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

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Date:	Mon, 27 Oct 2014 22:21:12 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Arnd Bergmann <arnd@...db.de>
cc:	Heena Sirwani <heenasirwani@...il.com>,
	linux-kernel@...r.kernel.org, john.stultz@...aro.org
Subject: Re: [PATCH v5] timekeeping: Added a function to return tv_sec portion
 of ktime_get_ts64()

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
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