linux-kernel - Re: [PATCH] perf: Increase MAX_NR

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Message-ID: <Z1HqnTwKNvWucrEd@hpe.com>
Date: Thu, 5 Dec 2024 12:02:05 -0600
From: Kyle Meyer <kyle.meyer@....com>
To: Ian Rogers <irogers@...gle.com>
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        namhyung@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        adrian.hunter@...el.com, kan.liang@...ux.intel.com,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Increase MAX_NR_CPUS to 4096

On Thu, Dec 05, 2024 at 09:36:07AM -0800, Ian Rogers wrote:
> On Thu, Dec 5, 2024 at 9:01 AM Kyle Meyer <kyle.meyer@....com> wrote:
> >
> > Systems have surpassed 2048 CPUs. Increase MAX_NR_CPUS to 4096.
> >
> > Bitmaps declared with MAX_NR_CPUS bits will increase from 256B to 512B,
> > and cpus_runtime will increase from 81960B to 163880B.
> >
> > Signed-off-by: Kyle Meyer <kyle.meyer@....com>
> 
> This is very interesting, thanks Kyle! Just noting, having the same
> #define in many places is clearly error prone and there are other
> redefinitions of this value:
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/bpf_skel/kwork_top.bpf.c?h=perf-tools-next#n21
> 
> I wonder I can refactor `tools/lib/perf/cpumap.c` to get rid of that
> constant as it is only used when parsing from a file/string.
> 
> Could the kwork developers perhaps look at their many uses? The other
> uses in the tool may be removable too.
> 
> Wrt this change, perhaps bump
> `tools/perf/util/bpf_skel/kwork_top.bpf.c` too and then we merge that
> while trying to remove other uses.

Sure, that sounds good to me. I'll send a second version.

Thanks,
Kyle Meyer

> > ---
> >
> > Tested on a 32 socket Sapphire Rapids system with 3840 CPUs.
> >
> >  tools/lib/perf/include/internal/cpumap.h | 2 +-
> >  tools/perf/perf.h                        | 2 +-
> >  2 files changed, 2 insertions(+), 2 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
> >
> >  struct perf_cpu_map *perf_cpu_map__alloc(int nr_cpus);
> > diff --git a/tools/perf/perf.h b/tools/perf/perf.h
> > index c004dd4e65a3..3cb40965549f 100644
> > --- a/tools/perf/perf.h
> > +++ b/tools/perf/perf.h
> > @@ -3,7 +3,7 @@
> >  #define _PERF_PERF_H
> >
> >  #ifndef MAX_NR_CPUS
> > -#define MAX_NR_CPUS                    2048
> > +#define MAX_NR_CPUS                    4096
> >  #endif
> >
> >  enum perf_affinity {
> > --
> > 2.47.1
> >