Re: [RFC PATCH 1/1] sched: ttwu_queue_cond: perform queued wakeups across different L2 caches

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 8/17/23 12:01, Vincent Guittot wrote:
> On Thu, 17 Aug 2023 at 17:34, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> Skipping queued wakeups for all logical CPUs sharing an LLC means that
>> on a 192 cores AMD EPYC 9654 96-Core Processor (over 2 sockets), groups
>> of 8 cores (16 hardware threads) end up grabbing runqueue locks of other
>> runqueues within the same group for each wakeup, causing contention on
>> the runqueue locks.
[...]
>>
>> -bool cpus_share_cache(int this_cpu, int that_cpu);
>> +bool cpus_share_cluster(int this_cpu, int that_cpu);   /* Share L2. */
>> +bool cpus_share_cache(int this_cpu, int that_cpu);     /* Share LLC. */
> 
> I think that Yicong is doing what you want with
> cpus_share_lowest_cache() which points to cluster when available or
> LLC otherwise
> https://lore.kernel.org/lkml/20220720081150.22167-1-yangyicong@hisilicon.com/t/#m0ab9fa0fe0c3779b9bbadcfbc1b643dce7cb7618
> 

AFAIU (please correct me if I'm wrong) my AMD EPYC machine has sockets 
consisting of 12 clusters, each cluster having its own L3 cache.

What I am trying to achieve here is really to implement "cpus_share_l2": 
I want this to match only when the cpus have a common L2 cache. L3 
appears to be a group which is either:

- too large (16 hw threads) or
- have a too high access latency.

I'm not certain which (or if both) of those reasons explain why
grouping by L2 is better here.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com