Re: [PATCH 1/1] perf tools: Fix thread_map event synthesizing in top and record

[Jeff Moyer <jmoyer@redhat.com>]

Arnaldo Carvalho de Melo <acme@infradead.org> writes:

> From: Arnaldo Carvalho de Melo <acme@redhat.com>
>
> Jeff Moyer reported these messages:
>
>   Warning:  ... trying to fall back to cpu-clock-ticks
>
> couldn't open /proc/-1/status
> couldn't open /proc/-1/maps
> [ls output]
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.008 MB perf.data (~363 samples) ]
>
> That lead me and David Ahern to see that something was fishy on the thread
> synthesizing routines, at least for the case where the workload is started
> from 'perf record', as -1 is the default for target_tid in 'perf record --tid'
> parameter, so somehow we were trying to synthesize the PERF_RECORD_MMAP and
> PERF_RECORD_COMM events for the thread -1, a bug.
>
> So I investigated this and noticed that when we introduced support for
> recording a process and its threads using --pid some bugs were introduced and
> that the way to fix it was to instead of passing the target_tid to the event
> synthesizing routines we should better pass the thread_map that has the list of
> threads for a --pid or just the single thread for a --tid.

Thanks for looking into this, acme.  Honestly, I can't for the life of
me figure out from your description how this problem would have affected
systems.  The fix makes the warnings go away, but I am still left with
the hung system I originally reported.

So, what does this patch actually fix?  And do you or others have any
interest in trying to work out why perf is hanging my system?

Thanks!
Jeff