On Wed, May 28, 2014 at 7:25 PM, Andi Kleen <a...@linux.intel.com> wrote: > On Wed, May 28, 2014 at 07:05:31PM +0200, Peter Zijlstra wrote: >> On Wed, May 28, 2014 at 09:08:47AM -0700, Andi Kleen wrote: >> > On Wed, May 28, 2014 at 05:35:48PM +0200, Peter Zijlstra wrote: >> > > On Wed, May 28, 2014 at 07:58:31AM -0700, Andi Kleen wrote: >> >> > > > When a PEBS counter overflows it clears its respective GLOBAL_STATUS >> > > > bit >> > > > automatically. >> > > >> > > That's an ambiguous statement; did you mean to say a PEBS enabled >> > > counter will not raise its bit in GLOBAL_STATUS on counter overflow? >> > >> > Let's revisit how PEBS works: >> > >> > - The counter overflows and sets the GLOBAL_STATUS bit >> > - The PEBS assist is armed >> > - The counter triggers again >> > - The PEBS assist fires and delivers a PEBS record >> > - Finally it clears the GLOBAL_STATUS >> > - When the threshold is reached it raises an PMI >> > >> > So the GLOBAL_STATUS bit is visible between the first overflow and the end >> > of the PEBS record delivery. >> >> OK, so that's something entirely different from what you initially said, >> but it is what I thought it did -- you said that it clears on overflow >> but it clears after recording. > > Fair enough. I should have said PEBS assist. > >> If we get the PMI (where denoted) we can actually reconstruct which >> event triggered, by looking at which bit(s) flipped between the recorded >> state and the current state (due to E coming before F) > > Normally when the PMI PEBS handler runs the GLOBAL_STATUS is already cleared > (as the PEBS assist will execute concurrently during the NMI entry) > Looking at the status won't help you much, it only has the PEBS bit > set. > > I don't think we need to do anything. It's a very unlikely situation > in normal operation, as the counter period is very long compared > to the race window. When it happens very rarely we can ignore it. > > It can happen mainly when you program two counters to count exactly > the same thing with the same threshold, but why would you do that? > That's a plausible scenario if you consider two distinct sessions of a tool, i.e., two users running perf top or perf record some precise events.
> I guess what would make sense is to add some debug counter somewhere > for this situation (more than one bit set) > > -Andi -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
