Kernel Bug Tracker – Bug 10410
"x86: tsc prevent time going backwards" broke suspend
Last modified: 2008-04-09 02:28:12 UTC
PC unable to resume after suspend to ram.
This patch make this regression for me:
Author: Thomas Gleixner <firstname.lastname@example.org>
Date: Tue Apr 1 19:45:18 2008 +0200
x86: tsc prevent time going backwards
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:
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
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
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: Thomas Gleixner <email@example.com>
Signed-off-by: Ingo Molnar <firstname.lastname@example.org>
fixed by commit 5b13d863573e746739ccfc24ac1a9473cfee8df1
Created attachment 15686 [details]
[ 2.128371] Magic number: 0:835:626
[ 2.128371] hash matches drivers/base/power/main.c:207
[ 2.128482] hash matches device 0000:00:1b.0
Created attachment 15687 [details]
Test patch which presets the tsc->cycle_last value on resume
can you test the attached patch on top of Linus latest ?
This patch working for me.
> ------- Comment #4 from email@example.com 2008-04-09 02:28 -------
> This patch working for me.
thanks for testing. I queue it for 2.6.26