.local, sds.busiest)) {
> > + /* Let non SMT CPU pull from SMT CPU sharing with sibling */
> > + goto force_balance;
> > + }
> >
> > if (busiest->group_weight > 1 &&
> > - local->idle_cpus <= (busiest->idle_cpus + 1))
> > + local->idle_cpus <= (busiest->idle_cpus + 1)) {
> > /*
> > * If the busiest group is not overloaded
> > * and there is no imbalance between this and busiest
> > @@ -10383,12 +10455,14 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
> > * there is more than 1 CPU per group.
> > */
> > goto out_balanced;
> > + }
> >
> > - if (busiest->sum_h_nr_running == 1)
> > + if (busiest->sum_h_nr_running == 1) {
> > /*
> > * busiest doesn't have any tasks waiting to run
> > */
> > goto out_balanced;
> > + }
> > }
> >
> > force_balance:
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