Re: [RFC PATCH 0/5] x86: dynamic indirect call promotion

[Andy Lutomirski <luto@kernel.org>]

On Wed, Nov 28, 2018 at 7:24 PM Andy Lutomirski <luto@amacapital.net> wrote:
>
>
> On Nov 28, 2018, at 6:06 PM, Nadav Amit <namit@vmware.com> wrote:
>
> >> On Nov 28, 2018, at 5:40 PM, Andy Lutomirski <luto@kernel.org> wrote:
> >>
> >>> On Wed, Nov 28, 2018 at 4:38 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >>> On Wed, Nov 28, 2018 at 07:34:52PM +0000, Nadav Amit wrote:
> >>>>> On Nov 28, 2018, at 8:08 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >>>>>
> >>>>>> On Wed, Oct 17, 2018 at 05:54:15PM -0700, Nadav Amit wrote:
> >>>>>> This RFC introduces indirect call promotion in runtime, which for the
> >>>>>> matter of simplification (and branding) will be called here "relpolines"
> >>>>>> (relative call + trampoline). Relpolines are mainly intended as a way
> >>>>>> of reducing retpoline overheads due to Spectre v2.
> >>>>>>
> >>>>>> Unlike indirect call promotion through profile guided optimization, the
> >>>>>> proposed approach does not require a profiling stage, works well with
> >>>>>> modules whose address is unknown and can adapt to changing workloads.
> >>>>>>
> >>>>>> The main idea is simple: for every indirect call, we inject a piece of
> >>>>>> code with fast- and slow-path calls. The fast path is used if the target
> >>>>>> matches the expected (hot) target. The slow-path uses a retpoline.
> >>>>>> During training, the slow-path is set to call a function that saves the
> >>>>>> call source and target in a hash-table and keep count for call
> >>>>>> frequency. The most common target is then patched into the hot path.
> >>>>>>
> >>>>>> The patching is done on-the-fly by patching the conditional branch
> >>>>>> (opcode and offset) that is used to compare the target to the hot
> >>>>>> target. This allows to direct all cores to the fast-path, while patching
> >>>>>> the slow-path and vice-versa. Patching follows 2 more rules: (1) Only
> >>>>>> patch a single byte when the code might be executed by any core. (2)
> >>>>>> When patching more than one byte, ensure that all cores do not run the
> >>>>>> to-be-patched-code by preventing this code from being preempted, and
> >>>>>> using synchronize_sched() after patching the branch that jumps over this
> >>>>>> code.
> >>>>>>
> >>>>>> Changing all the indirect calls to use relpolines is done using assembly
> >>>>>> macro magic. There are alternative solutions, but this one is
> >>>>>> relatively simple and transparent. There is also logic to retrain the
> >>>>>> software predictor, but the policy it uses may need to be refined.
> >>>>>>
> >>>>>> Eventually the results are not bad (2 VCPU VM, throughput reported):
> >>>>>>
> >>>>>>          base            relpoline
> >>>>>>          ----            ---------
> >>>>>> nginx      22898           25178 (+10%)
> >>>>>> redis-ycsb 24523           25486 (+4%)
> >>>>>> dbench     2144            2103 (+2%)
> >>>>>>
> >>>>>> When retpolines are disabled, and if retraining is off, performance
> >>>>>> benefits are up to 2% (nginx), but are much less impressive.
> >>>>>
> >>>>> Hi Nadav,
> >>>>>
> >>>>> Peter pointed me to these patches during a discussion about retpoline
> >>>>> profiling.  Personally, I think this is brilliant.  This could help
> >>>>> networking and filesystem intensive workloads a lot.
> >>>>
> >>>> Thanks! I was a bit held-back by the relatively limited number of responses.
> >>>
> >>> It is a rather, erm, ambitious idea, maybe they were speechless :-)
> >>>
> >>>> I finished another version two weeks ago, and every day I think: "should it
> >>>> be RFCv2 or v1”, ending up not sending it…
> >>>>
> >>>> There is one issue that I realized while working on the new version: I’m not
> >>>> sure it is well-defined what an outline retpoline is allowed to do. The
> >>>> indirect branch promotion code can change rflags, which might cause
> >>>> correction issues. In practice, using gcc, it is not a problem.
> >>>
> >>> Callees can clobber flags, so it seems fine to me.
> >>
> >> Just to check I understand your approach right: you made a macro
> >> called "call", and you're therefore causing all instances of "call" to
> >> become magic?  This is... terrifying.  It's even plausibly worse than
> >> "#define if" :)  The scariest bit is that it will impact inline asm as
> >> well.  Maybe a gcc plugin would be less alarming?
> >
> > It is likely to look less alarming. When I looked at the inline retpoline
> > implementation of gcc, it didn’t look much better than what I did - it
> > basically just emits assembly instructions.
>
> To be clear, that wasn’t a NAK.  It was merely a “this is alarming.”

Although... how do you avoid matching on things that really don't want
this treatment?  paravirt ops come to mind.