On Tue, May 14, 2024, 03:57 Zhao Liu <zhao1....@intel.com> wrote: > Hi Stefan, > > > QEMU uses clock_gettime(CLOCK_MONOTONIC) on Linux hosts. The man page > > says: > > > > All CLOCK_MONOTONIC variants guarantee that the time returned by > > consecutive calls will not go backwards, but successive calls > > may—depending on the architecture—return identical (not-in‐ > > creased) time values. > > > > trace_record_start() calls clock_gettime(CLOCK_MONOTONIC) so trace events > > should have monotonically increasing timestamps. > > > > I don't see a scenario where trace record A's timestamp is greater than > > trace record B's timestamp unless the clock is non-monotonic. > > > > Which host CPU architecture and operating system are you running? > > I tested on these 2 machines: > * CML (intel 10th) with Ubuntu 22.04 + kernel v6.5.0-28 > * MTL (intel 14th) with Ubuntu 22.04.2 + kernel v6.9.0 > > > Please attach to the QEMU process with gdb and print out the value of > > the use_rt_clock variable or add a printf in init_get_clock(). The value > > should be 1. > > Thanks, on both above machines, use_rt_clock is 1 and there're both > timestamp reversal issues with the following debug print: > > diff --git a/include/qemu/timer.h b/include/qemu/timer.h > index 9a366e551fb3..7657785c27dc 100644 > --- a/include/qemu/timer.h > +++ b/include/qemu/timer.h > @@ -831,10 +831,17 @@ extern int use_rt_clock; > > static inline int64_t get_clock(void) > { > + static int64_t clock = 0; >
Please try with a thread local variable (__thread) to check whether this happens within a single thread. If it only happens with a global variable then we'd need to look more closely at race conditions in the patch below. I don't think the patch is a reliable way to detect non-monotonic timestamps in a multi-threaded program. if (use_rt_clock) { > struct timespec ts; > clock_gettime(CLOCK_MONOTONIC, &ts); > - return ts.tv_sec * 1000000000LL + ts.tv_nsec; > + int64_t tmp = ts.tv_sec * 1000000000LL + ts.tv_nsec; > + if (tmp <= clock) { > + printf("get_clock: strange, clock: %ld, tmp: %ld\n", clock, > tmp); > + } > + assert(tmp > clock); > + clock = tmp; > + return clock; > } else { > /* XXX: using gettimeofday leads to problems if the date > changes, so it should be avoided. */ > diff --git a/util/qemu-timer-common.c b/util/qemu-timer-common.c > index cc1326f72646..3bf06eb4a4ce 100644 > --- a/util/qemu-timer-common.c > +++ b/util/qemu-timer-common.c > @@ -59,5 +59,6 @@ static void __attribute__((constructor)) > init_get_clock(void) > use_rt_clock = 1; > } > clock_start = get_clock(); > + printf("init_get_clock: use_rt_clock: %d\n", use_rt_clock); > } > #endif > > --- > The timestamp interval is very small, for example: > get_clock: strange, clock: 3302130503505, tmp: 3302130503503 > > or > > get_clock: strange, clock: 2761577819846455, tmp: 2761577819846395 > > I also tried to use CLOCK_MONOTONIC_RAW, but there's still the reversal > issue. > > Thanks, > Zhao > >