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Subject[PATCH 3/7] smp/hotplug: Rewrite AP state machine core
There is currently no explicit state change on rollback. That is,
st->bringup, st->rollback and st->target are not consistent when doing
the rollback.

Rework the AP state handling to be more coherent. This does mean we
have to do a second AP kick-and-wait for rollback, but since rollback
is the slow path of a slowpath, this really should not matter.

Take this opportunity to simplify the AP thread function to only run a
single callback per invocation. This unifies the three single/up/down
modes is supports. The looping it used to do for up/down are achieved
by retaining should_run and relying on the main smpboot_thread_fn()
loop.

(I have most of a patch that does the same for the BP state handling,
but that's not critical and gets a little complicated because
CPUHP_BRINGUP_CPU does the AP handoff from a callback, which gets
recursive @st usage, I still have de-fugly that.)

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/cpu.c | 260 +++++++++++++++++++++++++++++++++++++++--------------------
1 file changed, 176 insertions(+), 84 deletions(-)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -58,6 +58,7 @@ struct cpuhp_cpu_state {
bool single;
bool bringup;
struct hlist_node *node;
+ struct hlist_node *last;
enum cpuhp_state cb_state;
int result;
struct completion done;
@@ -112,6 +113,14 @@ static bool cpuhp_is_ap_state(enum cpuhp
return state > CPUHP_BRINGUP_CPU && state != CPUHP_TEARDOWN_CPU;
}

+/*
+ * The former STARTING/DYING states, ran with IRQs disabled and must not fail.
+ */
+static bool cpuhp_is_atomic_state(enum cpuhp_state state)
+{
+ return CPUHP_AP_IDLE_DEAD <= state && state < CPUHP_AP_ONLINE;
+}
+
static struct cpuhp_step *cpuhp_get_step(enum cpuhp_state state)
{
struct cpuhp_step *sp;
@@ -286,7 +295,72 @@ void cpu_hotplug_enable(void)
EXPORT_SYMBOL_GPL(cpu_hotplug_enable);
#endif /* CONFIG_HOTPLUG_CPU */

-static void __cpuhp_kick_ap_work(struct cpuhp_cpu_state *st);
+static inline enum cpuhp_state
+cpuhp_set_state(struct cpuhp_cpu_state *st, enum cpuhp_state target)
+{
+ enum cpuhp_state prev_state = st->state;
+
+ st->rollback = false;
+ st->last = NULL;
+
+ st->target = target;
+ st->single = false;
+ st->bringup = st->state < target;
+
+ return prev_state;
+}
+
+static inline void
+cpuhp_reset_state(struct cpuhp_cpu_state *st, enum cpuhp_state prev_state)
+{
+ st->rollback = true;
+
+ /*
+ * If we have st->last we need to undo partial multi_instance of this
+ * state first. Otherwise start undo at the previous state.
+ */
+ if (!st->last) {
+ if (st->bringup)
+ st->state--;
+ else
+ st->state++;
+ }
+
+ st->target = prev_state;
+ st->bringup = !st->bringup;
+}
+
+/* Regular hotplug invocation of the AP hotplug thread */
+static void __cpuhp_kick_ap(struct cpuhp_cpu_state *st)
+{
+ if (!st->single && st->state == st->target)
+ return;
+
+ st->result = 0;
+ /*
+ * Make sure the above stores are visible before should_run becomes
+ * true. Paired with the mb() above in cpuhp_thread_fun()
+ */
+ smp_mb();
+ st->should_run = true;
+ wake_up_process(st->thread);
+ wait_for_completion(&st->done);
+}
+
+static int cpuhp_kick_ap(struct cpuhp_cpu_state *st, enum cpuhp_state target)
+{
+ enum cpuhp_state prev_state;
+ int ret;
+
+ prev_state = cpuhp_set_state(st, target);
+ __cpuhp_kick_ap(st);
+ if ((ret = st->result)) {
+ cpuhp_reset_state(st, prev_state);
+ __cpuhp_kick_ap(st);
+ }
+
+ return ret;
+}

static int bringup_wait_for_ap(unsigned int cpu)
{
@@ -301,12 +375,10 @@ static int bringup_wait_for_ap(unsigned
stop_machine_unpark(cpu);
kthread_unpark(st->thread);

- /* Should we go further up ? */
- if (st->target > CPUHP_AP_ONLINE_IDLE) {
- __cpuhp_kick_ap_work(st);
- wait_for_completion(&st->done);
- }
- return st->result;
+ if (st->target <= CPUHP_AP_ONLINE_IDLE)
+ return 0;
+
+ return cpuhp_kick_ap(st, st->target);
}

static int bringup_cpu(unsigned int cpu)
@@ -404,71 +476,90 @@ static int cpuhp_should_run(unsigned int
return st->should_run;
}

