Messages in this thread Patch in this message | | | From | Michael Zhivich <> | Subject | [PATCH] Revert "clocksource: Make clocksource validation work for all clocksources" | Date | Mon, 12 Nov 2018 13:56:51 -0500 |
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Revert commit 1f45f1f33c8c ("clocksource: Make clocksource validation work for all clocksources") to restore correct clocksource_delta() computation for clocksources that wrap frequently, while retaining the check for tsc drifting.
Truncating result of clocksource_delta() to 0 causes incorrect behavior for clocksources that wrap frequently (e.g. acpi_pm which is only 24-bit wide). In particular, large time deltas (e.g. last = 0x000000, now = 0x800000) will be incorrectly computed as 0.
If acpi_pm is used as the clocksource watchdog, and machine is under heavy load, the time period for the watchdog check may be significantly longer than the requested 0.5 seconds. If the watchdog check is delayed by 2 seconds (observed behavior), then acpi_pm time delta will be
2.5 sec * 3579545 ticks/sec = 8948863 = 0x888c3f
which will be treated as negative and truncated to 0. This behavior will cause tsc to be incorrectly declared unstable in clocksource_watchdog(), as it no longer agrees with acpi_pm.
Signed-off-by: Michael Zhivich <mzhivich@akamai.com> --- kernel/time/timekeeping_internal.h | 6 +----- 1 file changed, 1 insertion(+), 5 deletions(-)
diff --git a/kernel/time/timekeeping_internal.h b/kernel/time/timekeeping_internal.h index bcbb52db2256..21463e20d008 100644 --- a/kernel/time/timekeeping_internal.h +++ b/kernel/time/timekeeping_internal.h @@ -18,11 +18,7 @@ static inline u64 clocksource_delta(u64 now, u64 last, u64 mask) { u64 ret = (now - last) & mask; - /* - * Prevent time going backwards by checking the MSB of mask in - * the result. If set, return 0. - */ - return ret & ~(mask >> 1) ? 0 : ret; + return (s64) ret > 0 ? ret : 0; } #else static inline u64 clocksource_delta(u64 now, u64 last, u64 mask) -- 2.19.1
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