From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnaldo Carvalho de Melo Subject: [PATCH 29/29] perf session: Add comment for perf_session__register_idle_thread() Date: Thu, 3 Jan 2019 09:46:09 -0300 Message-ID: <20190103124609.29672-30-acme@kernel.org> References: <20190103124609.29672-1-acme@kernel.org> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <20190103124609.29672-1-acme@kernel.org> Sender: linux-kernel-owner@vger.kernel.org To: Ingo Molnar Cc: Clark Williams , linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, Adrian Hunter , Jiri Olsa , Arnaldo Carvalho de Melo List-Id: linux-perf-users.vger.kernel.org From: Adrian Hunter 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 Acked-by: Jiri Olsa Link: http://lkml.kernel.org/r/20181221120620.9659-9-adrian.hunter@intel.com Signed-off-by: Arnaldo Carvalho de Melo --- tools/perf/util/session.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c index 78a067777144..5456c84c7dd1 100644 --- a/tools/perf/util/session.c +++ b/tools/perf/util/session.c @@ -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; -- 2.19.2