Re: [PATCH RFC V2 5/6] time: move leap second management into time keeping core

[Richard Cochran <richardcochran@gmail.com>]

On Mon, May 21, 2012 at 11:18:12AM -0700, John Stultz wrote:
> On 05/18/2012 07:09 AM, Richard Cochran wrote:
> >This patch refactors the timekeeping and ntp code in order to
> >improve leap second handling and to provide a TAI based clock.
> >The change has a number of aspects.
> >
> >* remove the NTP time_status variable
> >
> >Instead, compute this value on demand in adjtimex.
> >
> >* move TAI managment into timekeeping core from ntp
> >
> >Currently NTP manages the TAI offset. Since there's plans for a
> >CLOCK_TAI clockid, push the TAI management into the timekeeping
> >core.
> >
> >[ Original idea from: John Stultz<john.stultz@linaro.org>  ]
> >[ Changed by RC: ]
> >
> >   - replace u32 with time_t
> >   - fixup second call site of second_overflow()
> >
> >* replace modulus with integer test and schedule leap second
> >
> >On the day of a leap second, the NTP code performs a division on every
> >tick in order to know when to leap. This patch replaces the division
> >with an integer comparison, making the code faster and easier to
> >understand.
> >
> >Signed-off-by: Richard Cochran<richardcochran@gmail.com>
> >---
> >  include/linux/time.h      |    1 +
> >  kernel/time/ntp.c         |   86 +++++++++++++-------------------------------
> >  kernel/time/timekeeping.c |   54 ++++++++++++++++++++++++----
> >  3 files changed, 73 insertions(+), 68 deletions(-)
> >
> >diff --git a/include/linux/time.h b/include/linux/time.h
> >index 179f4d6..b6034b0 100644
> >--- a/include/linux/time.h
> >+++ b/include/linux/time.h
> >@@ -168,6 +168,7 @@ extern struct timespec timespec_trunc(struct timespec t, unsigned gran);
> >  extern int timekeeping_valid_for_hres(void);
> >  extern u64 timekeeping_max_deferment(void);
> >  extern int timekeeping_inject_offset(struct timespec *ts);
> >+extern void timekeeping_set_tai_offset(time_t tai_offset);
> >
> >  struct tms;
> >  extern void do_sys_times(struct tms *);
> >diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
> >index d4d48b0..53c3227 100644
> >--- a/kernel/time/ntp.c
> >+++ b/kernel/time/ntp.c
> >@@ -16,6 +16,7 @@
> >  #include<linux/mm.h>
> >  #include<linux/module.h>
> >
> >+#include "leap-seconds.h"
> >  #include "tick-internal.h"
> >
> >  /*
> >@@ -24,6 +25,7 @@
> >
> >  DEFINE_SPINLOCK(ntp_lock);
> >
> >+#define STA_LEAP		(STA_INS | STA_DEL)
> >
> >  /* USER_HZ period (usecs): */
> >  unsigned long			tick_usec = TICK_USEC;
> >@@ -42,19 +44,9 @@ static u64			tick_length_base;
> >   * phase-lock loop variables
> >   */
> >
> >-/*
> >- * clock synchronization status
> >- *
> >- * (TIME_ERROR prevents overwriting the CMOS clock)
> >- */
> >-static int			time_state = TIME_OK;
> >-
> >  /* clock status bits:							*/
> >  static int			time_status = STA_UNSYNC;
> >
> >-/* TAI offset (secs):							*/
> >-static long			time_tai;
> >-
> >  /* time adjustment (nsecs):						*/
> >  static s64			time_offset;
> >
> >@@ -386,57 +378,14 @@ u64 ntp_tick_length(void)
> >   * They were originally developed for SUN and DEC kernels.
> >   * All the kudos should go to Dave for this stuff.
> >   *
> >- * Also handles leap second processing, and returns leap offset
> >   */
> >  int second_overflow(unsigned long secs)
> 
> Might be good to rename second_overflow() to ntp_second_overflow()
> and drop passing the time_t as its now unused.

