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

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

On 2023-06-15 at 15:59:10 +0800, Yicong Yang wrote:
> On 2023/6/13 20:44, Chen Yu wrote:
> > On 2023-06-13 at 16:09:17 +0800, Yicong Yang wrote:
> >> On 2023/6/13 15:36, Yicong Yang wrote:
> >>> On 2023/6/9 18:50, Chen Yu wrote:
> >>>> On 2023-06-08 at 14:45:54 +0800, Yicong Yang wrote:
> >>>>> On 2023/6/8 11:26, Chen Yu wrote:
> >>>>>> 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?
> >>>>>
> >>>>> It depends on whether we're going to use it. currently only asym_smt_can_pull_tasks() uses
> >>>>> it within the SMT so I think update the lowest domain's group flag works. For correctness
> >>>>> all the domain group's flag should derives from its real child. I tried to solve this at group
> >>>>> building but seems hard to do, at that time we don't know whether a domain is going to degenerate
> >>>>> or not since the groups it not built.
> >>>>>
> >>>>>>
> >>>>>> 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;
> >>>>>
> >>>>> May need to test on some heterogeous platforms. Does it always stand that child domain of CPU from
> >>>>> remote group have the same flags with @tmp?
> >>>>>
> >>>> Good question, for heterogenous platforms sched groups within the same domain might not always
> >>>> have the same flags, because if group1 and group2 sit in big/small-core child domain, they could
> >>>> have different balance flags in theory. Maybe only update the local group's flag is accurate.
> >>>>
> >>>> I found Tim has proposed a fix for a similar scenario, and it is for SD_SHARE_CPUCAPACITY, and it
> >>>> should be in tip:
> >>>> https://lore.kernel.org/lkml/168372654916.404.6677242284447941021.tip-bot2@tip-bot2/
> >>>> We could adjust it based on his change to remove SD_CLUSTER, or we can
> >>>> replace groups->flag with tmp->flag directly, in case we have other flags to be
> >>>> adjusted in the future.
> >>>>
> >>>
> >>> Thanks for the reference. I think we can handle the SD_CLUSTER in the same way and only update
> >>> local groups flag should satisfy our needs. I tried to use the correct child domains to build the
> >>> sched groups then all the groups will have the correct flags, but it'll be a bit more complex.
> >>>
> >>
> >> something like below, detect the sched domain degeneration first and try to rebuild the groups if
> >> necessary.
> > Not sure if we need to rebuild the groups. With only
> > 
> > if (parent->flags & SD_CLUSTER)
> > 	parent->parent->groups->flags &= ~SD_CLUSTER;
> > 
> > I see the correct flags.
> > 
> > My understanding is that, although remote groups's flag might be incorrect,
> > later when other sched domain degenerates, these remote groups becomes local
> > groups for those sched domains, and the flags will be adjusted accordingly.
> 
> Maybe worth a try to build the groups correctly at very beginning rather
> correct it later when needed. Considering we've used it in several places[1][2]
> and this time we're going to use it for cluster.
> 
> [1] 16d364ba6ef2 ("sched/topology: Introduce sched_group::flags")
> [2] https://lore.kernel.org/lkml/168372654916.404.6677242284447941021.tip-bot2@tip-bot2/
>
Do you mean clearing the SD_CLUSTER during degenerating? Yup, that could
be enough for now. I guess you are going to send a new version with this
SD_CLUSTER flag cleared  + using group->flags to detect SD_CLUSTER rather
than percpu cluster id. I'd be happy to test once you send them out.

thanks,
Chenyu