x86: Reduce clock calibration time during slave cpu startup

Reduce the startup time for slave cpus.

Adds hooks for an arch-specific function for clock calibration.
These hooks are used on x86.  If a newly started cpu has the
same phys_proc_id as a core already active, uses the TSC for the
delay loop and has a CONSTANT_TSC, use the already-calculated
value of loops_per_jiffy.

This patch reduces the time required to start slave cpus on a
4096 cpu system from: 465 sec OLD 62 sec NEW

This reduces boot time on a 4096p system by almost 7 minutes.
Nice...

Signed-off-by: Jack Steiner <steiner@sgi.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: John Stultz <john.stultz@linaro.org>
[fix CONFIG_SMP=n build]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 15 insertions, 0 deletions

diff --git a/init/calibrate.c b/init/calibrate.c
index 24df7976816c..5f117ca9e069 100644
--- a/init/calibrate.c
+++ b/init/calibrate.c
@@ -246,6 +246,19 @@ recalibrate:
 
 static DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = { 0 };
 
+/*
+ * Check if cpu calibration delay is already known. For example,
+ * some processors with multi-core sockets may have all cores
+ * with the same calibration delay.
+ *
+ * Architectures should override this function if a faster calibration
+ * method is available.
+ */
+unsigned long __attribute__((weak)) __cpuinit calibrate_delay_is_known(void)
+{
+        return 0;
+}
+
 void __cpuinit calibrate_delay(void)
 {
         unsigned long lpj;
@@ -265,6 +278,8 @@ void __cpuinit calibrate_delay(void)
                 lpj = lpj_fine;
                 pr_info("Calibrating delay loop (skipped), "
                         "value calculated using timer frequency.. ");
+        } else if ((lpj = calibrate_delay_is_known())) {
+                ;
         } else if ((lpj = calibrate_delay_direct()) != 0) {
                 if (!printed)
                         pr_info("Calibrating delay using timer "