Okay
 
> >  {
> >  	s64 delta;
> >-	int leap = 0;
> >  	unsigned long flags;
> >
> >  	spin_lock_irqsave(&ntp_lock, flags);
> >
> >-	/*
> >-	 * Leap second processing. If in leap-insert state at the end of the
> >-	 * day, the system clock is set back one second; if in leap-delete
> >-	 * state, the system clock is set ahead one second.
> >-	 */
> >-	switch (time_state) {
> >-	case TIME_OK:
> >-		if (time_status&  STA_INS)
> >-			time_state = TIME_INS;
> >-		else if (time_status&  STA_DEL)
> >-			time_state = TIME_DEL;
> >-		break;
> >-	case TIME_INS:
> >-		if (secs % 86400 == 0) {
> >-			leap = -1;
> >-			time_state = TIME_OOP;
> >-			time_tai++;
> >-			printk(KERN_NOTICE
> >-				"Clock: inserting leap second 23:59:60 UTC\n");
> >-		}
> >-		break;
> >-	case TIME_DEL:
> >-		if ((secs + 1) % 86400 == 0) {
> >-			leap = 1;
> >-			time_tai--;
> >-			time_state = TIME_WAIT;
> >-			printk(KERN_NOTICE
> >-				"Clock: deleting leap second 23:59:59 UTC\n");
> >-		}
> >-		break;
> >-	case TIME_OOP:
> >-		time_state = TIME_WAIT;
> >-		break;
> >-
> >-	case TIME_WAIT:
> >-		if (!(time_status&  (STA_INS | STA_DEL)))
> >-			time_state = TIME_OK;
> >-		break;
> >-	}
> >-
> >-
> >  	/* Bump the maxerror field */
> >  	time_maxerror += MAXFREQ / NSEC_PER_USEC;
> >  	if (time_maxerror>  NTP_PHASE_LIMIT) {
> >@@ -478,7 +427,7 @@ int second_overflow(unsigned long secs)
> >  out:
> >  	spin_unlock_irqrestore(&ntp_lock, flags);
> >
> >-	return leap;
> >+	return 0;
> >  }
> [snip]
> 
> >-	getnstimeofday(&ts);
> >+	result = timekeeping_gettod_status(&ts,&orig_tai);
> >+	time_tai = orig_tai;
> >+
> >+	if (txc->modes&  ADJ_STATUS) {
> >+		/*
> >+		 * Check for new leap second commands.
> >+		 */
> >+		if (!(time_status&  STA_INS)&&  (txc->status&  STA_INS))
> >+			timekeeping_insert_leap_second();
> >+
> >+		else if (!(time_status&  STA_DEL)&&  (txc->status&  STA_DEL))
> >+			timekeeping_delete_leap_second();
> >+
> >+		else if ((time_status&  STA_LEAP)&&  !(txc->status&  STA_LEAP))
> >+			timekeeping_finish_leap_second();
> >+	}
> Doing this early doesn't run into any trouble with the tcx->modes
> rules, right?
> 
> I'm just worried about changes in behavior if  modes =
> ADJ_ADJTIME|ADJ_STATUS

I wouldn't worry too much. The whole ntp adjtimex thing is such a mess
anyhow. Garbage in, garbage out.

Let's turn the question around: What is the expected behavior of
ADJ_ADJTIME|ADJ_STATUS?

[ hint: not really well defined, I think you will find ]

> 
> 
> >  	spin_lock_irq(&ntp_lock);
> >
> >@@ -673,7 +637,7 @@ int do_adjtimex(struct timex *txc)
> >
> >  		/* If there are input parameters, then process them: */
> >  		if (txc->modes)
> >-			process_adjtimex_modes(txc);
> >+			process_adjtimex_modes(txc,&time_tai);
> >
> >  		txc->offset = shift_right(time_offset * NTP_INTERVAL_FREQ,
> >  				  NTP_SCALE_SHIFT);
> >@@ -681,7 +645,6 @@ int do_adjtimex(struct timex *txc)
> >  			txc->offset /= NSEC_PER_USEC;
> >  	}
> >
> >-	result = time_state;	/* mostly `TIME_OK' */
> >  	/* check for errors */
> >  	if (is_error_status(time_status))
> >  		result = TIME_ERROR;
> >@@ -702,6 +665,9 @@ int do_adjtimex(struct timex *txc)
> >
> >  	spin_unlock_irq(&ntp_lock);
> >
> >+	if (time_tai != orig_tai&&  result == TIME_OK)
> >+		timekeeping_set_tai_offset(time_tai);
> >+
> >  	txc->time.tv_sec = ts.tv_sec;
> >  	txc->time.tv_usec = ts.tv_nsec;
> >  	if (!(time_status&  STA_NANO))
> >diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> >index eab03fb..fdd1a48 100644
> >--- a/kernel/time/timekeeping.c
> >+++ b/kernel/time/timekeeping.c
> >@@ -444,6 +444,18 @@ int timekeeping_inject_offset(struct timespec *ts)
> >  EXPORT_SYMBOL(timekeeping_inject_offset);
> >
> >  /**
> >+ * timekeeping_set_tai_offset - Sets the current TAI offset from UTC
> >+ */
> >+void timekeeping_set_tai_offset(time_t tai_offset)
> >+{
> >+	unsigned long flags;
> >+
> >+	write_seqlock_irqsave(&timekeeper.lock, flags);
> >+	timekeeper.tai_offset = tai_offset;
> >+	write_sequnlock_irqrestore(&timekeeper.lock, flags);
> >+}
> >+
> >+/**
> >   * change_clocksource - Swaps clocksources if a new one is available
> >   *
> >   * Accumulates current time interval and initializes new clocksource
> >@@ -950,6 +962,38 @@ static void timekeeping_adjust(s64 offset)
> >  				timekeeper.ntp_error_shift;
> >  }
> >
> >+static void timekeeper_overflow(void)
> 
> Mind renaming this to timekeeper_second_overflow, just so readers
> don't mix it up with clocksource related overflows.

Okay

Thanks,
Richard