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[bpf-next,v2,2/2] bpf: Skip rcu_barrier() if rcu_trace_implies_rcu_gp() is true

Message ID 20221209010947.3130477-3-houtao@huaweicloud.com (mailing list archive)
State Accepted
Commit 822ed78fab13d5a54f8b8c030e8c5dc0fcd2cdae
Headers show
Series Misc optimizations for bpf mem allocator | expand

Commit Message

Hou Tao Dec. 9, 2022, 1:09 a.m. UTC 
From: Hou Tao <houtao1@huawei.com>

If there are pending rcu callback, free_mem_alloc() will use
rcu_barrier_tasks_trace() and rcu_barrier() to wait for the pending
__free_rcu_tasks_trace() and __free_rcu() callback.

If rcu_trace_implies_rcu_gp() is true, there will be no pending
__free_rcu(), so it will be OK to skip rcu_barrier() as well.

Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Hou Tao <houtao1@huawei.com>
---
 kernel/bpf/memalloc.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)
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Patch

diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
index 04d96d1b98a3..ebcc3dd0fa19 100644
--- a/kernel/bpf/memalloc.c
+++ b/kernel/bpf/memalloc.c
@@ -464,9 +464,17 @@  static void free_mem_alloc(struct bpf_mem_alloc *ma)
 {
 	/* waiting_for_gp lists was drained, but __free_rcu might
 	 * still execute. Wait for it now before we freeing percpu caches.
+	 *
+	 * rcu_barrier_tasks_trace() doesn't imply synchronize_rcu_tasks_trace(),
+	 * but rcu_barrier_tasks_trace() and rcu_barrier() below are only used
+	 * to wait for the pending __free_rcu_tasks_trace() and __free_rcu(),
+	 * so if call_rcu(head, __free_rcu) is skipped due to
+	 * rcu_trace_implies_rcu_gp(), it will be OK to skip rcu_barrier() by
+	 * using rcu_trace_implies_rcu_gp() as well.
 	 */
 	rcu_barrier_tasks_trace();
-	rcu_barrier();
+	if (!rcu_trace_implies_rcu_gp())
+		rcu_barrier();
 	free_mem_alloc_no_barrier(ma);
 }