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

[Yicong Yang <yangyicong@huawei.com>]

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.

Thanks.