netdev - Re: [PATCH net-next 15/24] net: Use nested-BH locking for XDP redirect.

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Message-ID: <20240118082754.9L_QFIgU@linutronix.de>
Date: Thu, 18 Jan 2024 09:27:54 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Toke Høiland-Jørgensen <toke@...hat.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Boqun Feng <boqun.feng@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Paolo Abeni <pabeni@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Cong Wang <xiyou.wangcong@...il.com>, Hao Luo <haoluo@...gle.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Jiri Olsa <jolsa@...nel.org>, Jiri Pirko <jiri@...nulli.us>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Ronak Doshi <doshir@...are.com>, Song Liu <song@...nel.org>,
	Stanislav Fomichev <sdf@...gle.com>,
	VMware PV-Drivers Reviewers <pv-drivers@...are.com>,
	Yonghong Song <yonghong.song@...ux.dev>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH net-next 15/24] net: Use nested-BH locking for XDP
 redirect.

On 2024-01-17 18:04:47 [-0800], Jakub Kicinski wrote:
> Oh, and I'm bringing it up here, because CONFIG_RT can throw
> in "need_resched()" into the napi_rx_has_budget(), obviously.

need_resched() does not work on PREEMPT_RT the way you think. This
context (the NAPI poll callback) is preemptible and (by default) runs at
SCHED_FIFO 50 (within a threaded IRQ) so a context switch can happen at
any time by a task with higher priority.
If threadA gets preempted and owns a lock that threadB, with higher
priority, wants then threadA will get back on CPU, inherit the priority
of the threadB and continue to run until it releases the lock.

If this is the per-CPU BH lock (which I want to remove) then it will
continue until all softirqs complete.

Sebastian