From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752258Ab2KJCFM (ORCPT ); Fri, 9 Nov 2012 21:05:12 -0500 Received: from e5.ny.us.ibm.com ([32.97.182.145]:49915 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751166Ab2KJCFH (ORCPT ); Fri, 9 Nov 2012 21:05:07 -0500 Message-ID: <509DB632.7070305@linaro.org> Date: Fri, 09 Nov 2012 18:04:34 -0800 From: John Stultz User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:16.0) Gecko/20121011 Thunderbird/16.0.1 MIME-Version: 1.0 To: Peter Zijlstra CC: Stephane Eranian , LKML , "mingo@elte.hu" , Paul Mackerras , Anton Blanchard , Will Deacon , "ak@linux.intel.com" , Pekka Enberg , Steven Rostedt , Robert Richter , tglx Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples References: <1350408232.2336.42.camel@laptop> In-Reply-To: <1350408232.2336.42.camel@laptop> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12111002-5930-0000-0000-00000E00BB06 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/16/2012 10:23 AM, Peter Zijlstra wrote: > On Tue, 2012-10-16 at 12:13 +0200, Stephane Eranian wrote: >> Hi, >> >> There are many situations where we want to correlate events happening at >> the user level with samples recorded in the perf_event kernel sampling buffer. >> For instance, we might want to correlate the call to a function or creation of >> a file with samples. Similarly, when we want to monitor a JVM with jitted code, >> we need to be able to correlate jitted code mappings with perf event samples >> for symbolization. >> >> Perf_events allows timestamping of samples with PERF_SAMPLE_TIME. >> That causes each PERF_RECORD_SAMPLE to include a timestamp >> generated by calling the local_clock() -> sched_clock_cpu() function. >> >> To make correlating user vs. kernel samples easy, we would need to >> access that sched_clock() functionality. However, none of the existing >> clock calls permit this at this point. They all return timestamps which are >> not using the same source and/or offset as sched_clock. >> >> I believe a similar issue exists with the ftrace subsystem. >> >> The problem needs to be adressed in a portable manner. Solutions >> based on reading TSC for the user level to reconstruct sched_clock() >> don't seem appropriate to me. >> >> One possibility to address this limitation would be to extend clock_gettime() >> with a new clock time, e.g., CLOCK_PERF. >> >> However, I understand that sched_clock_cpu() provides ordering guarantees only >> when invoked on the same CPU repeatedly, i.e., it's not globally synchronized. >> But we already have to deal with this problem when merging samples obtained >> from different CPU sampling buffer in per-thread mode. So this is not >> necessarily >> a showstopper. >> >> Alternatives could be to use uprobes but that's less practical to setup. >> >> Anyone with better ideas? > You forgot to CC the time people ;-) > > I've no problem with adding CLOCK_PERF (or another/better name). Hrm. I'm not excited about exporting that sort of internal kernel details to userland. The behavior and expectations from sched_clock() has changed over the years, so I'm not sure its wise to export it, since we'd have to preserve its behavior from then on. Also I worry that it will be abused in the same way that direct TSC access is, where the seemingly better performance from the more careful/correct CLOCK_MONOTONIC would cause developers to write fragile userland code that will break when moved from one machine to the next. I'd probably rather perf output timestamps to userland using sane clocks (CLOCK_MONOTONIC), rather then trying to introduce a new time domain to userland. But I probably could be convinced I'm wrong. thanks -john