linux-kernel - [PATCH] ftrace: Avoid synchronize_rcu_tasks

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Message-Id: <20210721114726.1545103-1-nsaenzju@redhat.com>
Date:   Wed, 21 Jul 2021 13:47:26 +0200
From:   Nicolas Saenz Julienne <nsaenzju@...hat.com>
To:     rostedt@...dmis.org, mingo@...hat.com
Cc:     linux-kernel@...r.kernel.org,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>
Subject: [PATCH] ftrace: Avoid synchronize_rcu_tasks_rude() call when not necessary

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@...hat.com>
---
 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