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

Arnd Bergmann
2014-10-25T18:24:43Z

On Saturday 25 October 2014 17:22:23 Thomas Gleixner wrote:
> 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.

Ah, very good point. That opens the question which type that function
should return. I really want to remove all uses of time_t from the
kernel, mostly so we know when we're done with this. However as you
say we know that we only need a 32-bit value here. Some possible
ideas:

- use time64_t here anyway and accept the slight inefficiency in return
for clarity
- introduce a monotonic_time_t (we probably also want a struct
monotonic_timespec if we do that) which is basically the old time_t
but is known to be y2038 safe because we only ever use it to store
monotonic times.
- return u32 and use the same type in the callers instead of
time_t/time64_t/monotonic_time_t.

> 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.

Heena already posted a first draft of that patch to the opw internal
mailing list, I found a small issue that needs to be resolved and
then she can post the new version to you for review.

Arnd