From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765021AbZEHAoX (ORCPT ); Thu, 7 May 2009 20:44:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1762984AbZEHAa7 (ORCPT ); Thu, 7 May 2009 20:30:59 -0400 Received: from e5.ny.us.ibm.com ([32.97.182.145]:55360 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759930AbZEHAa6 (ORCPT ); Thu, 7 May 2009 20:30:58 -0400 Subject: [RFC][PATCH] tsc_khz= boot option to avoid TSC calibration variance From: john stultz To: George Spelvin Cc: ulrich.windl@rz.uni-regensburg.de, linux-kernel@vger.kernel.org, tglx@linutronix.de, Clark Williams , zippel@linux-m68k.org, Ingo Molnar Content-Type: text/plain Date: Thu, 07 May 2009 17:30:47 -0700 Message-Id: <1241742647.7518.108.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.24.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org All, Despite recent tweaking, TSC calibration variance is still biting users who care about keeping close sync with NTP servers over reboots. Here's a recent example: http://lkml.indiana.edu/hypermail/linux/kernel/0905.0/02061.html The problem is, each reboot, we have to calibrate the TSC, and any error, regardless of how small, in the calibrated freq has to be corrected for by NTP. Assuming the error is within 500ppm NTP can correct this, but until it finds the proper correction value for the new TSC freq, users may see time offsets from the NTP server. In my experience, its fairly easy to see 100khz variance from reboot to reboot with 2.6.30-rc. While I think its worth trying to improve the calibration further, there will likely be a trade-off between very accurate calibration and fast boot times. To mitigate this, I wanted to provide a tsc_khz= boot option. This would allow users to set the tsc_khz value at boot-up, assuming they are within 1Mhz of the calibrated value (to protect against bad values). Once the tsc_khz value is set in grub, the box will always boot with the same value, so the NTP drift value prior to reboot will still be correct after rebooting. Thanks to George Spelvin for the idea: http://lkml.indiana.edu/hypermail/linux/kernel/0905.0/02807.html Thoughts or feedback? Signed-off-by: John Stultz diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index 90b3924..1f9ee19 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -2396,6 +2396,13 @@ and is between 256 and 4096 characters. It is defined in the file Used to enable high-resolution timer mode on older hardware, and in virtualized environment. + tsc_khz= [x86] Set the TSC freq value. + Format: + Used to avoid TSC calibration error. + The specified tsc_khz value must be accurate to the + calibrated tsc value by 1MHz. Otherwise the calibrated + value will be used. + turbografx.map[2|3]= [HW,JOY] TurboGraFX parallel port interface Format: diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index d57de05..93fc4c0 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -825,6 +825,16 @@ static void __init init_tsc_clocksource(void) clocksource_register(&clocksource_tsc); } +unsigned long tsc_khz_specified; +static int __init tsc_khz_specified_setup(char *str) +{ + tsc_khz_specified = simple_strtoul(str, NULL, 0); + return 1; +} + +__setup("tsc_khz=", tsc_khz_specified_setup); + + void __init tsc_init(void) { u64 lpj; @@ -834,6 +844,20 @@ void __init tsc_init(void) return; tsc_khz = calibrate_tsc(); + + /* + * If the calibrated TSC freq and user specified + * TSC freq are close enough, pick the what the + * user told us. + */ + if (tsc_khz_specified && ((tsc_khz+(tsc_khz/2))/1000 == + (tsc_khz_specified+(tsc_khz_specified/2))/1000)) { + printk(KERN_INFO "Using user defined TSC freq: %lu.%03lu MHz\n", + tsc_khz_specified/1000, + tsc_khz_specified%1000); + tsc_khz = tsc_khz_specified; + } + cpu_khz = tsc_khz; if (!tsc_khz) {