Re: [PATCH] BPF: Disable on PREEMPT_RT

From: Thomas Gleixner <tglx@linutronix.de>
To: Clark Williams <williams@redhat.com>
Cc: David Miller <davem@davemloft.net>,
	Sebastian Sewior <bigeasy@linutronix.de>,
	daniel@iogearbox.net, bpf@vger.kernel.org, ast@kernel.org,
	kafai@fb.com, songliubraving@fb.com, yhs@fb.com,
	Peter Zijlstra <peterz@infradead.org>,
	Arnaldo Carvalho de Melo <acme@redhat.com>
Subject: Re: [PATCH] BPF: Disable on PREEMPT_RT
Date: Fri, 18 Oct 2019 10:46:01 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1910181038130.1869@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20191017214917.18911f58@tagon>

Clark,

On Thu, 17 Oct 2019, Clark Williams wrote:
> On Thu, 17 Oct 2019 23:54:07 +0200 (CEST)
> Thomas Gleixner <tglx@linutronix.de> wrote:
> >   #2) BPF does allocations in atomic contexts, which is a dubious decision
> >       even for non RT. That's related to #1
> 
> I guess my question here is, are the allocations done on behalf of an about-to-run
> BPF program, or as a result of executing BPF code?  Is it something we might be able
> to satisfy from a pre-allocated pool rather than kmalloc()? Ok, I need to go dive
> into BPF a bit deeper.

Sebastion?

> >   #3) BPF uses the up_read_non_owner() hackery which was only invented to
> >       deal with already existing horrors and not meant to be proliferated.
> > 
> >       Yes, I know it's a existing facility ....
> 
> I'm sure I'll regret asking this, but why is up_read_non_owner() a horror? I mean,
> I get the fundamental wrongness of having someone that's not the owner of a semaphore
> performing an 'up' on it, but is there an RT-specific reason that it's bad? Is it
> totally a blocker for using BPF with RT or is it something we should fix over time?

RT has strict locker == unlocker semantics simply because the owner
(locker) is subject to priority inheritance and a non-owner unlock cannot
undo PI on behalf of the locker sanely. Also exposing the locker to PI if
the locker is not involved in unlocking is obviously a pointless exercise
and potentially a source of unbound priority inversion.

> I do think that we (RT) are going to have to co-exist with BPF, if only due to the
> increased use of XDP. I also think that other sub-systems will start to
> employ BPF for production purposes (as opposed to debug/analysis which is
> how we generally look at tracing, packet sniffing, etc.). I think we *have* to
> figure out how to co-exist.

I'm not saying that RT does not want BPF, quite the contrary, but for the
initial merge BPF is not a hard requirement, so disabling it was the
straight forward path.

I'm all ears to get pointers how to solve that right now.

Thanks,

	tglx