Patch "perf tools: Make perf_event__synthesize_mmap_events() scale" has been added to the 3.18-stable tree

This is a note to let you know that I've just added the patch titled

perf tools: Make perf_event__synthesize_mmap_events() scale

to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
perf-tools-make-perf_event__synthesize_mmap_events-scale.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Stephane Eranian <[email protected]>
Date: Wed, 15 Mar 2017 10:17:13 -0700
Subject: perf tools: Make perf_event__synthesize_mmap_events() scale

From: Stephane Eranian <[email protected]>

[ 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 you 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, therefore we can work around the issue by using /proc/pid/tasks/pid/maps. This entry does 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 limit.

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").

---
 tools/perf/util/event.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -167,8 +167,8 @@ int perf_event__synthesize_mmap_events(s
 	if (machine__is_default_guest(machine))
 		return 0;
 
-	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);
 
 	fp = fopen(filename, "r");
 	if (fp == NULL) {

Patches currently in stable-queue which might be from [email protected] are

queue-3.18/perf-session-don-t-rely-on-evlist-in-pipe-mode.patch queue-3.18/perf-tools-make-perf_event__synthesize_mmap_events-scale.patch queue-3.18/perf-inject-copy-events-when-reordering-events-in-pipe-mode.patch