Messages in this thread Patch in this message | | | Subject | Re: [PATCH] tsc_khz= boot option to avoid TSC calibration variance | From | john stultz <> | Date | Tue, 12 May 2009 16:31:47 -0700 |
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On Tue, 2009-05-12 at 17:20 +0400, Serge Belyshev wrote: > Please *please* don't set arbitrary limits. Just use user supplied value.
Ok, fair enough. As I was not trying to deal with incorrect calibration results, just calibration variance, I was hoping to avoid dealing with error reports where users pushed the tsc_khz value outside of a sane range.
But I guess throwing a warning when its outside of a sane range is better then ignoring user defined boot options.
thanks -john
So one more time....
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
Also thanks to George Spelvin for noticing and fixing the bogus frequency comparison check in my original RFC'ed patch. This version of the patch includes his much better comparison.
Also thanks to Serge Belyshev for suggesting instead of ignoring out of range values, to always use the user provided tsc_khz value but throw a warning.
Signed-off-by: John Stultz <johnstul@us.ibm.com>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index e87bdbf..cc5b2c1 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -2402,6 +2402,13 @@ 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: <khz> + Used to override the calibrated TSC freq. + This can be useful to avoid TSC calibration error + causing problems with NTP synchronization across + reboots. + 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..31a1b27 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 +844,25 @@ void __init tsc_init(void) return; tsc_khz = calibrate_tsc(); + + if (tsc_khz_specified) { + long difference = abs(tsc_khz - tsc_khz_specified); + /* + * Make a fair amount of noise if tsc_khz boot option + * is more then 0.1% off of the calibrated tsc_khz value + */ + if (difference > tsc_khz/1000) { + printk(KERN_WARNING "WARNING! Specified tsc_khz is" + " more then 0.1%% off from the calibrated TSC" + " freq.\n\tThis may cause severe time" + " problems.\n"); + } + 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) {
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