@@ -682,18 +682,18 @@ static void timekeeping_update_from_shadow(struct tk_data *tkd, unsigned int act
}
/**
- * timekeeping_forward_now - update clock to the current time
+ * timekeeping_forward - update clock to given cycle now value
* @tk: Pointer to the timekeeper to update
+ * @cycle_now: Current clocksource read value
*
* Forward the current clock to update its state since the last call to
* update_wall_time(). This is useful before significant clock changes,
* as it avoids having to deal with this time offset explicitly.
*/
-static void timekeeping_forward_now(struct timekeeper *tk)
+static void timekeeping_forward(struct timekeeper *tk, u64 cycle_now)
{
- u64 cycle_now, delta;
+ u64 delta;
- cycle_now = tk_clock_read(&tk->tkr_mono);
delta = clocksource_delta(cycle_now, tk->tkr_mono.cycle_last, tk->tkr_mono.mask,
tk->tkr_mono.clock->max_raw_delta);
tk->tkr_mono.cycle_last = cycle_now;
@@ -710,6 +710,21 @@ static void timekeeping_forward_now(struct timekeeper *tk)
}
}
+/**
+ * timekeeping_forward_now - update clock to the current time
+ * @tk: Pointer to the timekeeper to update
+ *
+ * Forward the current clock to update its state since the last call to
+ * update_wall_time(). This is useful before significant clock changes,
+ * as it avoids having to deal with this time offset explicitly.
+ */
+static void timekeeping_forward_now(struct timekeeper *tk)
+{
+ u64 cycle_now = tk_clock_read(&tk->tkr_mono);
+
+ timekeeping_forward(tk, cycle_now);
+}
+
/**
* ktime_get_real_ts64 - Returns the time of day in a timespec64.
* @ts: pointer to the timespec to be set
@@ -2151,6 +2166,45 @@ static u64 logarithmic_accumulation(struct timekeeper *tk, u64 offset,
return offset;
}
+static u64 timekeeping_accumulate(struct timekeeper *tk, u64 now, u64 offset,
+ unsigned int *clock_set)
+{
+ struct timekeeper *real_tk = &tk_core.timekeeper;
+ int shift = 0, maxshift;
+
+ /*
+ * If we have a sub-cycle_interval offset, we
+ * are likely doing a TK_FREQ_ADJ, so accumulate
+ * everything so we don't have a remainder offset
+ * when later adjusting the multiplier
+ */
+ if (offset < real_tk->cycle_interval) {
+ timekeeping_forward(tk, now);
+ *clock_set = 1;
+ return 0;
+ }
+
+ /*
+ * With NO_HZ we may have to accumulate many cycle_intervals
+ * (think "ticks") worth of time at once. To do this efficiently,
+ * we calculate the largest doubling multiple of cycle_intervals
+ * that is smaller than the offset. We then accumulate that
+ * chunk in one go, and then try to consume the next smaller
+ * doubled multiple.
+ */
+ shift = ilog2(offset) - ilog2(tk->cycle_interval);
+ shift = max(0, shift);
+ /* Bound shift to one less than what overflows tick_length */
+ maxshift = (64 - (ilog2(ntp_tick_length()) + 1)) - 1;
+ shift = min(shift, maxshift);
+ while (offset >= tk->cycle_interval) {
+ offset = logarithmic_accumulation(tk, offset, shift, clock_set);
+ if (offset < tk->cycle_interval << shift)
+ shift--;
+ }
+ return offset;
+}
+
/*
* timekeeping_advance - Updates the timekeeper to the current time and
* current NTP tick length
@@ -2160,8 +2214,7 @@ static bool timekeeping_advance(enum timekeeping_adv_mode mode)
struct timekeeper *tk = &tk_core.shadow_timekeeper;
struct timekeeper *real_tk = &tk_core.timekeeper;
unsigned int clock_set = 0;
- int shift = 0, maxshift;
- u64 offset;
+ u64 cycle_now, offset;
guard(raw_spinlock_irqsave)(&tk_core.lock);
@@ -2169,7 +2222,8 @@ static bool timekeeping_advance(enum timekeeping_adv_mode mode)
if (unlikely(timekeeping_suspended))
return false;
- offset = clocksource_delta(tk_clock_read(&tk->tkr_mono),
+ cycle_now = tk_clock_read(&tk->tkr_mono);
+ offset = clocksource_delta(cycle_now,
tk->tkr_mono.cycle_last, tk->tkr_mono.mask,
tk->tkr_mono.clock->max_raw_delta);
@@ -2177,24 +2231,7 @@ static bool timekeeping_advance(enum timekeeping_adv_mode mode)
if (offset < real_tk->cycle_interval && mode == TK_ADV_TICK)
return false;
- /*
- * With NO_HZ we may have to accumulate many cycle_intervals
- * (think "ticks") worth of time at once. To do this efficiently,
- * we calculate the largest doubling multiple of cycle_intervals
- * that is smaller than the offset. We then accumulate that
- * chunk in one go, and then try to consume the next smaller
- * doubled multiple.
