Commit 135225a3 authored by Adrian Hunter's avatar Adrian Hunter Committed by Thomas Gleixner
Browse files

timekeeping: Let timekeeping_cycles_to_ns() handle both under and overflow 



For the case !CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE, forego overflow
protection in the range (mask << 1) < delta <= mask, and interpret it
always as an inconsistency between CPU clock values. That allows
slightly neater code, and it is on a slow path so has no effect on
performance.

Suggested-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarAdrian Hunter <adrian.hunter@intel.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240325064023.2997-19-adrian.hunter@intel.com
parent fcf190c3

kernel/time/timekeeping.c

+13 −18
Original line number Diff line number Diff line
@@ -266,17 +266,14 @@ static inline u64 timekeeping_debug_get_ns(const struct tk_read_base *tkr) 
	 * Try to catch underflows by checking if we are seeing small

	 * mask-relative negative values.

	 */

	if (unlikely((~delta & mask) < (mask >> 3))) {

	if (unlikely((~delta & mask) < (mask >> 3)))

		tk->underflow_seen = 1;

		now = last;

	}



	/* Cap delta value to the max_cycles values to avoid mult overflows */

	if (unlikely(delta > max)) {

	/* Check for multiplication overflows */

	if (unlikely(delta > max))

		tk->overflow_seen = 1;

		now = last + max;

	}



	/* timekeeping_cycles_to_ns() handles both under and overflow */

	return timekeeping_cycles_to_ns(tkr, now);

}

#else
@@ -375,19 +372,17 @@ static inline u64 timekeeping_cycles_to_ns(const struct tk_read_base *tkr, u64 c 
	u64 mask = tkr->mask, delta = (cycles - tkr->cycle_last) & mask;



	/*

	 * This detects the case where the delta overflows the multiplication

	 * with tkr->mult.

	 * This detects both negative motion and the case where the delta

	 * overflows the multiplication with tkr->mult.

	 */

	if (unlikely(delta > tkr->clock->max_cycles)) {

		if (IS_ENABLED(CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE)) {

		/*

		 * Handle clocksource inconsistency between CPUs to prevent

		 * time from going backwards by checking for the MSB of the

		 * mask being set in the delta.

		 */

			if (unlikely(delta & ~(mask >> 1)))

		if (delta & ~(mask >> 1))

			return tkr->xtime_nsec >> tkr->shift;

		}



		return delta_to_ns_safe(tkr, delta);

	}