Re: Running JITed and interpreted programs simultaneously

From: Juraj Vijtiuk <juraj.vijtiuk@sartura.hr>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, Luka Perkov <luka.perkov@sartura.hr>,
	David Marcinkovic <david.marcinkovic@sartura.hr>
Subject: Re: Running JITed and interpreted programs simultaneously
Date: Mon, 19 Oct 2020 12:20:05 +0200	[thread overview]
Message-ID: <CAOjtDRXrSzqb4PTBXDAHMuCArYjpMoTcT0Maw2UqefJN2DbATA@mail.gmail.com> (raw)
In-Reply-To: <CAEf4BzazaFZQHLcNARGWn4TTJJTQPdBVbskg+bJGp-dds-t1xw@mail.gmail.com>

On Wed, Oct 14, 2020 at 12:05 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Fri, Oct 9, 2020 at 12:58 PM Juraj Vijtiuk <juraj.vijtiuk@sartura.hr> wrote:
> >
> > It would be great to hear if anyone has any thoughts on running a set
> > of BPF programs JITed while other programs are run by the interpreter.
> >
> > Something like that would be useful on 32-bit architectures, as the
> > JIT compiler there doesn't support some instructions, primarily
> > instructions that work with 64-bit data. As far as I can tell, it is
> > unlikely that support will be coming soon as it is a general issue for
> > all 32-bit architectures. Atomic operations like BPF_XADD look
> > especially problematic regarding support on 32 bit platforms. From
> > what I managed to see such a conclusion appeared in a few patches
> > where support for 32-bit JITs was added, for example [0].
> > That results in some programs being runnable with BPF JIT enabled, and
> > some failing during load time, but running successfully without JIT on
> > 32-bit platforms.
> >
> > The only way to run some programs with JIT and some without, that
> > seems possible right now, is to manually change
> > /proc/sys/net/core/bpf_jit_enable every time a program is loaded.
> > Although I've managed to do that and it seems to be working, it seems
> > pretty hacky and looks like it could cause race conditions if multiple
> > programs were loaded, especially by independent loaders.
>
> I agree, the global file is not flexible enough and can cause problems
> in production environment.
>
> I don't see any reason why we shouldn't allow to decide interpreted vs
> jitted mode per program during BPF_PROG_LOAD.
>
> See kernel/bpf/core.c, bpf_prog's jit_requested field determines
> whether a program is going to be jitted or not. It should be trivial
> to allow overriding that during BPF_PROG_LOAD command.
>
> We can probably also generalize this to allow to "force-jit" or
> "force-interpret" by users, which would fail if kernel didn't support
> requested mode.
>

Thanks for the suggestion, that makes sense. I've started working on a
patch today.
I'll post again when I get something working and test it.

> >
> > At first glance it seems that if something like this was to be added
> > to a loader, it would have to either somehow be aware of other BPF
> > programs being loaded or possibly implement some sort of locking
> > mechanism which also seems hacky. From what I understand, doing it in
> > the kernel looks even less promising as bpf_jit_enable is a system
> > wide setting, and I imagine that changing it to work on a per program
> > basis would pretty much require a rework of the current design, so
> > that looks even less promising.
> >
> > It looks like the best option right now is to just run everything in
> > interpreted mode, but I want to make sure that I am not missing
> > something. If someone has tried doing something similar, it would be
> > great to know about that.
> >
> > Thanks,
> > Juraj Vijtiuk
> >
> > [0] https://lore.kernel.org/netdev/20200305050207.4159-3-luke.r.nels@gmail.com/