linux-kernel - Re: [PATCH v3 1/4] sched/fair: add SD

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Message-ID: <xhsmheddphgx5.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 06 Feb 2024 14:16:06 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc: alexs@...nel.org, Ingo Molnar <mingo@...hat.com>, Peter Zijlstra
 <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, Vincent
 Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann
 <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben
 Segall <bsegall@...gle.com>, Daniel Bristot de Oliveira
 <bristot@...hat.com>, linux-kernel@...r.kernel.org, sshegde@...ux.ibm.com
Subject: Re: [PATCH v3 1/4] sched/fair: add SD_CLUSTER in comments

On 05/02/24 18:46, Ricardo Neri wrote:
> On Fri, Feb 02, 2024 at 03:27:32PM +0100, Valentin Schneider wrote:
>>
>> Subject nit: the prefix should be sched/topology
>>
>> On 01/02/24 19:54, alexs@...nel.org wrote:
>> > From: Alex Shi <alexs@...nel.org>
>> >
>> > The description of SD_CLUSTER is missing. Add it.
>> >
>> > Signed-off-by: Alex Shi <alexs@...nel.org>
>> > To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
>> > To: Valentin Schneider <vschneid@...hat.com>
>> > To: Vincent Guittot <vincent.guittot@...aro.org>
>> > To: Juri Lelli <juri.lelli@...hat.com>
>> > To: Peter Zijlstra <peterz@...radead.org>
>> > To: Ingo Molnar <mingo@...hat.com>
>> > ---
>> >  kernel/sched/topology.c | 1 +
>> >  1 file changed, 1 insertion(+)
>> >
>> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>> > index 10d1391e7416..8b45f16a1890 100644
>> > --- a/kernel/sched/topology.c
>> > +++ b/kernel/sched/topology.c
>> > @@ -1554,6 +1554,7 @@ static struct cpumask		***sched_domains_numa_masks;
>> >   * function:
>> >   *
>> >   *   SD_SHARE_CPUCAPACITY   - describes SMT topologies
>> > + *   SD_CLUSTER             - describes CPU Cluster topologies
>>
>> So I know this is the naming we've gone for the "Cluster" naming, but this
>> comment isn't really explaining anything.
>>
>> include/linux/sched/sd_flags.h has a bit more info already:
>>  * Domain members share CPU cluster (LLC tags or L2 cache)
>
> I also thought of this, but I didn't want to suggest to repeat in topolog.c
> what is described in sd_flags.h.
>
> Maybe it is better to remove the descriptions of all flags here and instead
> direct the reader to sd_flags.h?
>

Yeah I agree on less duplication.

>>
>> I had to go through a bit of git history to remember what the CLUSTER thing
>> was about, how about this:
>>
>> * SD_CLUSTER             - describes shared shared caches, cache tags or busses
>
> AFAIK, this describes a subset of CPUs in the package that share a
> resource, likely L2 cache.
>
>> * SD_SHARE_PKG_RESOURCES - describes shared LLC cache
>>
>> And looking at this it would make sense to:
>>   rename SD_CLUSTER into SD_SHARE_PKG_RESOURCES
>
> but not all CPUs in the package share the resource

But SD_CLUSTER never expands beyond the package, right?

Regardless, my main point is that having both SD_CLUSTER and
SD_SHARE_PKG_RESOURCES is a source of confusion (at the very least for
myself), and given SD_SHARE_PKG_RESOURCES is really used to mean "shares
LLC" (see update_top_cache_domain()), we could make that flag more explicit
and lift some ambiguity with SD_CLUSTER.

>
>>   rename SD_SHARE_PKG_RESOURCES into SD_SHARE_LLC
>> but that's another topic...
>>