Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754684AbaFPIwQ (ORCPT ); Mon, 16 Jun 2014 04:52:16 -0400 Received: from mail-we0-f169.google.com ([74.125.82.169]:48053 "EHLO mail-we0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754461AbaFPIwO (ORCPT ); Mon, 16 Jun 2014 04:52:14 -0400 Message-ID: <539EB03C.5080100@linaro.org> Date: Mon, 16 Jun 2014 10:52:12 +0200 From: Daniel Lezcano User-Agent: Mozilla/5.0 (X11; Linux i686; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 MIME-Version: 1.0 To: Doug Anderson CC: Kukjin Kim , Tomasz Figa , Vincent Guittot , Chirantan Ekbote , David Riley , Olof Johansson , linux-samsung-soc , Thomas Gleixner , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH 1/3] clocksource: exynos_mct: Fix ftrace References: <1401903034-20074-1-git-send-email-dianders@chromium.org> <539E0D8E.9080706@linaro.org> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/16/2014 06:40 AM, Doug Anderson wrote: > Daniel, > > On Sun, Jun 15, 2014 at 2:18 PM, Daniel Lezcano > wrote: >> On 06/04/2014 07:30 PM, Doug Anderson wrote: >>> >>> In (93bfb76 clocksource: exynos_mct: register sched_clock callback) we >>> supported using the MCT as a scheduler clock. We properly marked >>> exynos4_read_sched_clock() as notrace. However, we then went and >>> called another function that _wasn't_ notrace. That means if you do: >>> >>> cd /sys/kernel/debug/tracing/ >>> echo function_graph > current_tracer >>> >>> You'll get a crash. >>> >>> Fix this (but still let other readers of the MCT be trace-enabled) by >>> adding an extra function. It's important to keep other users of MCT >>> traceable because the MCT is actually quite slow. >> >> >> >> Hi Doug, >> >> could you elaborate ? I don't get the 'because the MCT ... slow' > > Sorry, I was trying to avoid duplication in the series and it's more > obvious when you look at parts 2 and 3 of the series. ;) > > Doing the math (please correct any miscalculations) using the numbers > from the other patches: You can see that the existing code takes > 1323852 us for 1000000 gettimeofday in userspace. The fastest > implementation (just shaving to a 32-bit timer) gets us as fast as > ~1000000 us for 1000000 gettimeofday in userspace. > > From profiling, I believe that gettimeofday from userspace is about > 50% overhead (system call, multiplication, copies, etc) and about 50% > MCT read. That means that the fastest you can possibly do an MCT read > is in .5us or 500ns. > > I believe an A15 has something like 1 or 2 cycles per instruction. If > it were 2 cycles per instruction, it can execute a normal instruction > on a 2GHz machine in .5ns. That means we can execute 1000 normal > instructions in the time it takes to do a since MCT access. > > ...so I guess that's what I'd call slow. ;) What do you think? I > know that the MCT read shows up in whole system profiles of > gettimeofday. Hi Dough, thanks for the explanation. I still don't get why it is important to keep others users of mct traceable because it is quite slow ? May be it is what you explained here, but I miss the connection between 'the other users' <-> 'traceable' <-> 'because slow'. -- Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog -- 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/