On Mon, Aug 24, 2009 at 5:44 PM, stephane eranian<eran...@googlemail.com> wrote: > On Mon, Aug 24, 2009 at 3:46 PM, Peter Zijlstra<a.p.zijls...@chello.nl> wrote: >> On Thu, 2009-08-20 at 15:49 +0200, stephane eranian wrote: >>> Hi, >>> >>> I am running into an issue trying to use enable_on_exec >>> in per-thread mode with an event group. >>> >>> My understanding is that enable_on_exec allows activation >>> of an event on first exec. This is useful for tools monitoring >>> other tasks and which you invoke as: tool my_program. In >>> other words, the tool forks+execs my_program. This option >>> allows developers to setup the events after the fork (to get >>> the pid) but before the exec(). Only execution after the exec >>> is monitored. This alleviates the need to use the >>> ptrace(PTRACE_TRACEME) call. >>> >>> My understanding is that an event group is scheduled only >>> if all events in the group are active (disabled=0). Thus, one >>> trick to activate a group with a single ioctl(PERF_IOC_ENABLE) >>> is to enable all events in the group except the leader. This works >>> well. But once you add enable_on_exec on on the events, >>> things go wrong. The non-leader events start counting before >>> the exec. If the non-leader events are created in disabled state, >>> then they never activate on exec. >>> >>> The attached test program demonstrates the problem. >>> simply invoke with a program that runs for a few seconds. >> >> OK, lots of issues here >> >> 1) your code is broken ;-) > > That's true. I knew about the missing synchro. But I think > the problem existed nonetheless. > >> 2) enable_on_exec on !leader counters is undefined > > then fail it. > >> 3) there is something fishy non the less >> > True. > >> >> 1. you fork() then create a counter group in both the parent and the >> child without sync, then read the parent group. This obviously doesn't >> do what is expected. See attached proglet for a better version. >> > I have modified the program based on your changes. See new version attached. > >> 2. enable_on_exec only works on leaders, Paul, was that intended? >> > All events in a group are scheduled together. If one event is not enabled > in a group, then the group is not dispatched. Setting enable_on_exec > just on leader makes sense. Then to enable the group on exec, you > enabled all events but the leader. The enable_on_exec will enable > the leader on exec and the group will be ready for dispatch. That's > how it should work in my mind. > > > As you indicated the issue is with the timing information and I think > it is not related to enable_on_exec. It is more related to the fact > that to enable a group with a single ioctl() you enable ALL BUT the > leader. But that means that the time_enabled for the !leader is > ticking. Thus scaling won't be as expected yet it is correct > given what happens internally. > > I think there needs to be a distinction between 'enabled immediately > but cannot run because group is not totally enabled' and 'cannot run > because the group has been multiplexed out yet all could be dispatched > because all events were dispatched'. In the former, it seems you don't > want time_enabled to tick, while in the latter you do. In other words, > time_enabled ticks for each event if the group is 'dispatch-able' (or > runnable in your terminology) otherwise it does not. time_enabled reflects > the fact that the group could run but did not have access to the PMU > resource because of contention with other groups. > In other words, I think timing_enabled is measuring the wrong thing. It should be instead called time_runnable and it should measure the time during which the event is runnable, i.e, its group is runnable. That means the event (group) could be dispatched if PMU was "free".
------------------------------------------------------------------------------ Let Crystal Reports handle the reporting - Free Crystal Reports 2008 30-Day trial. Simplify your report design, integration and deployment - and focus on what you do best, core application coding. Discover what's new with Crystal Reports now. http://p.sf.net/sfu/bobj-july _______________________________________________ perfmon2-devel mailing list perfmon2-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/perfmon2-devel
