Commit 17915131 authored by Borislav Petkov's avatar Borislav Petkov Committed by Thomas Gleixner
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clocksource: Improve comments for watchdog skew bounds 



Add more detail on the rationale for bounding the clocksource
->uncertainty_margin below at about 500ppm.

Signed-off-by: default avatarBorislav Petkov <bp@alien8.de>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240802154618.4149953-1-paulmck@kernel.org
parent f2655ac2

kernel/time/clocksource.c

+20 −8
Original line number Diff line number Diff line
@@ -125,6 +125,13 @@ static u64 suspend_start; 
 *

 * The default of 500 parts per million is based on NTP's limits.

 * If a clocksource is good enough for NTP, it is good enough for us!

 *

 * In other words, by default, even if a clocksource is extremely

 * precise (for example, with a sub-nanosecond period), the maximum

 * permissible skew between the clocksource watchdog and the clocksource

 * under test is not permitted to go below the 500ppm minimum defined

 * by MAX_SKEW_USEC.  This 500ppm minimum may be overridden using the

 * CLOCKSOURCE_WATCHDOG_MAX_SKEW_US Kconfig option.

 */

#ifdef CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US

#define MAX_SKEW_USEC	CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US
@@ -1146,14 +1153,19 @@ void __clocksource_update_freq_scale(struct clocksource *cs, u32 scale, u32 freq 
	}



	/*

	 * If the uncertainty margin is not specified, calculate it.

	 * If both scale and freq are non-zero, calculate the clock

	 * period, but bound below at 2*WATCHDOG_MAX_SKEW.  However,

	 * if either of scale or freq is zero, be very conservative and

	 * take the tens-of-milliseconds WATCHDOG_THRESHOLD value for the

	 * uncertainty margin.  Allow stupidly small uncertainty margins

	 * to be specified by the caller for testing purposes, but warn

	 * to discourage production use of this capability.

	 * If the uncertainty margin is not specified, calculate it.  If

	 * both scale and freq are non-zero, calculate the clock period, but

	 * bound below at 2*WATCHDOG_MAX_SKEW, that is, 500ppm by default.

	 * However, if either of scale or freq is zero, be very conservative

	 * and take the tens-of-milliseconds WATCHDOG_THRESHOLD value

	 * for the uncertainty margin.  Allow stupidly small uncertainty

	 * margins to be specified by the caller for testing purposes,

	 * but warn to discourage production use of this capability.

	 *

	 * Bottom line:  The sum of the uncertainty margins of the

	 * watchdog clocksource and the clocksource under test will be at

	 * least 500ppm by default.  For more information, please see the

	 * comment preceding CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US above.

	 */

	if (scale && freq && !cs->uncertainty_margin) {

		cs->uncertainty_margin = NSEC_PER_SEC / (scale * freq);