[PATCH v4 1/3] x86/time: latch to-be-written TSC value early in rendezvous loop

[Jan Beulich <jbeulich@xxxxxxxx>]

To reduce latency on time_calibration_tsc_rendezvous()'s last loop
iteration, read the value to be written on the last iteration at the end
of the loop body (i.e. in particular at the end of the second to last
iteration).

On my single-socket 18-core Skylake system this reduces the average loop
exit time on CPU0 (from the TSC write on the last iteration to until
after the main loop) from around 32k cycles to around 29k (albeit the
values measured on separate runs vary quite significantly).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v4: Different approach.
v3: New.
---
Of course it would also be nice to avoid the pretty likely branch
misprediction on the last iteration. But with the static prediction
hints having been rather short-lived in the architecture, I don't see
any good means to do so.

--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1683,7 +1683,7 @@ static void time_calibration_tsc_rendezv
     int i;
     struct calibration_rendezvous *r = _r;
     unsigned int total_cpus = cpumask_weight(&r->cpu_calibration_map);
-    uint64_t tsc = 0;
+    uint64_t tsc = 0, master_tsc = 0;
 
     /* Loop to get rid of cache effects on TSC skew. */
     for ( i = 4; i >= 0; i-- )
@@ -1708,7 +1708,7 @@ static void time_calibration_tsc_rendezv
             atomic_inc(&r->semaphore);
 
             if ( i == 0 )
-                write_tsc(r->master_tsc_stamp);
+                write_tsc(master_tsc);
 
             while ( atomic_read(&r->semaphore) != (2*total_cpus - 1) )
                 cpu_relax();
@@ -1730,7 +1730,7 @@ static void time_calibration_tsc_rendezv
             }
 
             if ( i == 0 )
-                write_tsc(r->master_tsc_stamp);
+                write_tsc(master_tsc);
 
             atomic_inc(&r->semaphore);
             while ( atomic_read(&r->semaphore) > total_cpus )
@@ -1739,9 +1739,17 @@ static void time_calibration_tsc_rendezv
 
         /* Just in case a read above ended up reading zero. */
         tsc += !tsc;
+
+        /*
+         * To reduce latency of the TSC write on the last iteration,
+         * fetch the value to be written into a local variable. To avoid
+         * introducing yet another conditional branch (which the CPU may
+         * have difficulty predicting well) do this on all iterations.
+         */
+        master_tsc = r->master_tsc_stamp;
     }
 
-    time_calibration_rendezvous_tail(r, tsc, r->master_tsc_stamp);
+    time_calibration_rendezvous_tail(r, tsc, master_tsc);
 }
 
 /* Ordinary rendezvous function which does not modify TSC values. */