Re: [PATCH] perf: Increase MAX_NR_CPUS to 4096

[Ian Rogers]

On Thu, Dec 5, 2024 at 9:01 AM Kyle Meyer <kyle.meyer@xxxxxxx> 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@xxxxxxx>

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.

Thanks,
Ian




> ---
>
> 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
>