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

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Date:	Sun, 26 Oct 2014 22:01:28 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Thomas Gleixner <tglx@...utronix.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 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
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