[PATCH] latency improvement in __smp_call_single_queue

[George Prekas <prekageo@amazon.com>]

If an interrupt arrives between llist_add and
send_call_function_single_ipi in the following code snippet, then the
remote CPU will not receive the IPI in a timely manner and subsequent
SMP calls even from other CPUs for other functions will be delayed:

     if (llist_add(node, &per_cpu(call_single_queue, cpu)))
         send_call_function_single_ipi(cpu);

Note: llist_add returns 1 if it was empty before the operation.

CPU 0                           | CPU 1                     | CPU 2
__smp_call_single_q(2,f1)       | __smp_call_single_q(2,f2) |
   llist_add returns 1           |                           |
   interrupted                   |   llist_add returns 0     |
       ...                       |   branch not taken        |
       ...                       |                           |
   resumed                       |                           |
   send_call_function_single_ipi |                           |
                                 |                           | f1
                                 |                           | f2

The call from CPU 1 for function f2 will be delayed because CPU 0 was
interrupted.

Signed-off-by: George Prekas <prekageo@amazon.com>
---
  kernel/smp.c | 4 ++++
  1 file changed, 4 insertions(+)

diff --git a/kernel/smp.c b/kernel/smp.c
index aa17eedff5be..9dc679466cf0 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -135,6 +135,8 @@ static 
DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);

  void __smp_call_single_queue(int cpu, struct llist_node *node)
  {
+    unsigned long flags;
+
      /*
       * The list addition should be visible before sending the IPI
       * handler locks the list to pull the entry off it because of
@@ -146,8 +148,10 @@ void __smp_call_single_queue(int cpu, struct 
llist_node *node)
       * locking and barrier primitives. Generic code isn't really
       * equipped to do the right thing...
       */
+    local_irq_save(flags);
      if (llist_add(node, &per_cpu(call_single_queue, cpu)))
          send_call_function_single_ipi(cpu);
+    local_irq_restore(flags);
  }

  /*
-- 
2.16.6