Re: [PATCH bpf] Use call_rcu_hurry() with synchronize_rcu_mult()

On 5/18/23 7:47 AM, Paul E. McKenney wrote:

The bpf_struct_ops_map_free() function must wait for both an RCU grace
period and an RCU Tasks grace period, and so it passes call_rcu() and
call_rcu_tasks() to synchronize_rcu_mult().  This works, but on ChromeOS
and Android platforms call_rcu() can have lazy semantics, resulting in
multi-second delays between call_rcu() invocation and invocation of the
corresponding callback.

Therefore, substitute call_rcu_hurry() for call_rcu().

My understanding on the net-effect is to free up the struct_ops resources faster.

I believe call_rcu() should be fine. struct_ops freeing should not happen very 
often. For example, when a bpf written tcp congestion control (struct_ops) is 
registered, it will stay in the kernel for a long time. A couple seconds delay 
in releasing the struct_ops should be acceptable.