2024-03-25 12:18:33

by Adrian Hunter

Subject: [PATCH V2 18/19] 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: Thomas Gleixner <[email protected]>
Signed-off-by: Adrian Hunter <[email protected]>
---
kernel/time/timekeeping.c | 31 +++++++++++++------------------
1 file changed, 13 insertions(+), 18 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 111dfdbd488f..4e18db1819f8 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -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)))
- return tkr->xtime_nsec >> tkr->shift;
- }
+ /*
+ * 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 (delta & ~(mask >> 1))
+ return tkr->xtime_nsec >> tkr->shift;

return delta_to_ns_safe(tkr, delta);
}
--
2.34.1

2024-04-08 13:11:13

by tip-bot2 for Tony Luck

Subject: [tip: timers/core] timekeeping: Let timekeeping_cycles_to_ns() handle both under and overflow

The following commit has been merged into the timers/core branch of tip:

Commit-ID: 135225a363ae67bc90bde7a2cbbe1ea0f152ba22
Gitweb: https://git.kernel.org/tip/135225a363ae67bc90bde7a2cbbe1ea0f152ba22
Author: Adrian Hunter <[email protected]>
AuthorDate: Mon, 25 Mar 2024 08:40:22 +02:00
Committer: Thomas Gleixner <[email protected]>
CommitterDate: Mon, 08 Apr 2024 15:03:08 +02:00

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: Thomas Gleixner <[email protected]>
Signed-off-by: Adrian Hunter <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

---
kernel/time/timekeeping.c | 31 +++++++++++++------------------
1 file changed, 13 insertions(+), 18 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 111dfdb..4e18db1 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -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)))
- return tkr->xtime_nsec >> tkr->shift;
- }
+ /*
+ * 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 (delta & ~(mask >> 1))
+ return tkr->xtime_nsec >> tkr->shift;

return delta_to_ns_safe(tkr, delta);
}