linux-kernel - Re: [PATCH V2] libperf evlist: Fix per-thread mmaps for multi-threaded targets

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Date:   Tue, 6 Sep 2022 14:59:46 +0200
From:   Jiri Olsa <olsajiri@...il.com>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH V2] libperf evlist: Fix per-thread mmaps for
 multi-threaded targets

On Mon, Sep 05, 2022 at 02:42:09PM +0300, Adrian Hunter wrote:

SNIP

> diff --git a/tools/lib/perf/evlist.c b/tools/lib/perf/evlist.c
> index e6c98a6e3908..6b1bafe267a4 100644
> --- a/tools/lib/perf/evlist.c
> +++ b/tools/lib/perf/evlist.c
> @@ -486,6 +486,7 @@ mmap_per_evsel(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
>  			if (ops->idx)
>  				ops->idx(evlist, evsel, mp, idx);
>  
> +			pr_debug("idx %d: mmapping fd %d\n", idx, *output);
>  			if (ops->mmap(map, mp, *output, evlist_cpu) < 0)
>  				return -1;
>  
> @@ -494,6 +495,7 @@ mmap_per_evsel(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
>  			if (!idx)
>  				perf_evlist__set_mmap_first(evlist, map, overwrite);
>  		} else {
> +			pr_debug("idx %d: set output fd %d -> %d\n", idx, fd, *output);
>  			if (ioctl(fd, PERF_EVENT_IOC_SET_OUTPUT, *output) != 0)
>  				return -1;
>  
> @@ -519,6 +521,48 @@ mmap_per_evsel(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
>  	return 0;
>  }
>  
> +static int
> +mmap_per_thread(struct perf_evlist *evlist, struct perf_evlist_mmap_ops *ops,
> +		struct perf_mmap_param *mp)
> +{
> +	int nr_threads = perf_thread_map__nr(evlist->threads);
> +	int nr_cpus    = perf_cpu_map__nr(evlist->all_cpus);
> +	int cpu, thread, idx = 0;
> +	int nr_mmaps = 0;
> +
> +	pr_debug("%s: nr cpu values (may include -1) %d nr threads %d\n",
> +		 __func__, nr_cpus, nr_threads);

-1 as cpu value is only for 'empty' perf_cpu_map, right?

> +
> +	/* per-thread mmaps */
> +	for (thread = 0; thread < nr_threads; thread++, idx++) {
> +		int output = -1;
> +		int output_overwrite = -1;
> +
> +		if (mmap_per_evsel(evlist, ops, idx, mp, 0, thread, &output,
> +				   &output_overwrite, &nr_mmaps))
> +			goto out_unmap;
> +	}
> +
> +	/* system-wide mmaps i.e. per-cpu */
> +	for (cpu = 1; cpu < nr_cpus; cpu++, idx++) {
> +		int output = -1;
> +		int output_overwrite = -1;
> +
> +		if (mmap_per_evsel(evlist, ops, idx, mp, cpu, 0, &output,
> +				   &output_overwrite, &nr_mmaps))
> +			goto out_unmap;
> +	}

will this loop be executed? we are in here because all_cpus is empty, right?

thanks,
jirka

> +
> +	if (nr_mmaps != evlist->nr_mmaps)
> +		pr_err("Miscounted nr_mmaps %d vs %d\n", nr_mmaps, evlist->nr_mmaps);
> +
> +	return 0;
> +
> +out_unmap:
> +	perf_evlist__munmap(evlist);
> +	return -1;
> +}

SNIP