This is a backport of the patch submitted by Thomas Gleixner to the x86 git tree, commit d8bb6f4c1670c8324e4135c61ef07486f7f17379 Comments from the original post : We already catch most of the TSC problems by sanity checks, but there is a subtle bug which has been in the code for ever. This can cause time jumps in the range of hours. This was reported in: http://lkml.org/lkml/2007/8/23/96 and http://lkml.org/lkml/2008/3/31/23 I was able to reproduce the problem with a gettimeofday loop test on a dual core and a quad core machine which both have sychronized TSCs. The TSCs seems not to be perfectly in sync though, but the kernel is not able to detect the slight delta in the sync check. Still there exists an extremly small window where this delta can be observed with a real big time jump. So far I was only able to reproduce this with the vsyscall gettimeofday implementation, but in theory this might be observable with the syscall based version as well. CPU 0 updates the clock source variables under xtime/vyscall lock and CPU1, where the TSC is slighty behind CPU0, is reading the time right after the seqlock was unlocked. The clocksource reference data was updated with the TSC from CPU0 and the value which is read from TSC on CPU1 is less than the reference data. This results in a huge delta value due to the unsigned subtraction of the TSC value and the reference value. This algorithm can not be changed due to the support of wrapping clock sources like pm timer. The huge delta is converted to nanoseconds and added to xtime, which is then observable by the caller. The next gettimeofday call on CPU1 will show the correct time again as now the TSC has advanced above the reference value. To prevent this TSC specific wreckage we need to compare the TSC value against the reference value and return the latter when it is larger than the actual TSC value. I pondered to mark the TSC unstable when the readout is smaller than the reference value, but this would render an otherwise good and fast clocksource unusable without a real good reason. Signed-off-by: Alok Kataria CC: Thomas Gleixner Index: linux-2.6.24.7/arch/x86/kernel/tsc_32.c =================================================================== --- linux-2.6.24.7.orig/arch/x86/kernel/tsc_32.c 2008-05-14 11:25:20.000000000 -0700 +++ linux-2.6.24.7/arch/x86/kernel/tsc_32.c 2008-05-14 11:26:21.000000000 -0700 @@ -268,14 +268,27 @@ /* clock source code */ static unsigned long current_tsc_khz = 0; +static struct clocksource clocksource_tsc; +/* + * We compare the TSC to the cycle_last value in the clocksource + * structure to avoid a nasty time-warp issue. This can be observed in + * a very small window right after one CPU updated cycle_last under + * xtime lock and the other CPU reads a TSC value which is smaller + * than the cycle_last reference value due to a TSC which is slighty + * behind. This delta is nowhere else observable, but in that case it + * results in a forward time jump in the range of hours due to the + * unsigned delta calculation of the time keeping core code, which is + * necessary to support wrapping clocksources like pm timer. + */ static cycle_t read_tsc(void) { cycle_t ret; rdtscll(ret); - return ret; + return ret >= clocksource_tsc.cycle_last ? + ret : clocksource_tsc.cycle_last; } static struct clocksource clocksource_tsc = { Index: linux-2.6.24.7/arch/x86/kernel/tsc_64.c =================================================================== --- linux-2.6.24.7.orig/arch/x86/kernel/tsc_64.c 2008-05-14 11:25:20.000000000 -0700 +++ linux-2.6.24.7/arch/x86/kernel/tsc_64.c 2008-05-14 11:26:21.000000000 -0700 @@ -10,6 +10,7 @@ #include #include +#include static int notsc __initdata = 0; @@ -246,18 +247,34 @@ __setup("notsc", notsc_setup); +static struct clocksource clocksource_tsc; -/* clock source code: */ +/* + * We compare the TSC to the cycle_last value in the clocksource + * structure to avoid a nasty time-warp. This can be observed in a + * very small window right after one CPU updated cycle_last under + * xtime/vsyscall_gtod lock and the other CPU reads a TSC value which + * is smaller than the cycle_last reference value due to a TSC which + * is slighty behind. This delta is nowhere else observable, but in + * that case it results in a forward time jump in the range of hours + * due to the unsigned delta calculation of the time keeping core + * code, which is necessary to support wrapping clocksources like pm + * timer. + */ static cycle_t read_tsc(void) { cycle_t ret = (cycle_t)get_cycles_sync(); - return ret; + + return ret >= clocksource_tsc.cycle_last ? + ret : clocksource_tsc.cycle_last; } static cycle_t __vsyscall_fn vread_tsc(void) { cycle_t ret = (cycle_t)get_cycles_sync(); - return ret; + + return ret >= __vsyscall_gtod_data.clock.cycle_last ? + ret : __vsyscall_gtod_data.clock.cycle_last; } static struct clocksource clocksource_tsc = { Index: linux-2.6.24.7/kernel/time/timekeeping.c =================================================================== --- linux-2.6.24.7.orig/kernel/time/timekeeping.c 2008-05-14 11:25:20.000000000 -0700 +++ linux-2.6.24.7/kernel/time/timekeeping.c 2008-05-14 11:26:21.000000000 -0700 @@ -189,6 +189,7 @@ if (clock == new) return; + new->cycle_last = 0; now = clocksource_read(new); nsec = __get_nsec_offset(); timespec_add_ns(&xtime, nsec); @@ -301,6 +302,7 @@ /* Make sure that we have the correct xtime reference */ timespec_add_ns(&xtime, timekeeping_suspend_nsecs); /* re-base the last cycle value */ + clock->cycle_last = 0; clock->cycle_last = clocksource_read(clock); clock->error = 0; timekeeping_suspended = 0;