On Thu, Mar 29, 2018 at 05:13:56PM +0100, Ben Hutchings wrote: > On Mon, 2018-03-19 at 19:04 +0100, Greg Kroah-Hartman wrote: > > 4.4-stable review patch. If anyone has any objections, please let me know. > > > > ------------------ > > > > From: Stephane Eranian <eran...@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 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"). > [...] > > I don't think so. It looks like this was fixed by commit 65376df58217 > ("proc: revert /proc/<pid>/maps [stack:TID] annotation") which we > already have in 4.4-stable. But older branches (3.16, 3.18, 4.1) don't > have that and probably should do.
Now added to 3.18.y > It looks like commit b18cb64ead40 ("fs/proc: Stop trying to report > thread stacks") is also a candidate for stable. Now added to 3.18.y and 4.4.y, thanks! greg k-h