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

[Yicong Yang]

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.

Thanks,
Yicong