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clocksource: Defer override invalidation unless clock is unstable

PATCH In review Kyle Walker 2 participants 2 messages Aug 6, 2016 – Aug 17, 2016 Timekeeping, clocksource core, NTP, alarmtimer

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Kyle Walker

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#0 lore raw

PATCH v2 1/1 clocksource: Defer override invalidation unless clock is unstable

Clocksources don't get the VALID_FOR_HRES flag until they have been checked by a watchdog. However, when using an override, the clocksource_select logic will clear the override value if the clocksource is not marked VALID_FOR_HRES during that inititial check. When using the boot arguments clocksource=<foo>, this selection can run before the watchdog, and can cause the override to be incorrectly cleared.

To address this condition, the override_name is only invalidated for unstable clocksources. Otherwise, the override is left intact until after the watchdog has validated the clocksource as stable/unstable.

Signed-off-by Kyle Walker <[email protected]>
Cc John Stultz <[email protected]>
Cc Thomas Gleixner <[email protected]>
Cc Martin Schwidefsky <[email protected]>
Cc [email protected]

---

Notes:
Changes from v1:
* Altered changelog description, many thanks to John Stultz for the assist!

 kernel/time/clocksource.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)
kernel/time/clocksource.c +12 -3
index 56ece14..4c1bb2a 100644
@@ -600,9 +600,18 @@ static void __clocksource_select(bool skipcur)
 		 */
 		if (!(cs->flags & CLOCK_SOURCE_VALID_FOR_HRES) && oneshot) {
 			/* Override clocksource cannot be used. */
-			pr_warn("Override clocksource %s is not HRT compatible - cannot switch while in HRT/NOHZ mode\n",
-				cs->name);
-			override_name[0] = 0;
+			if (cs->flags & CLOCK_SOURCE_UNSTABLE) {
+				pr_warn("Override clocksource %s is unstable and not HRT compatible - cannot switch while in HRT/NOHZ mode\n",
+					cs->name);
+				override_name[0] = 0;
+			} else {
+				/*
+				 * The override cannot be currently verified.
+				 * Deferring to let the watchdog check.
+				 */
+				pr_info("Override clocksource %s is not currently HRT compatible - deferring\n",
+					cs->name);
+			}
 		} else
 			/* Override clocksource can be used. */
 			best = cs;
Signature · 1 line
2.5.5
#1 lore raw
Quoted 8 lines · Kyle Walker

Clocksources don't get the VALID_FOR_HRES flag until they have been checked by a watchdog. However, when using an override, the clocksource_select logic will clear the override value if the clocksource is not marked VALID_FOR_HRES during that inititial check. When using the boot arguments clocksource=<foo>, this selection can run before the watchdog, and can cause the override to be incorrectly cleared.

Quoted 5 lines

To address this condition, the override_name is only invalidated for unstable clocksources. Otherwise, the override is left intact until after the watchdog has validated the clocksource as stable/unstable.

Signed-off-by Kyle Walker <[email protected]>

Queued for testing and targeting 4.9

thanks
-john