Re: [PATCH v1 1/8] perf: Increase MAX_NR_CPUS to 4096

[Ian Rogers]

On Fri, Dec 6, 2024 at 2:24 AM Leo Yan <leo.yan@xxxxxxx> wrote:
>
> Hi Ian,
>
> On Thu, Dec 05, 2024 at 08:40:28PM -0800, Ian Rogers wrote:
> >
> > From: Kyle Meyer <kyle.meyer@xxxxxxx>
> >
> > Systems have surpassed 2048 CPUs. Increase MAX_NR_CPUS to 4096.
> >
> > Bitmaps declared with MAX_NR_CPUS bits will increase from 256B to 512B,
> > cpus_runtime will increase from 81960B to 163880B, and max_entries will
> > increase from 8192B to 16384B.
> >
> > Signed-off-by: Kyle Meyer <kyle.meyer@xxxxxxx>
> > Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >  tools/lib/perf/include/internal/cpumap.h | 2 +-
> >  tools/perf/perf.h                        | 2 +-
> >  tools/perf/util/bpf_skel/kwork_top.bpf.c | 4 +++-
> >  3 files changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/lib/perf/include/internal/cpumap.h b/tools/lib/perf/include/internal/cpumap.h
> > index 49649eb51ce4..3cf28522004e 100644
> > --- a/tools/lib/perf/include/internal/cpumap.h
> > +++ b/tools/lib/perf/include/internal/cpumap.h
> > @@ -22,7 +22,7 @@ DECLARE_RC_STRUCT(perf_cpu_map) {
> >  };
> >
> >  #ifndef MAX_NR_CPUS
> > -#define MAX_NR_CPUS    2048
> > +#define MAX_NR_CPUS    4096
> >  #endif
>
> This series is fine for me.  Just wandering if we can use a central
> place to maintain the macro, e.g. lib/perf/include/perf/cpumap.h.  It
> is pointless to define exactly same macros in different headers.  As
> least, I think we can unify this except the kwork bpf program?
>
> P.s. for dynamically allocating per CPU maps in eBPF program, we can
> refer to the code samples/bpf/xdp_sample_user.c, but this is another
> topic.

Thanks Leo,

can I take this as an acked-by? Wrt a single constant I agree,
following these changes MAX_NR_CPUS is just used for a warning in
libperf's cpumap.c. I think we're agreed that getting rid of the
constant would be best. I also think the cpumap logic is duplicating
something that libc is providing in cpu_set:
https://man7.org/linux/man-pages/man3/CPU_SET.3.html
And we have more than one representation in perf for the sake of the
disk representation:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/lib/perf/include/perf/event.h?h=perf-tools-next#n227
Just changing the int to be a s16 would lower the memory overhead,
which is why I'd kind of like the abstraction to be minimal.

Thanks,
Ian