Re: [PATCH v8 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path

[Chen Yu <yu.c.chen@intel.com>]

On 2023-05-31 at 16:21:00 +0800, Yicong Yang wrote:
> On 2023/5/30 22:39, Yicong Yang wrote:
> > On 2023/5/30 19:55, Peter Zijlstra wrote:
> >> On Tue, May 30, 2023 at 03:02:53PM +0800, Yicong Yang wrote:
> >>
> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >>> index 373ff5f55884..b8c129ed8b47 100644
> >>> --- a/kernel/sched/fair.c
> >>> +++ b/kernel/sched/fair.c
> >>> @@ -6994,6 +6994,30 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >>>  		}
> >>>  	}
> >>>  
> >>> +	if (static_branch_unlikely(&sched_cluster_active)) {
> >>> +		struct sched_domain *sdc = rcu_dereference(per_cpu(sd_cluster, target));
> >>> +
> >>> +		if (sdc) {
> >>> +			for_each_cpu_wrap(cpu, sched_domain_span(sdc), target + 1) {
> >>> +				if (!cpumask_test_cpu(cpu, cpus))
> >>> +					continue;
> >>> +
> >>> +				if (has_idle_core) {
> >>> +					i = select_idle_core(p, cpu, cpus, &idle_cpu);
> >>> +					if ((unsigned int)i < nr_cpumask_bits)
> >>> +						return i;
> >>> +				} else {
> >>> +					if (--nr <= 0)
> >>> +						return -1;
> >>> +					idle_cpu = __select_idle_cpu(cpu, p);
> >>> +					if ((unsigned int)idle_cpu < nr_cpumask_bits)
> >>> +						return idle_cpu;
> >>> +				}
> >>> +			}
> >>> +			cpumask_andnot(cpus, cpus, sched_domain_span(sdc));
> >>> +		}
> >>> +	}
> >>
> >> Would not this:
> >>
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -6994,6 +6994,29 @@ static int select_idle_cpu(struct task_s
> >>  		}
> >>  	}
> >>  
> >> +	if (static_branch_unlikely(&sched_cluster_active)) {
> >> +		struct sched_group *sg = sd->groups;
> >> +		if (sg->flags & SD_CLUSTER) {
> >> +			for_each_cpu_wrap(cpu, sched_group_span(sg), target+1) {
> >> +				if (!cpumask_test_cpu(cpu, cpus))
> >> +					continue;
> >> +
> >> +				if (has_idle_core) {
> >> +					i = select_idle_core(p, cpu, cpus, &idle_cpu);
> >> +					if ((unsigned)i < nr_cpumask_bits)
> >> +						return 1;
> >> +				} else {
> >> +					if (--nr <= 0)
> >> +						return -1;
> >> +					idle_cpu = __select_idle_cpu(cpu, p);
> >> +					if ((unsigned)idle_cpu < nr_cpumask_bits)
> >> +						return idle_cpu;
> >> +				}
> >> +			}
> >> +			cpumask_andnot(cpus, cpus, sched_group_span(sg));
> >> +		}
> >> +	}
> >> +
> >>  	for_each_cpu_wrap(cpu, cpus, target + 1) {
> >>  		if (has_idle_core) {
> >>  			i = select_idle_core(p, cpu, cpus, &idle_cpu);
> >>
> >> also work? Then we can avoid the extra sd_cluster per-cpu variable.
> >>
> > 
> > I thought it will be fine since sg->flags is derived from the child domain. But practically it doesn't.
> > Tested on a 2P Skylake server with no clusters, add some debug messages to see how sg->flags appears:
> > 
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index 69968ed9ffb9..5c443b74abf5 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -90,8 +90,8 @@ static int sched_domain_debug_one(struct sched_domain *sd, int cpu, int level,
> > 
> >                 cpumask_or(groupmask, groupmask, sched_group_span(group));
> > 
> > -               printk(KERN_CONT " %d:{ span=%*pbl",
> > -                               group->sgc->id,
> > +               printk(KERN_CONT " %d:{ cluster: %s span=%*pbl",
> > +                               group->sgc->id, group->flags & SD_CLUSTER ? "true" : "false",
> >                                 cpumask_pr_args(sched_group_span(group)));
> > 
> >                 if ((sd->flags & SD_OVERLAP) &&
> > 
> > Unfortunately the result doesn't match what I expected, the MC domain's sg->flags still marked
> > as cluster:
> > 
> > [    8.886099] CPU0 attaching sched-domain(s):
> > [    8.889539]  domain-0: span=0,40 level=SMT
> > [    8.893538]   groups: 0:{ cluster: false span=0 }, 40:{ cluster: false span=40 }
> > [    8.897538]   domain-1: span=0-19,40-59 level=MC
> > [    8.901538]    groups: 0:{ cluster: true span=0,40 cap=2048 }, 1:{ cluster: true span=1,41 cap=2048 }, 2:{ cluster: true span=2,42 cap=2048 }, 3:{ cluster: true span=3,43 cap=2048 }, 4:{ cluster: true span=4,44 cap=2048 }, 5:{ cluster: true span=5,45 cap=2048 }, 6:{ cluster: true span=6,46 cap=2048 }, 7:{ cluster: true span=7,47 cap=2048 }, 8:{ cluster: true span=8,48 cap=2048 }, 9:{ cluster: true span=9,49 cap=2048 }, 10:{ cluster: true span=10,50 cap=2048 }, 11:{ cluster: true span=11,51 cap=2048 }, 12:{ cluster: true span=12,52 cap=2048 }, 13:{ cluster: true span=13,53 cap=2048 }, 14:{ cluster: true span=14,54 cap=2048 }, 15:{ cluster: true span=15,55 cap=2048 }, 16:{ cluster: true span=16,56 cap=2048 }, 17:{ cluster: true span=17,57 cap=2048 }, 18:{ cluster: true span=18,58 cap=2048 }, 19:{ cluster: true span=19,59 cap=2048 }
> > [    8.905538]    domain-2: span=0-79 level=NUMA
> > [    8.909538]     groups: 0:{ cluster: false span=0-19,40-59 cap=40960 }, 20:{ cluster: false span=20-39,60-79 cap=40960 }
> > 
> > I assume we didn't handle the sg->flags correctly on the domain degeneration. Simply checked the code seems
> > we've already make sg->flags = 0 on degeneration, maybe I need to check where's wrong.
> > 
> 
> Currently we only update the groups' flags to 0 for the final lowest domain in [1]. The upper
> domains' group won't be updated if degeneration happens. So we cannot use the suggested approach
> for cluster scanning and sd_cluster per-cpu variable is still needed.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/sched/topology.c?h=v6.4-rc4#n749
> 
> Thanks.
> 
>
Is this an issue? Suppose sched domain A's parent domain
is B, B's parent sched domain is C. When B degenerates, C's child domain
pointer is adjusted to A. However, currently the code does not adjust C's
sched groups' flags. Should we adjust C's sched groups flags to be the same
as A to keep consistency?

