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

Thomas Gleixner
2014-10-25T18:48:44Z

On Sat, 25 Oct 2014, Thomas Gleixner wrote:
> On Sat, 25 Oct 2014, Heena Sirwani wrote:
> > +time64_t ktime_get_seconds(void)
> > +{
> > + time64_t ts;
> > + struct timekeeper *tk = &tk_core.timekeeper;
> > + struct timespec64 tomono;
> > + s32 nsec;
> > + unsigned int seq;
> > +
> > + WARN_ON(timekeeping_suspended);
> > +
> > + do {
> > + seq = read_seqcount_begin(&tk_core.seq);
> > + ts = tk->xtime_sec;
> > + nsec = (long)(tk->tkr.xtime_nsec >> tk->tkr.shift);
> > + tomono = tk->wall_to_monotonic;
> > +
> > + } while (read_seqcount_retry(&tk_core.seq, seq));
> > +
> > + ts += tomono.tv_sec;
> > + if (nsec + tomono.tv_nsec >= NSEC_PER_SEC)
> > + ts += 1;
> > + return ts;
>
> 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?

Hmm. Thinking more about it. That's actually overkill. For ktime_sec a
32bit value is plenty enough unless we care about systems with more
than 136 years uptime. So if we calculate the seconds value of ktime,
i.e. CLOCK_MONOTONIC, in the update function, we can read it on both
32 and 64bit w/o the seqcount loop.

Where we really need the above readout mechanism is get_seconds() as
that will break in 2038 on 32bit. So there you need to change the
return value from unsigned long to time64_t and change the
implementation as above just xtime_sec instead of ktime_sec.

Thanks,

tglx