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
