Re: [PATCH v3 1/4] sched/fair: add SD_CLUSTER in comments

Ricardo Neri
2024-02-06T02:45:15Z

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@kernel.org wrote:
> > From: Alex Shi <alexs@kernel.org>
> >
> > The description of SD_CLUSTER is missing. Add it.
> >
> > Signed-off-by: Alex Shi <alexs@kernel.org>
> > To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > To: Valentin Schneider <vschneid@redhat.com>
> > To: Vincent Guittot <vincent.guittot@linaro.org>
> > To: Juri Lelli <juri.lelli@redhat.com>
> > To: Peter Zijlstra <peterz@infradead.org>
> > To: Ingo Molnar <mingo@redhat.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...
>