perf tools: Use /proc/<PID>/task/<TID>/status for PERF_RECORD_ event synthesis

To save memory usage, it needs to reduce the number of entries in the
proc filesystem.  It's using /proc/<PID>/task directory to traverse
threads in the process and then kernel creates /proc/<PID>/task/<TID>
entries.

After that it checks the thread info using the /proc/<TID>/status file
rather than /proc/<PID>/task/<TID>/status.  As far as I can see, they
are the same and contain all the info we need.

Using the latter eliminates the unnecessary /proc/<TID> entry.  This can
be useful especially a large number of threads are used in the system.
In my experiment around 1KB of memory on average was saved for each
thread (which is not a thread group leader).

To do this, pass both pid and tid to perf_event_prepare_comm() if it
knows them.  In case it doesn't know, passing 0 as pid will do the old
way.

Signed-off-by: Namhyung Kim <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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