[PATCH] ftrace: Avoid synchronize_rcu_tasks_rude() call when not necessary

[Nicolas Saenz Julienne]

synchronize_rcu_tasks_rude() triggers IPIs and forces rescheduling on
all CPUs. It is a costly operation and, when targeting nohz_full CPUs,
very disrupting (hence the name). So avoid calling it when 'old_hash'
doesn't need to be freed.

Signed-off-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx>
---
 kernel/trace/ftrace.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index e6fb3e6e1ffc..4fbcf560dd03 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5985,7 +5985,8 @@ ftrace_graph_release(struct inode *inode, struct file *file)
 		 * infrastructure to do the synchronization, thus we must do it
 		 * ourselves.
 		 */
-		synchronize_rcu_tasks_rude();
+		if (old_hash != EMPTY_HASH)
+			synchronize_rcu_tasks_rude();
 
 		free_ftrace_hash(old_hash);
 	}
-- 
2.31.1