Re: [PATCH v3] perf evsel: Enable ignore_missing_thread for pid option

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Dec 13, 2017 at 03:01:53PM +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.
> 
>         sys_perf_event_open(){
>            ...
>            if (group_fd != -1)
>                perf_fget_light()//to get corresponding group_leader by group_fd
>            ...
>            if (group_leader)
>               if (group_leader->ctx->task != ctx->task)//should on the same task
>                    goto err_context
>            ...
>         }
> 
> This patch also fixes this bug by introducing perf_evsel__remove_fd() and
> update_fds to allow removing fds for the missing thread.
> 
> Changes since v1:
> - Change group_fd__remove() into a more genetic way without changing code logic
> - Remove redundant condition
> 
> Changes since v2:
> - Use a proper function name and add some comment.
> - Multiline comment style fixes.

Acked-by: Jiri Olsa <jolsa@kernel.org>

thanks,
jirka