-/* Execute the teardown callbacks. Used to be CPU_DOWN_PREPARE */
-static int cpuhp_ap_offline(unsigned int cpu, struct cpuhp_cpu_state *st)
-{
- enum cpuhp_state target = max((int)st->target, CPUHP_TEARDOWN_CPU);
-
- return cpuhp_down_callbacks(cpu, st, target);
-}
-
-/* Execute the online startup callbacks. Used to be CPU_ONLINE */
-static int cpuhp_ap_online(unsigned int cpu, struct cpuhp_cpu_state *st)
-{
- return cpuhp_up_callbacks(cpu, st, st->target);
-}
-
/*
* Execute teardown/startup callbacks on the plugged cpu. Also used to invoke
* callbacks when a state gets [un]installed at runtime.
+ *
+ * Each invocation of this function by the smpboot thread does a single AP
+ * state callback.
+ *
+ * It has 3 modes of operation:
+ * - single: runs st->cb_state
+ * - up: runs ++st->state, while st->state < st->target
+ * - down: runs st->state--, while st->state > st->target
+ *
+ * When complete or on error, should_run is cleared and the completion is fired.
*/
static void cpuhp_thread_fun(unsigned int cpu)
{
struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
- int ret = 0;
+ bool bringup = st->bringup;
+ enum cpuhp_state state;

/*
- * Paired with the mb() in cpuhp_kick_ap_work and
- * cpuhp_invoke_ap_callback, so the work set is consistent visible.
+ * ACQUIRE for the cpuhp_should_run() load of ->should_run. Ensures
+ * that if we see ->should_run we also see the rest of the state.
*/
smp_mb();
- if (!st->should_run)
- return;

- st->should_run = false;
+ if (WARN_ON_ONCE(!st->should_run))
+ return;

lock_map_acquire(&cpuhp_state_lock_map);
- /* Single callback invocation for [un]install ? */
+
if (st->single) {
- if (st->cb_state < CPUHP_AP_ONLINE) {
- local_irq_disable();
- ret = cpuhp_invoke_callback(cpu, st->cb_state,
- st->bringup, st->node,
- NULL);
- local_irq_enable();
+ state = st->cb_state;
+ st->should_run = false;
+ } else {
+ if (bringup) {
+ st->state++;
+ state = st->state;
+ st->should_run = (st->state < st->target);
+ WARN_ON_ONCE(st->state > st->target);
} else {
- ret = cpuhp_invoke_callback(cpu, st->cb_state,
- st->bringup, st->node,
- NULL);
+ state = st->state;
+ st->state--;
+ st->should_run = (st->state > st->target);
+ WARN_ON_ONCE(st->state < st->target);
}
- } else if (st->rollback) {
- BUG_ON(st->state < CPUHP_AP_ONLINE_IDLE);
+ }
+
+ WARN_ON_ONCE(!cpuhp_is_ap_state(state));
+
+ if (st->rollback) {
+ struct cpuhp_step *step = cpuhp_get_step(state);
+ if (step->skip_onerr)
+ goto next;
+ }

- undo_cpu_down(cpu, st);
- st->rollback = false;
+ if (cpuhp_is_atomic_state(state)) {
+ local_irq_disable();
+ st->result = cpuhp_invoke_callback(cpu, state, bringup, st->node, &st->last);
+ local_irq_enable();
+
+ /*
+ * STARTING/DYING must not fail!
+ */
+ WARN_ON_ONCE(st->result);
} else {
- /* Cannot happen .... */
- BUG_ON(st->state < CPUHP_AP_ONLINE_IDLE);
+ st->result = cpuhp_invoke_callback(cpu, state, bringup, st->node, &st->last);
+ }

- /* Regular hotplug work */
- if (st->state < st->target)
- ret = cpuhp_ap_online(cpu, st);
- else if (st->state > st->target)
- ret = cpuhp_ap_offline(cpu, st);
+ if (st->result) {
+ /*
+ * If we fail on a rollback, we're up a creek without no
+ * paddle, no way forward, no way back. We loose, thanks for
+ * playing.
+ */
+ WARN_ON_ONCE(st->rollback);
+ st->should_run = false;
}
+
+next:
lock_map_release(&cpuhp_state_lock_map);
- st->result = ret;
- complete(&st->done);
+
+ if (!st->should_run)
+ complete(&st->done);
}

