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

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Message-ID: <e52d3f53-97dc-2081-5124-5e13e29d2603@efficios.com>
Date:   Thu, 17 Aug 2023 12:09:30 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Swapnil Sapkal <Swapnil.Sapkal@....com>,
        Aaron Lu <aaron.lu@...el.com>, x86@...nel.org
Subject: Re: [RFC PATCH 1/1] sched: ttwu_queue_cond: perform queued wakeups
 across different L2 caches

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