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

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Date: Thu, 18 Jan 2024 17:50:06 +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 2024-01-18 08:38:12 [-0800], Jakub Kicinski wrote:
> > If this is the per-CPU BH lock (which I want to remove) then it will
> > continue until all softirqs complete.
> 
> So there's no way for a process to know on RT that someone with higher
> prio is waiting for it to release its locks? :(

You could add a function to check if your current priority is inherited
from someone else and if so start dropping the locks you think are
responsible for it.
I made a PoC that appears to work for timer_list timer which is one of
the softirqs. This made me realise that I need in more spots and I am
doing it for the wrong reasons…

Sebastian