Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Oct 18, 2018 at 10:25 AM Nadav Amit <namit@vmware.com> wrote:
>
> at 10:00 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>
> >
> >
> >> On Oct 18, 2018, at 9:47 AM, Nadav Amit <namit@vmware.com> wrote:
> >>
> >> at 8:51 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >>
> >>>> On Wed, Oct 17, 2018 at 8:12 PM Nadav Amit <namit@vmware.com> wrote:
> >>>> at 6:22 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >>>>
> >>>>>> On Oct 17, 2018, at 5:54 PM, Nadav Amit <namit@vmware.com> wrote:
> >>>>>>
> >>>>>> It is sometimes beneficial to prevent preemption for very few
> >>>>>> instructions, or prevent preemption for some instructions that precede
> >>>>>> a branch (this latter case will be introduced in the next patches).
> >>>>>>
> >>>>>> To provide such functionality on x86-64, we use an empty REX-prefix
> >>>>>> (opcode 0x40) as an indication that preemption is disabled for the
> >>>>>> following instruction.
> >>>>>
> >>>>> Nifty!
> >>>>>
> >>>>> That being said, I think you have a few bugs. First, you can’t just ignore
> >>>>> a rescheduling interrupt, as you introduce unbounded latency when this
> >>>>> happens — you’re effectively emulating preempt_enable_no_resched(), which
> >>>>> is not a drop-in replacement for preempt_enable(). To fix this, you may
> >>>>> need to jump to a slow-path trampoline that calls schedule() at the end or
> >>>>> consider rewinding one instruction instead. Or use TF, which is only a
> >>>>> little bit terrifying…
> >>>>
> >>>> Yes, I didn’t pay enough attention here. For my use-case, I think that the
> >>>> easiest solution would be to make synchronize_sched() ignore preemptions
> >>>> that happen while the prefix is detected. It would slightly change the
> >>>> meaning of the prefix.
> >>
> >> So thinking about it further, rewinding the instruction seems the easiest
> >> and most robust solution. I’ll do it.
> >>
> >>>>> You also aren’t accounting for the case where you get an exception that
> >>>>> is, in turn, preempted.
> >>>>
> >>>> Hmm.. Can you give me an example for such an exception in my use-case? I
> >>>> cannot think of an exception that might be preempted (assuming #BP, #MC
> >>>> cannot be preempted).
> >>>
> >>> Look for cond_local_irq_enable().
> >>
> >> I looked at it. Yet, I still don’t see how exceptions might happen in my
> >> use-case, but having said that - this can be fixed too.
> >
> > I’m not totally certain there’s a case that matters.  But it’s worth checking
> >
> >> To be frank, I paid relatively little attention to this subject. Any
> >> feedback about the other parts and especially on the high-level approach? Is
> >> modifying the retpolines in the proposed manner (assembly macros)
> >> acceptable?
> >
> > It’s certainly a neat idea, and it could be a real speedup.
>
> Great. So I’ll try to shape things up, and I still wait for other comments
> (from others).
>
> I’ll just mention two more patches I need to cleanup (I know I still owe you some
> work, so obviously it will be done later):
>
> 1. Seccomp trampolines. On my Ubuntu, when I run Redis, systemd installs 17
> BPF filters on the Redis server process that are invoked on each
> system-call. Invoking each one requires an indirect branch. The patch keeps
> a per-process kernel code-page that holds trampolines for these functions.

I wonder how many levels of branches are needed before the branches
involved exceed the retpoline cost.

>
> 2. Binary-search for system-calls. Use the per-process kernel code-page also
> to hold multiple trampolines for the 16 common system calls of a certain
> process. The patch uses an indirection table and a binary-search to find the
> proper trampoline.

Same comment applies here.

>
> Thanks again,
> Nadav



-- 
Andy Lutomirski
AMA Capital Management, LLC