linux-kernel - Re: [PATCH v3 1/5] perf cpumap: If the die_id is missing use socket/physical_package

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Message-ID: <855f50a5-f397-4a08-8298-78bd040e5328@linaro.org>
Date: Wed, 18 Dec 2024 12:04:02 +0000
From: James Clark <james.clark@...aro.org>
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>, Sun Haiyong <sunhaiyong@...ngson.cn>,
 Yanteng Si <siyanteng@...ngson.cn>, linux-perf-users@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/5] perf cpumap: If the die_id is missing use
 socket/physical_package_id



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@...gle.com>
> ---
>   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;

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

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