- */
- shift = ilog2(offset) - ilog2(tk->cycle_interval);
- shift = max(0, shift);
- /* Bound shift to one less than what overflows tick_length */
- maxshift = (64 - (ilog2(ntp_tick_length())+1)) - 1;
- shift = min(shift, maxshift);
- while (offset >= tk->cycle_interval) {
- offset = logarithmic_accumulation(tk, offset, shift, &clock_set);
- if (offset < tk->cycle_interval<<shift)
- shift--;
- }
+ offset = timekeeping_accumulate(tk, cycle_now, offset, &clock_set);
/* Adjust the multiplier to correct NTP error */
timekeeping_adjust(tk, offset);
Lei Chen raised an issue with CLOCK_MONOTONIC_COARSE seeing time inconsistencies. Lei tracked down that this was being caused by the adjustment tk->tkr_mono.xtime_nsec -= offset; which is made to compensate for the unaccumulated cycles in offset when the mult value is adjusted forward, so that the non-_COARSE clockids don't see inconsistencies. However, the _COARSE clockids don't use the mult*offset value in their calculations, so this subtraction can cause the _COARSE clock ids to jump back a bit. Now, by design, this negative adjustment should be fine, because the logic run from timekeeping_adjust() is done after we accumulate approx mult*interval_cycles into xtime_nsec. The accumulated (mult*interval_cycles) will be larger then the (mult_adj*offset) value subtracted from xtime_nsec, and both operations are done together under the tk_core.lock, so the net change to xtime_nsec should always be positive. However, do_adjtimex() calls into timekeeping_advance() as well, since we want to apply the ntp freq adjustment immediately. In this case, we don't return early when the offset is smaller then interval_cycles, so we don't end up accumulating any time into xtime_nsec. But we do go on to call timekeeping_adjust(), which modifies the mult value, and subtracts from xtime_nsec to correct for the new mult value. Here because we did not accumulate anything, we have a window where the _COARSE clockids that don't utilize the mult*offset value, can see an inconsistency. So to fix this, rework the timekeeping_advance() logic a bit so that when we are called from do_adjtimex() and the offset is smaller then cycle_interval, that we call timekeeping_forward(), to first accumulate the sub-interval time into xtime_nsec. Then with no unaccumulated cycles in offset, we can do the mult adjustment without worry of the subtraction having an impact. NOTE: This was implemented as a potential alternative to Thomas' approach here: https://lore.kernel.org/lkml/87cyej5rid.ffs@tglx/ And similarly, it needs some additional review and testing, as it was developed while packing for conference travel. Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Stephen Boyd <sboyd@kernel.org> Cc: Anna-Maria Behnsen <anna-maria@linutronix.de> Cc: Frederic Weisbecker <frederic@kernel.org> Cc: Shuah Khan <shuah@kernel.org> Cc: Miroslav Lichvar <mlichvar@redhat.com> Cc: linux-kselftest@vger.kernel.org Cc: kernel-team@android.com Cc: Lei Chen <lei.chen@smartx.com> Fixes: da15cfdae033 ("time: Introduce CLOCK_REALTIME_COARSE") Reported-by: Lei Chen <lei.chen@smartx.com> Closes: https://lore.kernel.org/lkml/20250310030004.3705801-1-lei.chen@smartx.com/ Diagnosed-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: John Stultz <jstultz@google.com> --- kernel/time/timekeeping.c | 87 ++++++++++++++++++++++++++++----------- 1 file changed, 62 insertions(+), 25 deletions(-)