From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754302AbbGUKaI (ORCPT ); Tue, 21 Jul 2015 06:30:08 -0400 Received: from casper.infradead.org ([85.118.1.10]:40734 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753732AbbGUKaF (ORCPT ); Tue, 21 Jul 2015 06:30:05 -0400 Date: Tue, 21 Jul 2015 12:29:55 +0200 From: Peter Zijlstra To: Adrian Hunter Cc: Andi Kleen , Arnaldo Carvalho de Melo , Ingo Molnar , linux-kernel@vger.kernel.org, Jiri Olsa , Stephane Eranian , mathieu.poirier@linaro.org, Pawel Moll Subject: Re: [PATCH V4 1/5] perf: Add PERF_RECORD_SWITCH to indicate context switches Message-ID: <20150721102955.GM19282@twins.programming.kicks-ass.net> References: <1437471846-26995-1-git-send-email-adrian.hunter@intel.com> <1437471846-26995-2-git-send-email-adrian.hunter@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1437471846-26995-2-git-send-email-adrian.hunter@intel.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 21, 2015 at 12:44:02PM +0300, Adrian Hunter wrote: > There are already two events for context switches, namely > the tracepoint sched:sched_switch and the software event > context_switches. Unfortunately neither are suitable for > use by non-privileged users for the purpose of synchronizing > hardware trace data (e.g. Intel PT) to the context switch. > > Tracepoints are no good at all for non-privileged users > because they need either CAP_SYS_ADMIN or > /proc/sys/kernel/perf_event_paranoid <= -1. > > On the other hand, kernel software events need either > CAP_SYS_ADMIN or /proc/sys/kernel/perf_event_paranoid <= 1. > > Now many distributions do default perf_event_paranoid to 1 > making context_switches a contender, except it has another > problem (which is also shared with sched:sched_switch) > which is that it happens before perf schedules events out > instead of after perf schedules events in. Whereas a > privileged user can see all the events anyway, a > non-privileged user only sees events for their own processes, > in other words they see when their process was scheduled out > not when it was scheduled in. That presents two problems to > use the event: 1. the information comes too late, so tools > have to look ahead in the event stream to find out what the > current state is 2. if they are unlucky tracing might have > stopped before the context-switches event is recorded. > > This new PERF_RECORD_SWITCH event does not have those problems > and it also has a couple of other small advantages. It is > easier to use because it is an auxiliary event (like mmap, > comm and task events) which can be enabled by setting a single > bit. It is smaller than sched:sched_switch and easier to parse. > > To make the event useful for privileged users also, if the > context is cpu-wide then the event record will be > PERF_RECORD_SWITCH_CPU_WIDE which is the same as > PERF_RECORD_SWITCH except it also provides the next or > previous pid/tid. > > Signed-off-by: Adrian Hunter Acme, if you agree with the tools part, feel free to take the lot. Acked-by: Peter Zijlstra (Intel)