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 6198fa135176..fe3fd70f2313 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -713,14 +713,13 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 
 	/* Remove the sched domains which do not contribute to scheduling. */
 	for (tmp = sd; tmp; ) {
-		struct sched_domain *parent = tmp->parent;
+		struct sched_domain *parent = tmp->parent, *pparent;
 		if (!parent)
 			break;
 
 		if (sd_parent_degenerate(tmp, parent)) {
-			tmp->parent = parent->parent;
-			if (parent->parent)
-				parent->parent->child = tmp;
+			pparent = parent->parent;
+			tmp->parent = pparent;
 			/*
 			 * Transfer SD_PREFER_SIBLING down in case of a
 			 * degenerate parent; the spans match for this
@@ -728,6 +727,18 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 			 */
 			if (parent->flags & SD_PREFER_SIBLING)
 				tmp->flags |= SD_PREFER_SIBLING;
+
+			if (pparent) {
+				struct sched_group *sg = pparent->groups;
+
+				do {
+					sg->flags = tmp->flags;
+					sg = sg->next;
+				} while (sg != pparent->groups);
+
+				pparent->child = tmp;
+			}
+
 			destroy_sched_domain(parent);
 		} else
 			tmp = tmp->parent;
-- 
2.25.1

Besides per your description I found that something is not quite right:
 
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 6682535e37c8..6198fa135176 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -747,6 +747,7 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 			 */
 			do {
 				sg->flags = 0;
+				sg = sg->next;
 			} while (sg != sd->groups);
 
 			sd->child = NULL;
-- 
2.25.1

thanks,
Chenyu