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

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Sep 06, 2022 at 05:04:45PM +0300, Adrian Hunter wrote:
> 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

hm, but commit ae4f8ae16a07 does not have similar cpu loop

> 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 don't see how, if I'd see -1 together with other cpu values in
perf_cpu_map I'd think it's a bug, but I might be missing some
auxtrace usage,

I thought we use -1 just for empty cpu map, so in per-thread case
-1 is properly passed to perf_event_open syscall

jirka

> 
> 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
>