timekeeping: Provide NMI safe access to clock realtime

The configurable printk timestamping wants access to clock realtime. Right
now there is no ktime_get_real_fast_ns() accessor because reading the
monotonic base and the realtime offset cannot be done atomically. Contrary
to boot time this offset can change during runtime and cause half updated
readouts.

struct tk_read_base was fully packed when the fast timekeeper access was
implemented. commit ceea5e3771ed ("time: Fix clock->read(clock) race around
clocksource changes") removed the 'read' function pointer from the
structure, but of course left the comment stale.

So now the structure can fit a new 64bit member w/o violating the cache
line constraints.

Add real_base to tk_read_base and update it in the fast timekeeper update
sequence.

Implement an accessor which follows the same scheme as the accessor to
clock monotonic, but uses the new real_base to access clock real time.

The runtime overhead for updating real_base is minimal as it just adds two
cache hot values and stores them into an already dirtied cache line along
with the other fast timekeeper updates.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead,org>
Link: https://lkml.kernel.org/r/1505757060-2004-3-git-send-email-prarit@redhat.com

1 files changed, 5 insertions, 1 deletions

diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index 0a0a53daf2a2..98f645ee8409 100644
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -13,19 +13,22 @@
 /**
  * struct tk_read_base - base structure for timekeeping readout
  * @clock:	Current clocksource used for timekeeping.
- * @read:	Read function of @clock
  * @mask:	Bitmask for two's complement subtraction of non 64bit clocks
  * @cycle_last: @clock cycle value at last update
  * @mult:	(NTP adjusted) multiplier for scaled math conversion
  * @shift:	Shift value for scaled math conversion
  * @xtime_nsec: Shifted (fractional) nano seconds offset for readout
  * @base:	ktime_t (nanoseconds) base time for readout
+ * @base_real:	Nanoseconds base value for clock REALTIME readout
  *
  * This struct has size 56 byte on 64 bit. Together with a seqcount it
  * occupies a single 64byte cache line.
  *
  * The struct is separate from struct timekeeper as it is also used
  * for a fast NMI safe accessors.
+ *
+ * @base_real is for the fast NMI safe accessor to allow reading clock
+ * realtime from any context.
  */
 struct tk_read_base {
 	struct clocksource	*clock;
@@ -35,6 +38,7 @@ struct tk_read_base {
 	u32			shift;
 	u64			xtime_nsec;
 	ktime_t			base;
+	u64			base_real;
 };
 
 /**