user/sven/linux.git/include/linux/timekeeping.h, branch v3.18.22

timekeeping: Provide fast and NMI safe access to CLOCK_MONOTONIC

2014-07-23T22:01:55Z

Tracers want a correlated time between the kernel instrumentation and user space. We really do not want to export sched_clock() to user space, so we need to provide something sensible for this. Using separate data structures with an non blocking sequence count based update mechanism allows us to do that. The data structure required for the readout has a sequence counter and two copies of the timekeeping data. On the update side: smp_wmb(); tkf->seq++; smp_wmb(); update(tkf->base[0], tk); smp_wmb(); tkf->seq++; smp_wmb(); update(tkf->base[1], tk); On the reader side: do { seq = tkf->seq; smp_rmb(); idx = seq & 0x01; now = now(tkf->base[idx]); smp_rmb(); } while (seq != tkf->seq) So if a NMI hits the update of base[0] it will use base[1] which is still consistent, but this timestamp is not guaranteed to be monotonic across an update. The timestamp is calculated by: now = base_mono + clock_delta * slope So if the update lowers the slope, readers who are forced to the not yet updated second array are still using the old steeper slope. tmono ^ | o n | o n | u | o |o |12345678---> reader order o = old slope u = update n = new slope So reader 6 will observe time going backwards versus reader 5. While other CPUs are likely to be able observe that, the only way for a CPU local observation is when an NMI hits in the middle of the update. Timestamps taken from that NMI context might be ahead of the following timestamps. Callers need to be aware of that and deal with it. V2: Got rid of clock monotonic raw and reorganized the data structures. Folded in the barrier fix from Mathieu. Signed-off-by: Thomas Gleixner Cc: Peter Zijlstra Cc: Steven Rostedt Cc: Mathieu Desnoyers Signed-off-by: John Stultz

timekeeping: Provide ktime_get_raw()

2014-07-23T22:01:49Z

Provide a ktime_t based interface for raw monotonic time. Signed-off-by: Thomas Gleixner Signed-off-by: John Stultz

timekeeping: Simplify timekeeping_clocktai()

2014-07-23T22:01:48Z

timekeeping_clocktai() is not used in fast pathes, so the extra timespec conversion is not problematic. Signed-off-by: Thomas Gleixner Signed-off-by: John Stultz

timekeeping: Use ktime_get_boottime() for get_monotonic_boottime()

2014-07-23T22:01:47Z

get_monotonic_boottime() is not used in fast pathes, so the extra timespec conversion is not problematic. Signed-off-by: Thomas Gleixner Signed-off-by: John Stultz

timekeeping: Remove monotonic_to_bootbased

2014-07-23T22:01:46Z

No more users. Signed-off-by: Thomas Gleixner Signed-off-by: John Stultz

timekeeping: Provide ktime_get[*]_ns() helpers

2014-07-23T17:18:04Z

A lot of code converts either timespecs or ktime_t to nanoseconds. Provide helper functions. Signed-off-by: Thomas Gleixner Signed-off-by: John Stultz

timekeeping: Remove ktime_get_monotonic_offset()

2014-07-23T17:18:03Z

No more users. Signed-off-by: Thomas Gleixner Signed-off-by: John Stultz

timekeeping: Provide ktime_mono_to_any()

2014-07-23T17:18:01Z

ktime based conversion function to map a monotonic time stamp to a different CLOCK. Signed-off-by: Thomas Gleixner Signed-off-by: John Stultz

timekeeping: Use ktime_t based data for ktime_get_clocktai()

2014-07-23T17:18:00Z

Signed-off-by: Thomas Gleixner Signed-off-by: John Stultz

timekeeping; Use ktime_t based data for ktime_get_boottime()

2014-07-23T17:17:59Z

Signed-off-by: Thomas Gleixner Signed-off-by: John Stultz