[PATCH net-next] net: give more chances to rcu in netdev_wait_allrefs_any()

[PATCH net-next] net: give more chances to rcu in netdev_wait_allrefs_any()

[Eric Dumazet]

This came while reviewing commit c4e86b4363ac ("net: add two more
call_rcu_hurry()").

Paolo asked if adding one synchronize_rcu() would help.

While synchronize_rcu() does not help, making sure to call
rcu_barrier() before msleep(wait) is definitely helping
to make sure lazy call_rcu() are completed.

Instead of waiting ~100 seconds in my tests, the ref_tracker
splats occurs one time only, and netdev_wait_allrefs_any()
latency is reduced to the strict minimum.

Ideally we should audit our call_rcu() users to make sure
no refcount (or cascading call_rcu()) is held too long,
because rcu_barrier() is quite expensive.

Fixes: 0e4be9e57e8c ("net: use exponential backoff in netdev_wait_allrefs")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/all/28bbf698-befb-42f6-b561-851c67f464aa@kernel.org/T/#m76d73ed6b03cd930778ac4d20a777f22a08d6824
---
 net/core/dev.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index e09aa3785c159b4ab0fe7eb3546f9dd6797ebce2..c9e59eff8ec841f6267c2749489fdc7fe0d03430 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10566,8 +10566,9 @@ static struct net_device *netdev_wait_allrefs_any(struct list_head *list)
 			rebroadcast_time = jiffies;
 		}
 
+		rcu_barrier();
+
 		if (!wait) {
-			rcu_barrier();
 			wait = WAIT_REFS_MIN_MSECS;
 		} else {
 			msleep(wait);
-- 
2.44.0.769.g3c40516874-goog

Re: [PATCH net-next] net: give more chances to rcu in netdev_wait_allrefs_any()

[Jiri Pirko]

Fri, Apr 26, 2024 at 08:42:22AM CEST, edumazet@google.com wrote:
>This came while reviewing commit c4e86b4363ac ("net: add two more
>call_rcu_hurry()").
>
>Paolo asked if adding one synchronize_rcu() would help.
>
>While synchronize_rcu() does not help, making sure to call
>rcu_barrier() before msleep(wait) is definitely helping
>to make sure lazy call_rcu() are completed.
>
>Instead of waiting ~100 seconds in my tests, the ref_tracker
>splats occurs one time only, and netdev_wait_allrefs_any()
>latency is reduced to the strict minimum.
>
>Ideally we should audit our call_rcu() users to make sure
>no refcount (or cascading call_rcu()) is held too long,
>because rcu_barrier() is quite expensive.
>
>Fixes: 0e4be9e57e8c ("net: use exponential backoff in netdev_wait_allrefs")
>Signed-off-by: Eric Dumazet <edumazet@google.com>
>Link: https://lore.kernel.org/all/28bbf698-befb-42f6-b561-851c67f464aa@kernel.org/T/#m76d73ed6b03cd930778ac4d20a777f22a08d6824

Reviewed-by: Jiri Pirko <jiri@nvidia.com>

Re: [PATCH net-next] net: give more chances to rcu in netdev_wait_allrefs_any()

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by David S. Miller <davem@davemloft.net>:

On Fri, 26 Apr 2024 06:42:22 +0000 you wrote:
> This came while reviewing commit c4e86b4363ac ("net: add two more
> call_rcu_hurry()").
> 
> Paolo asked if adding one synchronize_rcu() would help.
> 
> While synchronize_rcu() does not help, making sure to call
> rcu_barrier() before msleep(wait) is definitely helping
> to make sure lazy call_rcu() are completed.
> 
> [...]

Here is the summary with links:
  - [net-next] net: give more chances to rcu in netdev_wait_allrefs_any()
    https://git.kernel.org/netdev/net-next/c/cd42ba1c8ac9

You are awesome, thank you!
-- 
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