Re: [PATCH v5] timekeeping: Added a function to return tv_sec portion of ktime_get_ts64()

Thomas Gleixner
2014-10-27T21:21:16Z

On Sun, 26 Oct 2014, Arnd Bergmann wrote:
> On Saturday 25 October 2014 17:12:48 Thomas Gleixner wrote:
> > I'd rather have an extra field in the timekeeper
> >
> > u64 xtime_sec;
> > + u64 ktime_sec;
> >
> > and update this in tk_update_ktime_data() so the readout function
> > boils down to
> >
> > time64_t ktime_get_seconds(void)
> > {
> > #if BITS_PER_LONG < 64
> > u64 sec;
> > int seq;
> >
> > do {
> > seq = read_seqcount_begin(&tk_core.seq);
> > sec = tk->ktime_sec;
> > } while (read_seqcount_retry(&tk_core.seq, seq));
> >
> > return sec;
> > #else
> > return tk->ktime_sec;
> > #endif
> > }
> >
> > So 64bit can do w/o the seqcount and 32bit avoids all extra math, right?
> >
>
> But where would you set ktime_sec? My first thought was tk_update_ktime_data(),
> but would then either be slightly wrong because it doesn't take the current
> xtime_nsec + tk->wall_to_monotonic.tv_nsec into account, if we do
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index ec1791fae965..d72a456c6bd3 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -426,8 +426,8 @@ static inline void tk_update_ktime_data(struct timekeeper *tk)
> * nsec = base_mono + now();
> * ==> base_mono = (xtime_sec + wtm_sec) * 1e9 + wtm_nsec
> */
> - nsec = (s64)(tk->xtime_sec + tk->wall_to_monotonic.tv_sec);
> - nsec *= NSEC_PER_SEC;
> + tk->ktime_sec = (s64)(tk->xtime_sec + tk->wall_to_monotonic.tv_sec);
> + nsec = tk->ktime_sec * NSEC_PER_SEC;
> nsec += tk->wall_to_monotonic.tv_nsec;
> tk->tkr.base_mono = ns_to_ktime(nsec);
>
>
> or we'd have to add an expensive timekeeping_get_ns() call:
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index ec1791fae965..d4c95b815de6 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -426,9 +426,11 @@ static inline void tk_update_ktime_data(struct timekeeper *tk)
> * nsec = base_mono + now();
> * ==> base_mono = (xtime_sec + wtm_sec) * 1e9 + wtm_nsec
> */
> - nsec = (s64)(tk->xtime_sec + tk->wall_to_monotonic.tv_sec);
> - nsec *= NSEC_PER_SEC;
> - nsec += tk->wall_to_monotonic.tv_nsec;
> + tk->ktime_sec = (s64)(tk->xtime_sec + tk->wall_to_monotonic.tv_sec);
> + nsec = tk->wall_to_monotonic.tv_nsec;
> + if ((nsec + timekeeping_get_ns()) > NSEC_PER_SEC)
> + tk->ktime_sec += 1;
> + nsec += tk->ktime_sec * NSEC_PER_SEC;
> tk->tkr.base_mono = ns_to_ktime(nsec);

No. Why would you do that? Look at the math you implemented and just
do the same thing in the update function.

Thanks,

tglx