linux-kernel - Re: [PATCH v5] timekeeping: Added a function to return tv_sec portion of ktime_get

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Date:	Sat, 25 Oct 2014 17:22:23 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Heena Sirwani <heenasirwani@...il.com>
cc:	linux-kernel@...r.kernel.org, john.stultz@...aro.org, arnd@...db.de
Subject: Re: [PATCH v5] timekeeping: Added a function to return tv_sec portion
 of ktime_get_ts64()

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
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