linux-kernel - Re: [PATCH bpf-next v1 12/22] rqspinlock: Add basic support for CONFIG

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Message-ID: <CAADnVQKDg3=cKmjqrQ7YraaW6STckj_w=1yU4oDZ7T+miMvgpA@mail.gmail.com>
Date: Wed, 8 Jan 2025 19:53:29 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Waiman Long <llong@...hat.com>
Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, 
	Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <martin.lau@...nel.org>, 
	Eduard Zingerman <eddyz87@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, Tejun Heo <tj@...nel.org>, 
	Barret Rhoden <brho@...gle.com>, Josh Don <joshdon@...gle.com>, Dohyun Kim <dohyunkim@...gle.com>, 
	Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH bpf-next v1 12/22] rqspinlock: Add basic support for CONFIG_PARAVIRT

On Wed, Jan 8, 2025 at 7:46 PM Waiman Long <llong@...hat.com> wrote:
>
> >>>> As for the locking semantics allowed by the BPF verifier, is it possible
> >>>> to enforce the strict locking rules for PREEMPT_RT kernel and use the
> >>>> relaxed semantics for non-PREEMPT_RT kernel. We don't want the loading
> >>>> of an arbitrary BPF program to break the latency guarantee of a
> >>>> PREEMPT_RT kernel.
> >>> Not really.
> >>> root can load silly bpf progs that take significant
> >>> amount time without abusing spinlocks.
> >>> Like 100k integer divides or a sequence of thousands of calls to map_update.
> >>> Long runtime of broken progs is a known issue.
> >>> We're working on a runtime termination check/watchdog that
> >>> will detect long running progs and will terminate them.
> >>> Safe termination is tricky, as you can imagine.
> >> Right.
> >>
> >> In that case, we just have to warn users that they can load BPF prog at
> >> their own risk and PREEMPT_RT kernel may break its latency guarantee.
> > Let's not open this can of worms.
> > There will be a proper watchdog eventually.
> > If we start to warn, when do we warn? On any bpf program loaded?
> > How about classic BPF ? tcpdump and seccomp ? They are limited
> > to 4k instructions, but folks can abuse that too.
>
> My intention is to document this somewhere, not to print out a warning
> in the kernel dmesg log.

Document what exactly?
"Loading arbitrary BPF program may break the latency guarantee of PREEMPT_RT"
?
That's not helpful to anyone.
Especially it undermines the giant effort we did together
with RT folks to make bpf behave well on RT.
For a long time bpf was the only user of migrate_disable().
Some of XDP bits got friendly to RT only in the last release. Etc.