linux-kernel - Re: [PATCH v2] perf evlist: Avoid iteration for empty evlist.

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Date:   Fri, 18 Mar 2022 21:40:04 +0100
From:   Jiri Olsa <olsajiri@...il.com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] perf evlist: Avoid iteration for empty evlist.

On Thu, Mar 17, 2022 at 04:16:43PM -0700, Ian Rogers wrote:
> As seen with 'perf stat --null ..' and reported in:
> https://lore.kernel.org/lkml/YjCLcpcX2peeQVCH@kernel.org/
> 
> v2. Avoids setting evsel in the empty list case as suggested by Jiri
>     Olsa <jolsa@...nel.org>.
> Fixes: 472832d2c000 ("perf evlist: Refactor evlist__for_each_cpu()")
> Signed-off-by: Ian Rogers <irogers@...gle.com>

Acked-by: Jiri Olsa <jolsa@...nel.org>

thanks,
jirka

> ---
>  tools/perf/util/evlist.c | 28 +++++++++++++++++-----------
>  1 file changed, 17 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index 8134d45e2164..9bb79e049957 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -346,7 +346,7 @@ struct evlist_cpu_iterator evlist__cpu_begin(struct evlist *evlist, struct affin
>  {
>  	struct evlist_cpu_iterator itr = {
>  		.container = evlist,
> -		.evsel = evlist__first(evlist),
> +		.evsel = NULL,
>  		.cpu_map_idx = 0,
>  		.evlist_cpu_map_idx = 0,
>  		.evlist_cpu_map_nr = perf_cpu_map__nr(evlist->core.all_cpus),
> @@ -354,16 +354,22 @@ struct evlist_cpu_iterator evlist__cpu_begin(struct evlist *evlist, struct affin
>  		.affinity = affinity,
>  	};
>  
> -	if (itr.affinity) {
> -		itr.cpu = perf_cpu_map__cpu(evlist->core.all_cpus, 0);
> -		affinity__set(itr.affinity, itr.cpu.cpu);
> -		itr.cpu_map_idx = perf_cpu_map__idx(itr.evsel->core.cpus, itr.cpu);
> -		/*
> -		 * If this CPU isn't in the evsel's cpu map then advance through
> -		 * the list.
> -		 */
> -		if (itr.cpu_map_idx == -1)
> -			evlist_cpu_iterator__next(&itr);
> +	if (evlist__empty(evlist)) {
> +		/* Ensure the empty list doesn't iterate. */
> +		itr.evlist_cpu_map_idx = itr.evlist_cpu_map_nr;
> +	} else {
> +		itr.evsel = evlist__first(evlist);
> +		if (itr.affinity) {
> +			itr.cpu = perf_cpu_map__cpu(evlist->core.all_cpus, 0);
> +			affinity__set(itr.affinity, itr.cpu.cpu);
> +			itr.cpu_map_idx = perf_cpu_map__idx(itr.evsel->core.cpus, itr.cpu);
> +			/*
> +			 * If this CPU isn't in the evsel's cpu map then advance
> +			 * through the list.
> +			 */
> +			if (itr.cpu_map_idx == -1)
> +				evlist_cpu_iterator__next(&itr);
> +		}
>  	}
>  	return itr;
>  }
> -- 
> 2.35.1.894.gb6a874cedc-goog
>