Messages in this thread | | | Date | Fri, 17 Oct 2014 14:23:02 -0400 | From | Prarit Bhargava <> | Subject | Re: [PATCH] clocksource, Add warning to clocksource_delta() validation code |
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On 10/17/2014 02:17 PM, John Stultz wrote: > On Fri, Oct 17, 2014 at 6:57 AM, Prarit Bhargava <prarit@redhat.com> wrote: >> A bug report came in against an older kernel which output "backward time" >> messages and the report noted that the upstream kernel worked. After some >> investigation it turned out that one of the sockets was bad on the system >> and the "backward time" messages were caused by a real, but intermittent, >> hardware failure. >> >> Commit 09ec54429c6d10f87d1f084de53ae2c1c3a81108 ("clocksource: Move >> cycle_last validation to core code") modifies the x86 clocksource such that >> if a negative delta between two reads of time is calculated the >> clocksource_delta() code will return 0. There is no warning when this >> occurs and there really should be one in order to catch not only hardware >> issues like the issue above, but potential coding issues as the code is >> modified. This patch introduces a WARN() which will also dump a stack >> trace to the console so the exact code path can be evaluated. >> >> I tested this by booting on the broken hardware and left the system idle >> until a negative clocksource_delta() event occurred. >> >> Cc: John Stultz <john.stultz@linaro.org> >> Cc: Thomas Gleixner <tglx@linutronix.de> >> Signed-off-by: Prarit Bhargava <prarit@redhat.com> >> --- >> kernel/time/timekeeping_internal.h | 7 ++++++- >> 1 file changed, 6 insertions(+), 1 deletion(-) >> >> diff --git a/kernel/time/timekeeping_internal.h b/kernel/time/timekeeping_internal.h >> index 4ea005a..abe6bc8 100644 >> --- a/kernel/time/timekeeping_internal.h >> +++ b/kernel/time/timekeeping_internal.h >> @@ -17,7 +17,12 @@ static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask) >> { >> cycle_t ret = (now - last) & mask; >> >> - return (s64) ret > 0 ? ret : 0; >> + if ((s64)ret > 0) >> + return ret; >> + >> + WARN(1, "Clocksource calculated negative delta, %lld. last = %llu, now = %llu, mask = %llx\n", >> + (s64)ret, last, now, mask); >> + return 0; > > > I realize you followed up that this wasn't finished, but just as some > feedback, there's a number of types of hardware where there may be a > very slight skew between cpu TSC, and this will briefly trigger right > after each timekeeping update if a system is reading the clock > frequently (think of the case where the update happens on the cpu > thats just a little bit ahead, while a timestamping loop is running on > a cpu that is a little bit behind).
Ah, interesting. Okay ... drop this patch then. Thanks for the info John.
P.
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