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

Arnd Bergmann
2014-10-26T21:01:39Z

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);

/* Update the monotonic raw base */

Arnd