[patch v3 03/36] x86/smpboot: Avoid pointless delay calibration if TSC is synchronized

[Thomas Gleixner <tglx@linutronix.de>]

From: Thomas Gleixner <tglx@linutronix.de>

When TSC is synchronized across sockets then there is no reason to
calibrate the delay for the first CPU which comes up on a socket.

Just reuse the existing calibration value.

This removes 100ms pointlessly wasted time from CPU hotplug per socket.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mikelley@microsoft.com>


---
 arch/x86/kernel/smpboot.c |   38 ++++++++++++++++++++++++--------------
 arch/x86/kernel/tsc.c     |   20 ++++++++++++++++----
 2 files changed, 40 insertions(+), 18 deletions(-)
---

--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -178,10 +178,7 @@ static void smp_callin(void)
 	 */
 	apic_ap_setup();
 
-	/*
-	 * Save our processor parameters. Note: this information
-	 * is needed for clock calibration.
-	 */
+	/* Save our processor parameters. */
 	smp_store_cpu_info(cpuid);
 
 	/*
@@ -192,14 +189,6 @@ static void smp_callin(void)
 
 	ap_init_aperfmperf();
 
-	/*
-	 * Get our bogomips.
-	 * Update loops_per_jiffy in cpu_data. Previous call to
-	 * smp_store_cpu_info() stored a value that is close but not as
-	 * accurate as the value just calculated.
-	 */
-	calibrate_delay();
-	cpu_data(cpuid).loops_per_jiffy = loops_per_jiffy;
 	pr_debug("Stack at about %p\n", &cpuid);
 
 	wmb();
@@ -212,8 +201,24 @@ static void smp_callin(void)
 	cpumask_set_cpu(cpuid, cpu_callin_mask);
 }
 
+static void ap_calibrate_delay(void)
+{
+	/*
+	 * Calibrate the delay loop and update loops_per_jiffy in cpu_data.
+	 * smp_store_cpu_info() stored a value that is close but not as
+	 * accurate as the value just calculated.
+	 *
+	 * As this is invoked after the TSC synchronization check,
+	 * calibrate_delay_is_known() will skip the calibration routine
+	 * when TSC is synchronized across sockets.
+	 */
+	calibrate_delay();
+	cpu_data(smp_processor_id()).loops_per_jiffy = loops_per_jiffy;
+}
+
 static int cpu0_logical_apicid;
 static int enable_start_cpu0;
+
 /*
  * Activate a secondary processor.
  */
@@ -240,10 +245,15 @@ static void notrace start_secondary(void
 
 	/* otherwise gcc will move up smp_processor_id before the cpu_init */
 	barrier();
+	/* Check TSC synchronization with the control CPU: */
+	check_tsc_sync_target();
+
 	/*
-	 * Check TSC synchronization with the boot CPU:
+	 * Calibrate the delay loop after the TSC synchronization check.
+	 * This allows to skip the calibration when TSC is synchronized
+	 * across sockets.
 	 */
-	check_tsc_sync_target();
+	ap_calibrate_delay();
 
 	speculative_store_bypass_ht_init();
 
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1598,10 +1598,7 @@ void __init tsc_init(void)
 
 #ifdef CONFIG_SMP
 /*
- * If we have a constant TSC and are using the TSC for the delay loop,
- * we can skip clock calibration if another cpu in the same socket has already
- * been calibrated. This assumes that CONSTANT_TSC applies to all
- * cpus in the socket - this should be a safe assumption.
+ * Check whether existing calibration data can be reused.
  */
 unsigned long calibrate_delay_is_known(void)
 {
@@ -1609,6 +1606,21 @@ unsigned long calibrate_delay_is_known(v
 	int constant_tsc = cpu_has(&cpu_data(cpu), X86_FEATURE_CONSTANT_TSC);
 	const struct cpumask *mask = topology_core_cpumask(cpu);
 
+	/*
+	 * If TSC has constant frequency and TSC is synchronized across
+	 * sockets then reuse CPU0 calibration.
+	 */
+	if (constant_tsc && !tsc_unstable)
+		return cpu_data(0).loops_per_jiffy;
+
+	/*
+	 * If TSC has constant frequency and TSC is not synchronized across
+	 * sockets and this is not the first CPU in the socket, then reuse
+	 * the calibration value of an already online CPU on that socket.
+	 *
+	 * This assumes that CONSTANT_TSC is consistent for all CPUs in a
+	 * socket.
+	 */
 	if (!constant_tsc || !mask)
 		return 0;