From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756624Ab0JSRKD (ORCPT ); Tue, 19 Oct 2010 13:10:03 -0400 Received: from canuck.infradead.org ([134.117.69.58]:57702 "EHLO canuck.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754071Ab0JSRKB convert rfc822-to-8bit (ORCPT ); Tue, 19 Oct 2010 13:10:01 -0400 Subject: Re: [PATCH] perf_events: fix time tracking in samples From: Peter Zijlstra To: Stephane Eranian Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, paulus@samba.org, davem@davemloft.net, fweisbec@gmail.com, perfmon2-devel@lists.sf.net, eranian@gmail.com, robert.richter@amd.com In-Reply-To: References: <4cbdcbea.8491d80a.25b0.ffffece8@mx.google.com> <1287507178.1998.3440.camel@laptop> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Tue, 19 Oct 2010 19:09:47 +0200 Message-ID: <1287508187.1998.3445.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2010-10-19 at 19:01 +0200, Stephane Eranian wrote: > On Tue, Oct 19, 2010 at 6:52 PM, Peter Zijlstra wrote: > > On Tue, 2010-10-19 at 18:47 +0200, Stephane Eranian wrote: > >> This patch corrects time tracking in samples. Without this patch > >> both time_enabled and time_running may be reported as zero when > >> user asks for PERF_SAMPLE_READ. > >> > >> You use PERF_SAMPLE_READ when you want to sample the values of > >> other counters in each sample. Because of multiplexing, it is > >> necessary to know both time_enable, time_running to be able > >> to scale counts correctly. > >> > >> We defer updating timing until we know it is really needed, i.e., > >> only when we have PERF_SAMPLE_READ. > >> > >> With this patch, the libpfm4 example task_smpl now reports > >> correct counts (shown on 2.4GHz Core 2): > >> > >> $ task_smpl -p 2400000000 -e unhalted_core_cycles:u,instructions_retired:u,baclears noploop 5 > >> noploop for 5 seconds > >> IIP:0x000000004006d6 PID:5596 TID:5596 TIME:466,210,211,430 STREAM_ID:33 PERIOD:2,400,000,000 ENA=1,010,157,814 RUN=1,010,157,814 NR=3 > >> 2,400,000,254 unhalted_core_cycles:u (33) > >> 2,399,273,744 instructions_retired:u (34) > >> 53,340 baclears (35) > >> > >> Signed-off-by: Stephane Eranian > >> > >> --- > >> > >> diff --git a/kernel/perf_event.c b/kernel/perf_event.c > >> index f309e80..04611dd 100644 > >> --- a/kernel/perf_event.c > >> +++ b/kernel/perf_event.c > >> @@ -3494,6 +3494,9 @@ static void perf_output_read_group(struct perf_output_handle *handle, > >> static void perf_output_read(struct perf_output_handle *handle, > >> struct perf_event *event) > >> { > >> + update_context_time(event->ctx); > >> + update_event_times(event); > >> + > >> if (event->attr.read_format & PERF_FORMAT_GROUP) > >> perf_output_read_group(handle, event); > >> else > > > > > > Right, except that this can actually corrupt the time measurements... :/ > > > > Usually context times are updated under ctx->lock, and this is called > > from NMI context, which can interrupt ctx->lock.. > > > Ok, I missed that. But I don't understand why you need the lock to > udpate the time. The lower-level clock is lockless if I recall. Can't you > use an atomic ops in update_context_time()? atomic ops would slow down those code paths, also, I don't think you can fully get the ordering between ->tstamp_$foo and ->total_time_$foo just right. > > I was thinking about updating a local copy of the times, in that case > > you can only get funny times from samples, but it won't corrupt the > > actual running data. > > > You want time to be correct in every sample How would you detect > bogus timing? Not sure, but barring 64bit atomics for all these, 32bit archs and NMI are going to be 'interesting'