[RFC PATCH 2/2] smp: Reduce NMI traffic from CSD waiters to CSD destination.

[Imran Khan <imran.f.khan@oracle.com>]

On systems with hundreds of CPUs, if few hundred or most of the CPUs
detect a CSD hang, then all of these waiters endup sending an NMI to
destination CPU to dump its backtrace.
Depending on the number of such NMIs, destination CPU can spent
a significant amount of time handling these NMIs and thus making
it more difficult for this CPU to address those pending CSDs timely.
In worst case it can happen that by the time destination CPU is done
handling all of the above mentioned backtrace NMIs, csd wait time
may have elapsed and all of the waiters start sending backtrace NMI
again and this behaviour continues in loop.

To avoid the above mentioned scenario, issue backtrace NMI only from
first waiter. The other waiters to same CSD destination can make use
of backtrace obtained via fist waiter's NMI.

Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
---
 kernel/smp.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index b7ccba677a0a0..a1cd21ea8b308 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -43,6 +43,8 @@ static DEFINE_PER_CPU_ALIGNED(struct call_function_data, cfd_data);
 
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct llist_head, call_single_queue);
 
+static DEFINE_PER_CPU(atomic_t, trigger_backtrace) = ATOMIC_INIT(1);
+
 static void __flush_smp_call_function_queue(bool warn_cpu_offline);
 
 int smpcfd_prepare_cpu(unsigned int cpu)
@@ -242,7 +244,8 @@ static bool csd_lock_wait_toolong(struct __call_single_data *csd, u64 ts0, u64 *
 			 *bug_id, !cpu_cur_csd ? "unresponsive" : "handling this request");
 	}
 	if (cpu >= 0) {
-		dump_cpu_task(cpu);
+		if (atomic_cmpxchg_acquire(&per_cpu(trigger_backtrace, cpu), 1, 0))
+			dump_cpu_task(cpu);
 		if (!cpu_cur_csd) {
 			pr_alert("csd: Re-sending CSD lock (#%d) IPI from CPU#%02d to CPU#%02d\n", *bug_id, raw_smp_processor_id(), cpu);
 			arch_send_call_function_single_ipi(cpu);
@@ -423,9 +426,14 @@ static void __flush_smp_call_function_queue(bool warn_cpu_offline)
 	struct llist_node *entry, *prev;
 	struct llist_head *head;
 	static bool warned;
+	atomic_t *tbt;
 
 	lockdep_assert_irqs_disabled();
 
+	/* Allow waiters to send backtrace NMI from here onwards */
+	tbt = this_cpu_ptr(&trigger_backtrace);
+	atomic_set_release(tbt, 1);
+
 	head = this_cpu_ptr(&call_single_queue);
 	entry = llist_del_all(head);
 	entry = llist_reverse_order(entry);
-- 
2.34.1