linux-kernel - Re: [PATCH v3 1/5] perf cpumap: If the die_id is missing use socket/physical_package

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Message-ID: <CAP-5=fUUUZcE8D3q3iNkrym=9g6UQxgfDihC24+1J879fj4kww@mail.gmail.com>
Date: Thu, 19 Dec 2024 09:20:39 -0800
From: Ian Rogers <irogers@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: James Clark <james.clark@...aro.org>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, Kan Liang <kan.liang@...ux.intel.com>, 
	Sun Haiyong <sunhaiyong@...ngson.cn>, Yanteng Si <siyanteng@...ngson.cn>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/5] perf cpumap: If the die_id is missing use socket/physical_package_id

On Wed, Dec 18, 2024 at 9:32 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Wed, Dec 18, 2024 at 09:42:27AM -0800, Ian Rogers wrote:
> > On Wed, Dec 18, 2024 at 4:04 AM James Clark <james.clark@...aro.org> wrote:
> > >
> > >
> > >
> > > On 16/12/2024 11:24 pm, Ian Rogers wrote:
> > > > An error value for a missing die_id may be written into things like
> > > > the cpu_topology_map. As the topology needs to be fully written out,
> > > > including the die_id, to allow perf.data file features to be aligned
> > > > we can't allow error values to be written out. Instead base the
> > > > missing die_id value off of the socket/physical_package_id assuming
> > > > they correlate 1:1.
> > > >
> > > > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > > > ---
> > > >   tools/perf/util/cpumap.c | 3 ++-
> > > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/tools/perf/util/cpumap.c b/tools/perf/util/cpumap.c
> > > > index 27094211edd8..d362272f8466 100644
> > > > --- a/tools/perf/util/cpumap.c
> > > > +++ b/tools/perf/util/cpumap.c
> > > > @@ -283,7 +283,8 @@ int cpu__get_die_id(struct perf_cpu cpu)
> > > >   {
> > > >       int value, ret = cpu__get_topology_int(cpu.cpu, "die_id", &value);
> > > >
> > > > -     return ret ?: value;
> > > > +        /* If die_id is missing fallback on using the socket/physical_package_id. */
> > > > +     return ret || value < 0 ? cpu__get_socket_id(cpu) : value;
> > > >   }
> > > >
> > > >   struct aggr_cpu_id aggr_cpu_id__die(struct perf_cpu cpu, void *data)
> > >
> > > Hi Ian,
> > >
> > > I sent a fix for the same or a similar problem here [1]. For this one
> > > I'm not sure why we'd want to use the socket ID for die when it's always
> > > been 0 for not present. I wonder if this change is mingling two things:
> > > fixing the negative error value appearing and replacing die with socket ID.
> > >
> > > Personally I would prefer to keep the 0 to fix the error value, that way
> > > nobody gets surprised by the change.
> > >
> > > Also it looks like cpu__get_cluster_id() can suffer from the same issue,
> > > and if we do it this way we should drop these as they aren't valid anymore:
> > >
> > >         /* There is no die_id on legacy system. */
> > >         if (die < 0)
> > >                 die = 0;
> >
> > I think this breaks the assumption here:
> > https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/tests/expr.c?h=perf-tools-next#n244
>
> Hmm.. I'm not sure how it worked before.  The code is already there and
> it just changed the condition from == -1 to < 0, right?

You'd need to be testing on a multi-socket machine to see the issue.
If you had say a dual socket Ampere chip and the die_id was missing,
does it make sense for there to be two sockets/packages but only 1
die? I think it is best we assume 1 die per socket when the die_id is
missing, and to some crippled extent (because of the s390 workaround)
the expr test is doing the sanity check.

Thanks,
Ian