Commit 4ac1dd32 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Thomas Gleixner
Browse files

clocksource: Set cs_watchdog_read() checks based on .uncertainty_margin 



Right now, cs_watchdog_read() does clocksource sanity checks based
on WATCHDOG_MAX_SKEW, which sets a floor on any clocksource's
.uncertainty_margin.  These sanity checks can therefore act
inappropriately for clocksources with large uncertainty margins.

One reason for a clocksource to have a large .uncertainty_margin is when
that clocksource has long read-out latency, given that it does not make
sense for the .uncertainty_margin to be smaller than the read-out latency.
With the current checks, cs_watchdog_read() could reject all normal
reads from a clocksource with long read-out latencies, such as those
from legacy clocksources that are no longer implemented in hardware.

Therefore, recast the cs_watchdog_read() checks in terms of the
.uncertainty_margin values of the clocksources involved in the timespan in
question.  The first covers two watchdog reads and one cs read, so use
twice the watchdog .uncertainty_margin plus that of the cs.  The second
covers only a pair of watchdog reads, so use twice the watchdog
.uncertainty_margin.

Reported-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/20240802154618.4149953-4-paulmck@kernel.org
parent f33a5d4b

kernel/time/clocksource.c

+5 −4
Original line number Diff line number Diff line
@@ -244,6 +244,7 @@ enum wd_read_status { 


static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)

{

	int64_t md = 2 * watchdog->uncertainty_margin;

	unsigned int nretries, max_retries;

	int64_t wd_delay, wd_seq_delay;

	u64 wd_end, wd_end2;
@@ -258,7 +259,7 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, 
		local_irq_enable();



		wd_delay = cycles_to_nsec_safe(watchdog, *wdnow, wd_end);

		if (wd_delay <= WATCHDOG_MAX_SKEW) {

		if (wd_delay <= md + cs->uncertainty_margin) {

			if (nretries > 1 && nretries >= max_retries) {

				pr_warn("timekeeping watchdog on CPU%d: %s retried %d times before success\n",

					smp_processor_id(), watchdog->name, nretries);
@@ -271,12 +272,12 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, 
		 * there is too much external interferences that cause

		 * significant delay in reading both clocksource and watchdog.

		 *

		 * If consecutive WD read-back delay > WATCHDOG_MAX_SKEW/2,

		 * report system busy, reinit the watchdog and skip the current

		 * If consecutive WD read-back delay > md, report

		 * system busy, reinit the watchdog and skip the current

		 * watchdog test.

		 */

		wd_seq_delay = cycles_to_nsec_safe(watchdog, wd_end, wd_end2);

		if (wd_seq_delay > WATCHDOG_MAX_SKEW/2)

		if (wd_seq_delay > md)

			goto skip_test;

	}