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