Hi Team, We have a query on the implementation linux timer implementation.
We have been running our application on linux kernel 4.9.168 (debian - stretch) version. We use intel Rangeley 4 cpu system which has tsc timer. Our application is an multi-threaded program which creates a monotonic timer-fd object, sets one second expiry time and does read on the timer object infinitely. Once in a while, we face an anomaly in the return value of timerfd read call. As per the man page, read on timer fd (http://man7.org/linux/man-pages/man2/timerfd_create.2.html) returns 8 or -1 but we observed read returns 0. While debugging, we found timerfd_read function returns 0 when the amount of negative tsc clock drift goes above certain boundary condition. Through instrumentation, we found the following sequence of code flow gets hit and causes timerfd_read to return 0. Please find the below sequence of code that leads to this anomaly. read(timerfd,…) --> timerfd_read() ---> hrtimer_forward_now ---> hrtimer_forward() return 0 if expiry time is greater than current time. Timer object expire time becomes greater than current time due to negative tsc clock drift. We like to know whether returning 0 from timerfd_read is expected? - If yes, can we document it in timerfd read man page? - If no, can we do the needful to address this problem in Linux? Please cc me in all the discussions happening on this post. Regards, Arul
