linux-kernel - Re: [PATCH v5 net 3/3] bpf: report RCU QS in cpumap kthread

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Date: Tue, 19 Mar 2024 14:32:56 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Yan Zhai <yan@...udflare.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jiri Pirko <jiri@...nulli.us>, Simon Horman <horms@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Lorenzo Bianconi <lorenzo@...nel.org>,
	Coco Li <lixiaoyan@...gle.com>, Wei Wang <weiwan@...gle.com>,
	Alexander Duyck <alexanderduyck@...com>,
	linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
	bpf@...r.kernel.org, kernel-team@...udflare.com,
	Joel Fernandes <joel@...lfernandes.org>,
	Toke Høiland-Jørgensen <toke@...hat.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>, mark.rutland@....com,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v5 net 3/3] bpf: report RCU QS in cpumap kthread

On Tue, Mar 19, 2024 at 01:44:40PM -0700, Yan Zhai wrote:
> When there are heavy load, cpumap kernel threads can be busy polling
> packets from redirect queues and block out RCU tasks from reaching
> quiescent states. It is insufficient to just call cond_resched() in such
> context. Periodically raise a consolidated RCU QS before cond_resched
> fixes the problem.
> 
> Fixes: 6710e1126934 ("bpf: introduce new bpf cpu map type BPF_MAP_TYPE_CPUMAP")
> Reviewed-by: Jesper Dangaard Brouer <hawk@...nel.org>
> Signed-off-by: Yan Zhai <yan@...udflare.com>

Acked-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  kernel/bpf/cpumap.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
> index 9ee8da477465..a8e34416e960 100644
> --- a/kernel/bpf/cpumap.c
> +++ b/kernel/bpf/cpumap.c
> @@ -263,6 +263,7 @@ static int cpu_map_bpf_prog_run(struct bpf_cpu_map_entry *rcpu, void **frames,
>  static int cpu_map_kthread_run(void *data)
>  {
>  	struct bpf_cpu_map_entry *rcpu = data;
> +	unsigned long last_qs = jiffies;
>  
>  	complete(&rcpu->kthread_running);
>  	set_current_state(TASK_INTERRUPTIBLE);
> @@ -288,10 +289,12 @@ static int cpu_map_kthread_run(void *data)
>  			if (__ptr_ring_empty(rcpu->queue)) {
>  				schedule();
>  				sched = 1;
> +				last_qs = jiffies;
>  			} else {
>  				__set_current_state(TASK_RUNNING);
>  			}
>  		} else {
> +			rcu_softirq_qs_periodic(last_qs);
>  			sched = cond_resched();
>  		}
>  
> -- 
> 2.30.2
> 
>