Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753111AbaFPEkn (ORCPT ); Mon, 16 Jun 2014 00:40:43 -0400 Received: from mail-ve0-f177.google.com ([209.85.128.177]:59816 "EHLO mail-ve0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751833AbaFPEkk (ORCPT ); Mon, 16 Jun 2014 00:40:40 -0400 MIME-Version: 1.0 In-Reply-To: <539E0D8E.9080706@linaro.org> References: <1401903034-20074-1-git-send-email-dianders@chromium.org> <539E0D8E.9080706@linaro.org> Date: Sun, 15 Jun 2014 21:40:39 -0700 X-Google-Sender-Auth: ps-JKqSI2JeoBLw5H77fSd01zig Message-ID: Subject: Re: [PATCH 1/3] clocksource: exynos_mct: Fix ftrace From: Doug Anderson To: Daniel Lezcano 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" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -Doug -- 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/