[PATCH v17 6/8] x86/smpboot: Send INIT/SIPI/SIPI to secondary CPUs in parallel

[Usama Arif <usama.arif@bytedance.com>]

From: David Woodhouse <dwmw@amazon.co.uk>

When the APs can find their own APIC ID without assistance, perform the
AP bringup in parallel.

Register a CPUHP_BP_PARALLEL_STARTUP stage "x86/cpu:kick" which just
calls do_boot_cpu() to deliver INIT/SIPI/SIPI to each AP in turn before
the normal native_cpu_up() does the rest of the hand-holding.

The APs will then take turns through the real mode code (which has its
own bitlock for exclusion) until they make it to their own stack, then
proceed through the first few lines of start_secondary() and execute
these parts in parallel:

 start_secondary()
    -> cr4_init()
    -> (some 32-bit only stuff so not in the parallel cases)
    -> cpu_init_secondary()
       -> cpu_init_exception_handling()
       -> cpu_init()
          -> wait_for_master_cpu()

At this point they wait for the BSP to set their bit in cpu_callout_mask
(from do_wait_cpu_initialized()), and release them to continue through
the rest of cpu_init() and beyond.

This reduces the time taken for bringup on my 28-thread Haswell system
from about 120ms to 80ms. On a socket 96-thread Skylake it takes the
bringup time from 500ms to 100ms.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Usama Arif <usama.arif@bytedance.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Kim Phillips <kim.phillips@amd.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
---
 arch/x86/kernel/smpboot.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 45f3d08321fe..0003f5e1740c 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -57,6 +57,7 @@
 #include <linux/pgtable.h>
 #include <linux/overflow.h>
 #include <linux/stackprotector.h>
+#include <linux/smpboot.h>
 
 #include <asm/acpi.h>
 #include <asm/cacheinfo.h>
@@ -993,7 +994,8 @@ static void announce_cpu(int cpu, int apicid)
 		node_width = num_digits(num_possible_nodes()) + 1; /* + '#' */
 
 	if (cpu == 1)
-		printk(KERN_INFO "x86: Booting SMP configuration:\n");
+		printk(KERN_INFO "x86: Booting SMP configuration in %s:\n",
+		       do_parallel_bringup ? "parallel" : "series");
 
 	if (system_state < SYSTEM_RUNNING) {
 		if (node != current_node) {
@@ -1326,9 +1328,12 @@ int native_cpu_up(unsigned int cpu, struct task_struct *tidle)
 {
 	int ret;
 
-	ret = do_cpu_up(cpu, tidle);
-	if (ret)
-		goto out;
+	/* If parallel AP bringup isn't enabled, perform the first steps now. */
+	if (!do_parallel_bringup) {
+		ret = do_cpu_up(cpu, tidle);
+		if (ret)
+			goto out;
+	}
 
 	ret = do_wait_cpu_initialized(cpu);
 	if (ret)
@@ -1348,6 +1353,12 @@ int native_cpu_up(unsigned int cpu, struct task_struct *tidle)
 	return ret;
 }
 
+/* Bringup step one: Send INIT/SIPI to the target AP */
+static int native_cpu_kick(unsigned int cpu)
+{
+	return do_cpu_up(cpu, idle_thread_get(cpu));
+}
+
 /**
  * arch_disable_smp_support() - disables SMP support for x86 at runtime
  */
@@ -1516,6 +1527,8 @@ static bool prepare_parallel_bringup(void)
 		smpboot_control = STARTUP_APICID_CPUID_01;
 	}
 
+	cpuhp_setup_state_nocalls(CPUHP_BP_PARALLEL_STARTUP, "x86/cpu:kick",
+				  native_cpu_kick, NULL);
 	return true;
 }
 
-- 
2.25.1