Re: [PATCH] perf evsel: Enable ignore_missing_thread for pid option

[Jiri Olsa]

On Tue, Dec 05, 2017 at 05:03:33PM +0800, Mengting Zhang wrote:
> While monitoring a multithread process with pid option, perf sometimes
> may return sys_perf_event_open failure with 3(No such process) if any
> of the process's threads die before we open the event. However, we want
> perf continue monitoring the remaining threads and do not exit with error.
> 
> Here, the patch enables perf_evsel::ignore_missing_thread for -p option
> to ignore complete failure if any of threads die before we open the event.
> But it may still return sys_perf_event_open failure with 22(Invalid) if we
> monitors several event groups.
> 
> 	sys_perf_event_open: pid 28960  cpu 40  group_fd 118202  flags 0x8
> 	sys_perf_event_open: pid 28961  cpu 40  group_fd 118203  flags 0x8
> 	WARNING: Ignored open failure for pid 28962
> 	sys_perf_event_open: pid 28962  cpu 40  group_fd [118203]  flags 0x8
> 	sys_perf_event_open failed, error -22
> 
> That is because when we ignore a missing thread, we change the thread_idx
> without dealing with its fds, FD(evsel, cpu, thread). Then get_group_fd()
> may return a wrong group_fd for the next thread and sys_perf_event_open()
> return with 22.

oops, nice catch

SNIP

> +static int group_fd__remove(struct perf_evsel *evsel,
> +			    int nr_cpus, int cpu_idx,
> +			    int nr_threads, int thread_idx)

please call this something more generic like update_fds,
I think it affects more stuff than just group_fds

> +{
> +	struct perf_evsel *pos;
> +	struct perf_evlist *evlist = evsel->evlist;
> +
> +	if (nr_cpus < 1 || nr_threads < 1)
> +		return -EINVAL;

we already have check for threads->nr == 1 in ignore_missing_thread
also not sure how possible is to get nr_cpus < 1, but ok

> +
> +	if (cpu_idx >= nr_cpus || thread_idx >= nr_threads)
> +		return -EINVAL;
> +
> +	evlist__for_each_entry(evlist, pos) {
> +		if (pos != evsel) {
> +			for (int cpu = 0; cpu < nr_cpus; cpu++)
> +				for (int thread = thread_idx; thread < nr_threads; thread++)
> +					FD(pos, cpu, thread) = FD(pos, cpu, thread + 1);
> +		}
> +		else {
> +			for (int cpu = 0; cpu < cpu_idx; cpu++)
> +				for (int thread = thread_idx; thread < nr_threads; thread++)
> +					FD(pos, cpu, thread) = FD(pos, cpu, thread + 1);
> +			break;
> +		}
> +	}

could you please put this into some generic function, like:

	void perf_evsel__remove_thread(evsel, nr_cpus, nr_threads, int thread_idx)
	{
		for (int cpu = 0; cpu < nr_cpus; cpu++)
			for (int thread = thread_idx; thread < nr_threads; thread++)
				FD(pos, cpu, thread) = FD(pos, cpu, thread + 1);
	}


with the loop would be like:

	evlist__for_each_entry(evlist, pos) {
		int nr_cpus = pos != evsel ? nr_cpus : cpu_idx;

		perf_evsel__remove_thread(evsel, nr_cpus, nr_threads, thread_idx)
	}

or something along those lines...


thanks for catching this

jirka