From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755093AbbAZNvk (ORCPT ); Mon, 26 Jan 2015 08:51:40 -0500 Received: from pandora.arm.linux.org.uk ([78.32.30.218]:54962 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751244AbbAZNvh (ORCPT ); Mon, 26 Jan 2015 08:51:37 -0500 Date: Mon, 26 Jan 2015 13:51:23 +0000 From: Russell King - ARM Linux To: Will Deacon Cc: Felipe Balbi , Peter Zijlstra , Tony Lindgren , Linux Kernel Mailing List , Arnaldo Carvalho de Melo , Ingo Molnar , Paul Mackerras , Linux OMAP Mailing List , Linux ARM Kernel Mailing List Subject: Re: perf not capturing stack traces Message-ID: <20150126135123.GC11502@n2100.arm.linux.org.uk> References: <20150123195128.GE26557@saruman.tx.rr.com> <20150123205959.GD3073@kernel.org> <20150123223745.GJ26557@saruman.tx.rr.com> <20150124151204.GF3073@kernel.org> <20150124222342.GA26941@saruman.tx.rr.com> <20150125155651.GB11502@n2100.arm.linux.org.uk> <20150126102711.GC15598@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150126102711.GC15598@arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 26, 2015 at 10:27:11AM +0000, Will Deacon wrote: > On Sun, Jan 25, 2015 at 03:56:52PM +0000, Russell King - ARM Linux wrote: > > On Sat, Jan 24, 2015 at 04:23:42PM -0600, Felipe Balbi wrote: > > > yeah, I'll try a few older kernels, also see if I can reproduce on other > > > boards. > > > > Perf works for me with CONFIG_FRAME_POINTER=y, but that's only for kernel > > space, and for userspace where the programs have been built for ARM mode > > with frame pointers. > > > > The kernel may work without CONFIG_FRAME_POINTER set, but I've never > > tested that, and I'd suggest that (given my experience looking at oops > > dumps) it's not all that reliable. > > > > Lastly, userspace without frame pointers is pretty much hopeless. > > FWIW, perf can now use libunwind for unwinding the userspace side of > things, so it's not quite as bad as it used to be. For the kernel side, > if the unwinder isn't working properly it would be nice to know *why*, > but I agree that it tends to be far flakier than the frame-pointer method. I don't see how userspace could be unwound without capturing the entire userspace stack on every perf event - and that could be a considerable size. We have no way to know within the kernel which words on the userspace stack are part of the callchain and which aren't - the only way we'd know is by loading the userspace's unwind tables, having the kernel parsing them and generate a list of functions. -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net.