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

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Message-ID: <60b5c9bf-4ec9-957e-17dd-aa0a50411ff9@intel.com>
Date:   Tue, 6 Sep 2022 17:04:45 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Jiri Olsa <olsajiri@...il.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 6/09/22 15:59, Jiri Olsa wrote:
> 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?

The cpu map is a map of valid 3rd arguments to perf_event_open, so -1
means all CPUs which is per-thread by necessity.

> 
>> +
>> +	/* 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?

Yes it is executed.  I put back the code that was there before ae4f8ae16a07
("libperf evlist: Allow mixing per-thread and per-cpu mmaps"), which uses
perf_cpu_map__empty() which only checks the first entry is -1:

bool perf_cpu_map__empty(const struct perf_cpu_map *map)
{
	return map ? map->map[0].cpu == -1 : true;
}

But there can be more CPUs in the map, so perf_cpu_map__empty()
returns true for the per-thread case, as desired, even if there
are also system-wide CPUs.

I guess perf_cpu_map__empty() needs renaming.

> 
> 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