Messages in this thread | | | Date | Fri, 17 Oct 2014 11:17:39 -0700 | Subject | Re: [PATCH] clocksource, Add warning to clocksource_delta() validation code | From | John Stultz <> |
| |
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).
So this warning would trigger quite often and be a bit alarming when its really expected (well... more like unfortunate but handled) behavior.
thanks -john
| |