Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762924Ab3DCRvF (ORCPT ); Wed, 3 Apr 2013 13:51:05 -0400 Received: from mail-pa0-f43.google.com ([209.85.220.43]:62535 "EHLO mail-pa0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762748Ab3DCRvD (ORCPT ); Wed, 3 Apr 2013 13:51:03 -0400 Message-ID: <515C6C01.9070905@linaro.org> Date: Wed, 03 Apr 2013 10:50:57 -0700 From: John Stultz User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130308 Thunderbird/17.0.4 MIME-Version: 1.0 To: Pawel Moll CC: Peter Zijlstra , David Ahern , Stephane Eranian , Thomas Gleixner , LKML , "mingo@elte.hu" , Paul Mackerras , Anton Blanchard , Will Deacon , "ak@linux.intel.com" , Pekka Enberg , Steven Rostedt , Robert Richter , Richard Cochran Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples References: <1350408232.2336.42.camel@laptop> <1359728280.8360.15.camel@hornet> <51118797.9080800@linaro.org> <5123C3AF.8060100@linaro.org> <1361356160.10155.22.camel@laptop> <51285BF1.2090208@linaro.org> <1361801441.4007.40.camel@laptop> <1363291021.3100.144.camel@hornet> <51586315.7080006@gmail.com> <5159D221.70304@linaro.org> <1364889256.16858.1.camel@laptop> <515B0502.8070408@linaro.org> <1365009558.26858.19.camel@hornet> <515C66FE.7030501@linaro.org> <1365010502.26858.32.camel@hornet> In-Reply-To: <1365010502.26858.32.camel@hornet> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2271 Lines: 57 On 04/03/2013 10:35 AM, Pawel Moll wrote: > On Wed, 2013-04-03 at 18:29 +0100, John Stultz wrote: >> On 04/03/2013 10:19 AM, Pawel Moll wrote: >>> On Tue, 2013-04-02 at 17:19 +0100, John Stultz wrote: >>>> But if we're going to have to do >>>> this via a clockid, I'm going to want it to be done via a dynamic posix >>>> clockid, so its clear its tightly tied with perf and not considered a >>>> generic interface (and I can clearly point folks having problems to the >>>> perf maintainers ;). >>> Ok, so how about the code below? >>> >>> There are two distinct parts of the "solution": >>> >>> 1. The dynamic posix clock, as you suggested. Then one can get the perf >>> timestamp by doing: >>> >>> clock_fd = open("/dev/perf-clock", O_RDONLY); >>> clock_gettime(FD_TO_CLOCKID(clock_fd), &ts) >>> >>> 2. A sort-of-hack in the get_posix_clock() function making it possible >>> to do the same using the perf event file descriptor, eg.: >>> >>> fd = sys_perf_event_open(&attr, -1, 0, -1, 0); >>> clock_gettime(FD_TO_CLOCKID(fd), &ts) >> #2 makes my nose wrinkle. > To make myself clear: I consider the code as it is a hack. > >> Forgive me for being somewhat ignorant on the >> perf interfaces, but why is the second portion necessary or beneficial? > My thinking: the perf syscall returns a file descriptor already, so it > would make sense to re-use it in the clock_gettime() call instead of > jumping through loops to open a character device file, which may not > exist at all (eg. no udev) or may be placed or named in a random way > (eg. some local udev rule). > > I'm open for different opinions :-) Cc'ing Richard for his thoughts here. I get the reasoning around reusing the fd we already have, but is the possibility of a dynamic chardev pathname really a big concern? I'm guessing the private_data on the perf file is already used? Maybe can we extend the dynamic posix clock code to work on more then just the chardev? Although I worry about multiplexing too much functionality on the file. thanks -john -- 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/