Messages in this thread | | | Date | Wed, 3 Dec 2008 04:48:16 +0100 (CET) | From | Roman Zippel <> | Subject | Re: [RFC][PATCH] Catch xtime_nsec underflows and fix them |
| |
Hi,
On Wed, 3 Dec 2008, Zhang, Yanmin wrote:
> > This doesn't explain the problem entirely, I considered a negative > > xtime_nsec before, but xtime_nsec+offset should still be positive > xtime_nsec underflows after clocksource_adjust. Before clocksource_adjust, > xtime_nsec is a small positive. > > When xtime_nsec underflows at the first time, xtime.tv_nsec becomes -1. > Later on when the second tick arrives, below statement in the while loop > clock->xtime_nsec += clock->xtime_interval; > will cause clock->xtime_nsec becomes positive again. So the second tick > appears a going-backward time.
Yes, but only by 1nsec, so normally it wouldn't be noticable.
> > and > > produce the correct result, at least I can't find anything in > > getnstimeofday(). > The testing uses vsyscall to get call gettimeofday. vsyscall_gtod_data.wall_time_nsec > is a u32 while timespec->tv_nsec is a signed long.
Ok, I was missing this part, I looked at the 32bit version of getnstimeofday() and there xtime.tv_nsec was correctly sign extended. To be safe for the future wall_time_nsec should also be a s32.
bye, Roman | |