[PATCH v2] rcu: Deoffload rdp if rcuop/rcuog kthreads spawn failed

Zqiang
2022-03-14T02:32:45Z

When CONFIG_RCU_NOCB_CPU is enabled and 'rcu_nocbs' is set, the rcuop
and rcuog kthreads is created. however the rcuop or rcuog kthreads
creation may fail, if failed, deoffload per-cpu rdp which belong
to rcu_nocb_mask.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
---
v1->v2:
Invert the locking dependency order between
rcu_state.barrier_mutex and hotplug lock.

Holding nocb_gp_kthread_mutex, ensure that
the nocb_gp_kthread exists.

kernel/rcu/tree_nocb.h | 63 +++++++++++++++++++++++++++++++-----------
1 file changed, 47 insertions(+), 16 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 46694e13398a..4ec96d0c11de 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -972,10 +972,7 @@ static int rdp_offload_toggle(struct rcu_data *rdp,
}
raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);

- if (wake_gp)
- wake_up_process(rdp_gp->nocb_gp_kthread);
-
- return 0;
+ return wake_gp;
}

static long rcu_nocb_rdp_deoffload(void *arg)
@@ -984,11 +981,18 @@ static long rcu_nocb_rdp_deoffload(void *arg)
struct rcu_segcblist *cblist = &rdp->cblist;
unsigned long flags;
int ret;
+ struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;

+ /*
+ *When rcuog or rcuop spawn fail, direct call rcu_nocb_rdp_deoffload().
+ *due to the target CPU(rdp->cpu) is not online(cpu_online(rdp->cpu)
+ *return false) yet. this warning will be triggered.
+ */
WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());

pr_info("De-offloading %d\n", rdp->cpu);

+ mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
rcu_nocb_lock_irqsave(rdp, flags);
/*
* Flush once and for all now. This suffices because we are
@@ -1010,9 +1014,19 @@ static long rcu_nocb_rdp_deoffload(void *arg)
rcu_segcblist_set_flags(cblist, SEGCBLIST_RCU_CORE);
invoke_rcu_core();
ret = rdp_offload_toggle(rdp, false, flags);
- swait_event_exclusive(rdp->nocb_state_wq,
- !rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB |
+
+ if (rdp_gp->nocb_gp_kthread) {
+ if (ret)
+ wake_up_process(rdp_gp->nocb_gp_kthread);
+ swait_event_exclusive(rdp->nocb_state_wq,
+ !rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB |
SEGCBLIST_KTHREAD_GP));
+ } else {
+ rcu_nocb_lock_irqsave(rdp, flags);
+ rcu_segcblist_clear_flags(&rdp->cblist,
+ SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP);
+ rcu_nocb_unlock_irqrestore(rdp, flags);
+ }
/* Stop nocb_gp_wait() from iterating over this structure. */
list_del_rcu(&rdp->nocb_entry_rdp);
/*
@@ -1030,12 +1044,12 @@ static long rcu_nocb_rdp_deoffload(void *arg)
* rcu_nocb_unlock_irqrestore() anymore.
*/
raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
-
+ mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
/* Sanity check */
WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));

- return ret;
+ return 0;
}

int rcu_nocb_cpu_deoffload(int cpu)
@@ -1043,8 +1057,8 @@ int rcu_nocb_cpu_deoffload(int cpu)
struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
int ret = 0;

- mutex_lock(&rcu_state.barrier_mutex);
cpus_read_lock();
+ mutex_lock(&rcu_state.barrier_mutex);
if (rcu_rdp_is_offloaded(rdp)) {
if (cpu_online(cpu)) {
ret = work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp);
@@ -1055,8 +1069,8 @@ int rcu_nocb_cpu_deoffload(int cpu)
ret = -EINVAL;
}
}
- cpus_read_unlock();
mutex_unlock(&rcu_state.barrier_mutex);
+ cpus_read_unlock();

return ret;
}
@@ -1068,6 +1082,7 @@ static long rcu_nocb_rdp_offload(void *arg)
struct rcu_segcblist *cblist = &rdp->cblist;
unsigned long flags;
int ret;
+ struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;

WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
/*
@@ -1077,6 +1092,12 @@ static long rcu_nocb_rdp_offload(void *arg)
if (!rdp->nocb_gp_rdp)
return -EINVAL;

+ mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
+ if (!rdp_gp->nocb_gp_kthread) {
+ mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
+ return -EINVAL;
+ }
+
pr_info("Offloading %d\n", rdp->cpu);

/*
@@ -1112,6 +1133,8 @@ static long rcu_nocb_rdp_offload(void *arg)
* rcu_nocb_unlock() rcu_nocb_unlock()
*/
ret = rdp_offload_toggle(rdp, true, flags);
+ if (ret)
+ wake_up_process(rdp_gp->nocb_gp_kthread);
swait_event_exclusive(rdp->nocb_state_wq,
rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB) &&
rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP));
@@ -1123,8 +1146,8 @@ static long rcu_nocb_rdp_offload(void *arg)
rcu_nocb_lock_irqsave(rdp, flags);
rcu_segcblist_clear_flags(cblist, SEGCBLIST_RCU_CORE);
rcu_nocb_unlock_irqrestore(rdp, flags);
-
- return ret;
+ mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
+ return 0;
}

int rcu_nocb_cpu_offload(int cpu)
@@ -1132,8 +1155,8 @@ int rcu_nocb_cpu_offload(int cpu)
struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
int ret = 0;

- mutex_lock(&rcu_state.barrier_mutex);
cpus_read_lock();
+ mutex_lock(&rcu_state.barrier_mutex);
if (!rcu_rdp_is_offloaded(rdp)) {
if (cpu_online(cpu)) {
ret = work_on_cpu(cpu, rcu_nocb_rdp_offload, rdp);
@@ -1144,8 +1167,8 @@ int rcu_nocb_cpu_offload(int cpu)
ret = -EINVAL;
}
}
- cpus_read_unlock();
mutex_unlock(&rcu_state.barrier_mutex);
+ cpus_read_unlock();

return ret;
}
@@ -1246,7 +1269,7 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
"rcuog/%d", rdp_gp->cpu);
if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo GP kthread, OOM is now expected behavior\n", __func__)) {
mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
- return;
+ goto end;
}
WRITE_ONCE(rdp_gp->nocb_gp_kthread, t);
if (kthread_prio)
@@ -1258,12 +1281,20 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
t = kthread_run(rcu_nocb_cb_kthread, rdp,
"rcuo%c/%d", rcu_state.abbr, cpu);
if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo CB kthread, OOM is now expected behavior\n", __func__))
- return;
+ goto end;

if (kthread_prio)
sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
WRITE_ONCE(rdp->nocb_cb_kthread, t);
WRITE_ONCE(rdp->nocb_gp_kthread, rdp_gp->nocb_gp_kthread);
+ return;
+end:
+ mutex_lock(&rcu_state.barrier_mutex);
+ if (rcu_rdp_is_offloaded(rdp)) {
+ rcu_nocb_rdp_deoffload(rdp);
+ cpumask_clear_cpu(cpu, rcu_nocb_mask);
+ }
+ mutex_unlock(&rcu_state.barrier_mutex);
}

/* How many CB CPU IDs per GP kthread? Default of -1 for sqrt(nr_cpu_ids). */
--
2.25.1