Received-SPF: pass (google.com: domain of linux-kernel+bounces-30356-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Date: Thu, 18 Jan 2024 08:38:12 -0800
From: Jakub Kicinski <kuba@kernel.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Toke =?UTF-8?B?SMO4aWxhbmQtSsO4cmdlbnNlbg==?= <toke@redhat.com>, Alexei
 Starovoitov <alexei.starovoitov@gmail.com>, LKML
 <linux-kernel@vger.kernel.org>, Network Development
 <netdev@vger.kernel.org>, "David S. Miller" <davem@davemloft.net>, Boqun
 Feng <boqun.feng@gmail.com>, Daniel Borkmann <daniel@iogearbox.net>, Eric
 Dumazet <edumazet@google.com>, Frederic Weisbecker <frederic@kernel.org>,
 Ingo Molnar <mingo@redhat.com>, Paolo Abeni <pabeni@redhat.com>, Peter
 Zijlstra <peterz@infradead.org>, Thomas Gleixner <tglx@linutronix.de>,
 Waiman Long <longman@redhat.com>, Will Deacon <will@kernel.org>, Alexei
 Starovoitov <ast@kernel.org>, Andrii Nakryiko <andrii@kernel.org>, Cong
 Wang <xiyou.wangcong@gmail.com>, Hao Luo <haoluo@google.com>, Jamal Hadi
 Salim <jhs@mojatatu.com>, Jesper Dangaard Brouer <hawk@kernel.org>, Jiri
 Olsa <jolsa@kernel.org>, Jiri Pirko <jiri@resnulli.us>, John Fastabend
 <john.fastabend@gmail.com>, KP Singh <kpsingh@kernel.org>, Martin KaFai Lau
 <martin.lau@linux.dev>, Ronak Doshi <doshir@vmware.com>, Song Liu
 <song@kernel.org>, Stanislav Fomichev <sdf@google.com>, VMware PV-Drivers
 Reviewers <pv-drivers@vmware.com>, Yonghong Song <yonghong.song@linux.dev>,
 bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH net-next 15/24] net: Use nested-BH locking for XDP
 redirect.
Message-ID: <20240118083812.1b91ba88@kernel.org>
In-Reply-To: <20240118082754.9L_QFIgU@linutronix.de>
References: <20231215171020.687342-1-bigeasy@linutronix.de>
	<20231215171020.687342-16-bigeasy@linutronix.de>
	<CAADnVQKJBpvfyvmgM29FLv+KpLwBBRggXWzwKzaCT9U-4bgxjA@mail.gmail.com>
	<87r0iw524h.fsf@toke.dk>
	<20240112174138.tMmUs11o@linutronix.de>
	<87ttnb6hme.fsf@toke.dk>
	<20240117180447.2512335b@kernel.org>
	<20240118082754.9L_QFIgU@linutronix.de>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit

On Thu, 18 Jan 2024 09:27:54 +0100 Sebastian Andrzej Siewior wrote:
> 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.

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? :(