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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
CC:     Stephane Eranian <eranian@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.9 028/219] perf tools: Make
 perf_event__synthesize_mmap_events() scale
Thread-Topic: [PATCH AUTOSEL for 4.9 028/219] perf tools: Make
 perf_event__synthesize_mmap_events() scale
Thread-Index: AQHTsz7qFH7IlbHWoE2wAhj8JY6qlg==
Date:   Sat, 3 Mar 2018 22:28:12 +0000
Message-ID: <20180303222716.26640-28-alexander.levin@microsoft.com>
References: <20180303222716.26640-1-alexander.levin@microsoft.com>
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From: Stephane Eranian <eranian@google.com>

[ Upstream commit 88b897a30c525c2eee6e7f16e1e8d0f18830845e ]

This patch significantly improves the execution time of
perf_event__synthesize_mmap_events() when running perf record on systems
where processes have lots of threads.

It just happens that cat /proc/pid/maps support uses a O(N^2) algorithm to
generate each map line in the maps file.  If you have 1000 threads, then yo=
u
have necessarily 1000 stacks.  For each vma, you need to check if it
corresponds to a thread's stack.  With a large number of threads, this can =
take
a very long time. I have seen latencies >> 10mn.

As of today, perf does not use the fact that a mapping is a stack, therefor=
e we
can work around the issue by using /proc/pid/tasks/pid/maps.  This entry do=
es
not try to map a vma to stack and is thus much faster with no loss of
functonality.

The proc-map-timeout logic is kept in case users still want some upper limi=
t.

In V2, we fix the file path from /proc/pid/tasks/pid/maps to actual
/proc/pid/task/pid/maps, tasks -> task.  Thanks Arnaldo for catching this.

Committer note:

This problem seems to have been elliminated in the kernel since commit :
b18cb64ead40 ("fs/proc: Stop trying to report thread stacks").

Signed-off-by: Stephane Eranian <eranian@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170315135059.GC2177@redhat.com
Link: http://lkml.kernel.org/r/1489598233-25586-1-git-send-email-eranian@go=
ogle.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 tools/perf/util/event.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
index 8ab0d7da956b..663192395780 100644
--- a/tools/perf/util/event.c
+++ b/tools/perf/util/event.c
@@ -255,8 +255,8 @@ int perf_event__synthesize_mmap_events(struct perf_tool=
 *tool,
 	if (machine__is_default_guest(machine))
 		return 0;
=20
-	snprintf(filename, sizeof(filename), "%s/proc/%d/maps",
-		 machine->root_dir, pid);
+	snprintf(filename, sizeof(filename), "%s/proc/%d/task/%d/maps",
+		 machine->root_dir, pid, pid);
=20
 	fp =3D fopen(filename, "r");
 	if (fp =3D=3D NULL) {
--=20
2.14.1