From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753223Ab3LSLOQ (ORCPT ); Thu, 19 Dec 2013 06:14:16 -0500 Received: from mga09.intel.com ([134.134.136.24]:17364 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751537Ab3LSLOO (ORCPT ); Thu, 19 Dec 2013 06:14:14 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.95,512,1384329600"; d="scan'208";a="427153903" From: Alexander Shishkin To: Peter Zijlstra Cc: Arnaldo Carvalho de Melo , Ingo Molnar , linux-kernel@vger.kernel.org, David Ahern , Frederic Weisbecker , Jiri Olsa , Mike Galbraith , Namhyung Kim , Paul Mackerras , Stephane Eranian , Andi Kleen Subject: Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow tracing units In-Reply-To: <20131219102625.GC30183@twins.programming.kicks-ass.net> References: <1386765443-26966-1-git-send-email-alexander.shishkin@linux.intel.com> <1386765443-26966-5-git-send-email-alexander.shishkin@linux.intel.com> <20131217161126.GL13532@twins.programming.kicks-ass.net> <8761qmthr6.fsf@ashishki-desk.ger.corp.intel.com> <20131218133439.GR21999@twins.programming.kicks-ass.net> <8738lqtg0v.fsf@ashishki-desk.ger.corp.intel.com> <20131218141125.GT21999@twins.programming.kicks-ass.net> <87zjnys0gj.fsf@ashishki-desk.ger.corp.intel.com> <20131218150900.GU21999@twins.programming.kicks-ass.net> <87wqj1s2d3.fsf@ashishki-desk.ger.corp.intel.com> <20131219102625.GC30183@twins.programming.kicks-ass.net> User-Agent: Notmuch/0.15.2+182~gd0bd88f (http://notmuchmail.org) Emacs/23.4.1 (x86_64-pc-linux-gnu) Date: Thu, 19 Dec 2013 13:14:09 +0200 Message-ID: <87r499rt32.fsf@ashishki-desk.ger.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Peter Zijlstra writes: > On Thu, Dec 19, 2013 at 09:53:44AM +0200, Alexander Shishkin wrote: >> Peter Zijlstra writes: >> > The thing is; why can't you zero-copy whatever buffer the hardware >> > writes into, into the normal buffer? >> >> I'm not sure I understand. You mean, have the buffer split between perf >> data and trace data? > > Yep, I don't see any reason why this wouldn't work. > > When the hardware thing sends an interrupt to notify us its buffer is > 'full', stop the recorder, try to create a single record in the buffer > that's big enough + 1 page, then swizzle the hardware pages and the > buffer pages for that record, using the +1 page to page align the actual > data. Then (re)start the hardware on the 'new' pages. We configure the hardware thing to send an interrupt *before* the buffer is full, keep the recorder running while userspace saves stuff to perf.data file. Recording only stops if perf fails to read the trace data out fast enough and the buffer fills up. So you'd have a complete trace. Also, we have what we call a "snapshot" mode, where we keep the hardware thing running, writing data to a circular buffer till it's stopped, in case we're only interested in the most recent trace data to see what it is that takes too long to respond, etc. And while it is running, we're getting new records in the perf stream all the time (mmaps, etc). Put simple: perf data and trace data are two different separate types of information that originate from two different sources, can exist and make sense separately from one another and should not be mixed. Regards, -- Alex