vppinfra: improve timebase resilience

Deal with arbitrary kernel reference time changes: for example,
yanking the kernel reference clock back to a time before vpp started.

Best practice involves aligning the kernel reference clock with
reality prior to starting apps which use 10us granularity timers.

Compute change in the reference and cpu clocks. Recompute the vpp
start time reference and and total cpu clock count, using the current
clock tick rate.

Next, compute a new clock rate sample. If the sample seems sane,
factor it into the exponentially smoothed clock rate and recalculate
total cpu ticks based on the new clock rate.

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: Ib6132ffbbe0e6d140725676de5e35be112a31dfe

1 files changed, 20 insertions, 3 deletions

diff --git a/src/vppinfra/time.c b/src/vppinfra/time.c
index edeca1f9c2f..0b00a11421d 100644
--- a/src/vppinfra/time.c
+++ b/src/vppinfra/time.c
@@ -251,15 +251,32 @@ void
 clib_time_verify_frequency (clib_time_t * c)
 {
   f64 now_reference, delta_reference, delta_reference_max;
-  u64 delta_clock;
+  f64 delta_clock_in_seconds;
+  u64 now_clock, delta_clock;
   f64 new_clocks_per_second, delta;
 
   /* Ask the kernel and the CPU what time it is... */
   now_reference = unix_time_now ();
-  c->last_cpu_time = clib_cpu_time_now ();
+  now_clock = clib_cpu_time_now ();
 
-  /* Calculate a new clock rate sample */
+  /* Compute change in the reference clock */
   delta_reference = now_reference - c->last_verify_reference_time;
+
+  /* And change in the CPU clock */
+  delta_clock_in_seconds = (f64) (now_clock - c->last_verify_cpu_time) *
+    c->seconds_per_clock;
+
+  /*
+   * Recompute vpp start time reference, and total clocks
+   * using the current clock rate
+   */
+  c->init_reference_time += (delta_reference - delta_clock_in_seconds);
+  c->total_cpu_time = (now_reference - c->init_reference_time)
+    * c->clocks_per_second;
+
+  c->last_cpu_time = now_clock;
+
+  /* Calculate a new clock rate sample */
   delta_clock = c->last_cpu_time - c->last_verify_cpu_time;
 
   c->last_verify_cpu_time = c->last_cpu_time;