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perf session: Add comment for perf_session__register_idle_thread()

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Add a comment to perf_session__register_idle_thread() to bring attention to
a pitfall with the idle task thread structure. The pitfall is that there
should really be a 'struct thread' for the idle task of each cpu, but there
is only one that can have pid == tid == 0.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20181221120620.9659-9-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
Adrian Hunter 2018-12-21 14:06:20 +02:00 committed by Arnaldo Carvalho de Melo
parent 256d92bc93
commit b25756df5b
1 changed files with 7 additions and 0 deletions
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7
tools/perf/util/session.c
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@ -1527,6 +1527,13 @@ struct thread *perf_session__findnew(struct perf_session *session, pid_t pid)
return machine__findnew_thread(&session->machines.host, -1, pid);
}
/*
* Threads are identified by pid and tid, and the idle task has pid == tid == 0.
* So here a single thread is created for that, but actually there is a separate
* idle task per cpu, so there should be one 'struct thread' per cpu, but there
* is only 1. That causes problems for some tools, requiring workarounds. For
* example get_idle_thread() in builtin-sched.c, or thread_stack__per_cpu().
*/
int perf_session__register_idle_thread(struct perf_session *session)
{
struct thread *thread;