netdev - Re: [PATCH bpf] Use call_rcu_hurry() with synchronize_rcu

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Message-ID: <2cc44c11-bcdc-4ae4-a42d-d6d46cf1da6e@paulmck-laptop>
Date: Thu, 18 May 2023 10:25:05 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>, Song Liu <song@...nel.org>,
	Yonghong Song <yhs@...com>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	bpf@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf] Use call_rcu_hurry() with synchronize_rcu_mult()

On Thu, May 18, 2023 at 09:51:39AM -0700, Martin KaFai Lau wrote:
> 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.

Very good, and sorry for the noise!

							Thanx, Paul