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

[Mathieu Desnoyers]

On 8/17/23 12:09, Mathieu Desnoyers wrote:
> On 8/17/23 12:01, Vincent Guittot wrote:
> > On Thu, 17 Aug 2023 at 17:34, Mathieu Desnoyers
> > <mathieu.desnoyers@xxxxxxxxxxxx> 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@xxxxxxxxxxxxx/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.

Re-reading the patch you pointed me to, I notice:

"+ * Whether CPUs are share lowest cache, which means LLC on non-cluster
 + * machines and LLC tag or L2 on machines with clusters."

So this "share lowest cache" really means lowest in terms of number, 
e.g. L2 < L3, and not "lowest in the hierarchy" as is "closest to 
memory", correct ?

Thanks,

Mathieu

> Thanks,
>
> Mathieu

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