Re: [PATCH v3 1/5] perf cpumap: If the die_id is missing use socket/physical_package_id

[Ian Rogers]

On Wed, Dec 18, 2024 at 4:04 AM James Clark <james.clark@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >   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
hence using the number of packages. I think we need to make sure there
is order otherwise we'll end up with "if no dies value present" or "if
dies value is 0 ignore" everywhere. What is strange to me is that ARM
may have the die_id present but have it contain the value -1.

> And last minor comment this could do with a fixes: 05be17eed774

Thanks,
Ian

> [1]:
> https://lore.kernel.org/linux-perf-users/20241218115552.912517-1-james.clark@xxxxxxxxxx/T/#u
>