[PATCH v3 3/9] cpu/hotplug: Add dynamic parallel bringup states before CPUHP_BRINGUP_CPU

[David Woodhouse <dwmw2@infradead.org>]

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

If the platform registers these states, bring all CPUs to each registered
state in turn, before the final bringup to CPUHP_BRINGUP_CPU. This allows
the architecture to parallelise the slow asynchronous tasks like sending
INIT/SIPI and waiting for the AP to come to life.

There is a subtlety here: even with an empty CPUHP_BP_PARALLEL_DYN step,
this means that *all* CPUs are brought through the prepare states and to
CPUHP_BP_PREPARE_DYN before any of them are taken to CPUHP_BRINGUP_CPU
and then are allowed to run for themselves to CPUHP_ONLINE.

So any combination of prepare/start calls which depend on A-B ordering
for each CPU in turn, such as the X2APIC code which used to allocate a
cluster mask 'just in case' and store it in a global variable in the
prep stage, then potentially consume that preallocated structure from
the AP and set the global pointer to NULL to be reallocated in
CPUHP_X2APIC_PREPARE for the next CPU... would explode horribly.

We believe that X2APIC was the only such case, for x86. But this is why
it remains an architecture opt-in. For now.

Note that the new parallel stages do *not* yet bring each AP to the
CPUHP_BRINGUP_CPU state. The final loop in bringup_nonboot_cpus() is
untouched, bringing each AP in turn from the final PARALLEL_DYN state
(or all the way from CPUHP_OFFLINE) to CPUHP_BRINGUP_CPU and then
waiting for that AP to do its own processing and reach CPUHP_ONLINE
before releasing the next. Parallelising that part by bringing them all
to CPUHP_BRINGUP_CPU and then waiting for them all is an exercise for
the future.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 include/linux/cpuhotplug.h |  2 ++
 kernel/cpu.c               | 27 +++++++++++++++++++++++++--
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 773c83730906..45c327538321 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -131,6 +131,8 @@ enum cpuhp_state {
 	CPUHP_MIPS_SOC_PREPARE,
 	CPUHP_BP_PREPARE_DYN,
 	CPUHP_BP_PREPARE_DYN_END		= CPUHP_BP_PREPARE_DYN + 20,
+	CPUHP_BP_PARALLEL_DYN,
+	CPUHP_BP_PARALLEL_DYN_END		= CPUHP_BP_PARALLEL_DYN + 4,
 	CPUHP_BRINGUP_CPU,
 
 	/*
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 192e43a87407..1a46eb57d8f7 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1462,6 +1462,24 @@ int bringup_hibernate_cpu(unsigned int sleep_cpu)
 void bringup_nonboot_cpus(unsigned int setup_max_cpus)
 {
 	unsigned int cpu;
+	int n = setup_max_cpus - num_online_cpus();
+
+	/* ∀ parallel pre-bringup state, bring N CPUs to it */
+	if (n > 0) {
+		enum cpuhp_state st = CPUHP_BP_PARALLEL_DYN;
+
+		while (st <= CPUHP_BP_PARALLEL_DYN_END &&
+		       cpuhp_hp_states[st].name) {
+			int i = n;
+
+			for_each_present_cpu(cpu) {
+				cpu_up(cpu, st);
+				if (!--i)
+					break;
+			}
+			st++;
+		}
+	}
 
 	for_each_present_cpu(cpu) {
 		if (num_online_cpus() >= setup_max_cpus)
@@ -1829,6 +1847,10 @@ static int cpuhp_reserve_state(enum cpuhp_state state)
 		step = cpuhp_hp_states + CPUHP_BP_PREPARE_DYN;
 		end = CPUHP_BP_PREPARE_DYN_END;
 		break;
+	case CPUHP_BP_PARALLEL_DYN:
+		step = cpuhp_hp_states + CPUHP_BP_PARALLEL_DYN;
+		end = CPUHP_BP_PARALLEL_DYN_END;
+		break;
 	default:
 		return -EINVAL;
 	}
@@ -1853,14 +1875,15 @@ static int cpuhp_store_callbacks(enum cpuhp_state state, const char *name,
 	/*
 	 * If name is NULL, then the state gets removed.
 	 *
-	 * CPUHP_AP_ONLINE_DYN and CPUHP_BP_PREPARE_DYN are handed out on
+	 * CPUHP_AP_ONLINE_DYN and CPUHP_BP_P*_DYN are handed out on
 	 * the first allocation from these dynamic ranges, so the removal
 	 * would trigger a new allocation and clear the wrong (already
 	 * empty) state, leaving the callbacks of the to be cleared state
 	 * dangling, which causes wreckage on the next hotplug operation.
 	 */
 	if (name && (state == CPUHP_AP_ONLINE_DYN ||
-		     state == CPUHP_BP_PREPARE_DYN)) {
+		     state == CPUHP_BP_PREPARE_DYN ||
+		     state == CPUHP_BP_PARALLEL_DYN)) {
 		ret = cpuhp_reserve_state(state);
 		if (ret < 0)
 			return ret;
-- 
2.31.1