linux-kernel - Re: [PATCH v1 1/8] perf: Increase MAX_NR

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Message-ID: <20241206230321.GA5430@e132581.arm.com>
Date: Fri, 6 Dec 2024 23:03:21 +0000
From: Leo Yan <leo.yan@....com>
To: Ian Rogers <irogers@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...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>,
	James Clark <james.clark@...aro.org>,
	Kyle Meyer <kyle.meyer@....com>, Ben Gainey <ben.gainey@....com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/8] perf: Increase MAX_NR_CPUS to 4096

Hi Ian,

On Fri, Dec 06, 2024 at 08:25:06AM -0800, Ian Rogers wrote:

[...]

> > 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?

Yeah.  I will give my review tags in the cover letter.

> 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.
>
> And we have more than one representation in perf for the sake of the
> disk representation:

Thanks for sharing the info.

> 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.

Here I am not clear what for "changing the int to be a s16".  Could you
elaberate a bit for this?

Lastly, I also found multiple files use "MAX_CPUS" rather than
"MAX_NR_CPUS".  Polish them in a new series?

Thanks,
Leo