Messages in this thread | | | Date | Thu, 17 Aug 2023 12:14:47 -0400 | Subject | Re: [RFC PATCH 1/1] sched: ttwu_queue_cond: perform queued wakeups across different L2 caches | From | 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@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.
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
| |