Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757059Ab1F1LHi (ORCPT ); Tue, 28 Jun 2011 07:07:38 -0400 Received: from merlin.infradead.org ([205.233.59.134]:50356 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755727Ab1F1LGK convert rfc822-to-8bit (ORCPT ); Tue, 28 Jun 2011 07:06:10 -0400 Subject: Re: [tip:perf/core] perf: Ignore non-sampling overflows From: Peter Zijlstra To: Robert Richter Cc: "linux-tip-commits@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "hpa@zytor.com" , "mingo@redhat.com" , "francis.moro@gmail.com" , "tglx@linutronix.de" , "mingo@elte.hu" In-Reply-To: <20110628105335.GA17199@erda.amd.com> References:

<20110628105335.GA17199@erda.amd.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Tue, 28 Jun 2011 13:05:05 +0200 Message-ID: <1309259105.6701.210.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1879 Lines: 45 On Tue, 2011-06-28 at 12:53 +0200, Robert Richter wrote: > > --- a/kernel/perf_event.c > > +++ b/kernel/perf_event.c > > @@ -4240,6 +4240,13 @@ static int __perf_event_overflow(struct perf_event *event, int nmi, > > struct hw_perf_event *hwc = &event->hw; > > int ret = 0; > > > > + /* > > + * Non-sampling counters might still use the PMI to fold short > > + * hardware counters, ignore those. > > + */ > > + if (unlikely(!is_sampling_event(event))) > > + return 0; > > + > do you remember the background of this change. This check silently > drops data of non-sampling events. I want to use perf_event_overflow() > to write to the buffer and want to modify the check, but don't see > which 'accidentally' interrupts may occur that must be ignored. IIRC this is because we always program the interrupt bit, such that when the counter overflows we can account and reprogram the thing. This is needed because no hardware counter is in fact 64 bits wide. Therefore we have to program the counter to its max width and properly account the state and reprogram on overflow. Imagine a 32bit cycle counter (@1GHz), if we were not to program that as taking interrupts and nobody would read that counter for about 4.2 seconds, we'd have overflowed and lost the actual count value for the thing. So what we do is program is at 31bits (so that the msb can toggle and trigger the interrupt), and on interrupt add to event->count, and reset the hardware to start counting again. Now some arch/*/perf_event.c implementations unconditionally called perf_event_overflow() from their IRQ handler, even for such non-sampling counters. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/