/* Invoke a single callback on a remote cpu */
@@ -477,6 +568,7 @@ cpuhp_invoke_ap_callback(int cpu, enum c
struct hlist_node *node)
{
struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
+ int ret;

if (!cpu_online(cpu))
return 0;
@@ -491,48 +583,43 @@ cpuhp_invoke_ap_callback(int cpu, enum c
if (!st->thread)
return cpuhp_invoke_callback(cpu, state, bringup, node, NULL);

+ st->rollback = false;
+ st->last = NULL;
+
+ st->node = node;
+ st->bringup = bringup;
st->cb_state = state;
st->single = true;
- st->bringup = bringup;
- st->node = node;

- /*
- * Make sure the above stores are visible before should_run becomes
- * true. Paired with the mb() above in cpuhp_thread_fun()
- */
- smp_mb();
- st->should_run = true;
- wake_up_process(st->thread);
- wait_for_completion(&st->done);
- return st->result;
-}
+ __cpuhp_kick_ap(st);

-/* Regular hotplug invocation of the AP hotplug thread */
-static void __cpuhp_kick_ap_work(struct cpuhp_cpu_state *st)
-{
- st->result = 0;
- st->single = false;
/*
- * Make sure the above stores are visible before should_run becomes
- * true. Paired with the mb() above in cpuhp_thread_fun()
+ * If we failed and did a partial, do a rollback.
*/
- smp_mb();
- st->should_run = true;
- wake_up_process(st->thread);
+ if ((ret = st->result) && st->last) {
+ st->rollback = true;
+ st->bringup = !bringup;
+
+ __cpuhp_kick_ap(st);
+ }
+
+ return ret;
}

static int cpuhp_kick_ap_work(unsigned int cpu)
{
struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
- enum cpuhp_state state = st->state;
+ enum cpuhp_state prev_state = st->state;
+ int ret;

- trace_cpuhp_enter(cpu, st->target, state, cpuhp_kick_ap_work);
lock_map_acquire(&cpuhp_state_lock_map);
lock_map_release(&cpuhp_state_lock_map);
- __cpuhp_kick_ap_work(st);
- wait_for_completion(&st->done);
- trace_cpuhp_exit(cpu, st->state, state, st->result);
- return st->result;
+
+ trace_cpuhp_enter(cpu, st->target, prev_state, cpuhp_kick_ap_work);
+ ret = cpuhp_kick_ap(st, st->target);
+ trace_cpuhp_exit(cpu, st->state, prev_state, ret);
+
+ return ret;
}

static struct smp_hotplug_thread cpuhp_threads = {
@@ -716,13 +803,13 @@ static int __ref _cpu_down(unsigned int

cpuhp_tasks_frozen = tasks_frozen;

- prev_state = st->state;
- st->target = target;
+ prev_state = cpuhp_set_state(st, target);
/*
* If the current CPU state is in the range of the AP hotplug thread,
* then we need to kick the thread.
*/
if (st->state > CPUHP_TEARDOWN_CPU) {
+ st->target = max((int)target, CPUHP_TEARDOWN_CPU);
ret = cpuhp_kick_ap_work(cpu);
/*
* The AP side has done the error rollback already. Just
@@ -737,6 +824,8 @@ static int __ref _cpu_down(unsigned int
*/
if (st->state > CPUHP_TEARDOWN_CPU)
goto out;
+
+ st->target = target;
}
/*
* The AP brought itself down to CPUHP_TEARDOWN_CPU. So we need
@@ -744,9 +833,8 @@ static int __ref _cpu_down(unsigned int
*/
ret = cpuhp_down_callbacks(cpu, st, target);
if (ret && st->state > CPUHP_TEARDOWN_CPU && st->state < prev_state) {
- st->target = prev_state;
- st->rollback = true;
- cpuhp_kick_ap_work(cpu);
+ cpuhp_reset_state(st, prev_state);
+ __cpuhp_kick_ap(st);
}

out:
@@ -846,7 +934,7 @@ static int _cpu_up(unsigned int cpu, int

cpuhp_tasks_frozen = tasks_frozen;

- st->target = target;
+ cpuhp_set_state(st, target);
/*
* If the current CPU state is in the range of the AP hotplug thread,
* then we need to kick the thread once more.
@@ -1313,6 +1401,10 @@ static int cpuhp_issue_call(int cpu, enu
struct cpuhp_step *sp = cpuhp_get_step(state);
int ret;

+ /*
+ * If there's nothing to do, we done.
+ * Relies on the union for multi_instance.
+ */
if ((bringup && !sp->startup.single) ||
(!bringup && !sp->teardown.single))
return 0;

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