Re: [RFC PATCH 1/2] rcu: Add rcu_read_lock_notrace()

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 17 Jul 2025 07:57:27 -0700
Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> I still don't understand what problem is being solved.
> As current tracepoint code stands there is no issue with it at all
> on PREEMPT_RT from bpf pov.
> bpf progs that attach to tracepoints are not sleepable.
> They don't call rt_spinlock either.
> Recognizing tracepoints that can sleep/fault and allow
> sleepable bpf progs there is on our to do list,
> but afaik it doesn't need any changes to tracepoint infra.
> There is no need to replace existing preempt_disable wrappers
> with sleepable srcu_fast or anything else.

From the PREEMPT_RT point of view, it wants BPF to be preemptable. It
may stop migration, but if someone adds a long running BPF program
(when I say long running, it could be anything more than 10us), and it
executes on a low priority task. If that BPF program is not preemptable
it can delay a high priority task from running. That defeats the
purpose of PREEMPT_RT.

If this is unsolvable, then we will need to make PREEMPT_RT and BPF
mutually exclusive in the configs.

-- Steve