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

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Date: Mon, 5 Feb 2024 18:46:33 -0800
From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To: Valentin Schneider <vschneid@...hat.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 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?

> 
> 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

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