Re: [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads

[Song Liu]

On Mon, Jan 30, 2023 at 11:48 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> On Mon, Jan 30, 2023 at 06:36:32PM +0000, Mark Rutland wrote:
> > On Mon, Jan 30, 2023 at 01:40:18PM +0100, Peter Zijlstra wrote:
> > > On Fri, Jan 27, 2023 at 02:11:31PM -0800, Josh Poimboeuf wrote:
> > > > @@ -8500,8 +8502,10 @@ EXPORT_STATIC_CALL_TRAMP(might_resched);
> > > >  static DEFINE_STATIC_KEY_FALSE(sk_dynamic_cond_resched);
> > > >  int __sched dynamic_cond_resched(void)
> > > >  {
> > > > - if (!static_branch_unlikely(&sk_dynamic_cond_resched))
> > > > + if (!static_branch_unlikely(&sk_dynamic_cond_resched)) {
> > > > +         klp_sched_try_switch();
> > > >           return 0;
> > > > + }
> > > >   return __cond_resched();
> > > >  }
> > > >  EXPORT_SYMBOL(dynamic_cond_resched);
> > >
> > > I would make the klp_sched_try_switch() not depend on
> > > sk_dynamic_cond_resched, because __cond_resched() is not a guaranteed
> > > pass through __schedule().
> > >
> > > But you'll probably want to check with Mark here, this all might
> > > generate crap code on arm64.
> >
> > IIUC here klp_sched_try_switch() is a static call, so on arm64 this'll generate
> > at least a load, a conditional branch, and an indirect branch. That's not
> > ideal, but I'd have to benchmark it to find out whether it's a significant
> > overhead relative to the baseline of PREEMPT_DYNAMIC.
> >
> > For arm64 it'd be a bit nicer to have another static key check, and a call to
> > __klp_sched_try_switch(). That way the static key check gets turned into a NOP
> > in the common case, and the call to __klp_sched_try_switch() can be a direct
> > call (potentially a tail-call if we made it return 0).
>
> Hm, it might be nice if our out-of-line static call implementation would
> automatically do a static key check as part of static_call_cond() for
> NULL-type static calls.
>
> But the best answer is probably to just add inline static calls to
> arm64.  Is the lack of objtool the only thing blocking that?
>
> Objtool is now modular, so all the controversial CFG reverse engineering
> is now optional, so it shouldn't be too hard to just enable objtool for
> static call inlines.

This might be a little off topic, and maybe I missed some threads:
How far are we from officially supporting livepatch on arm64?

IIUC, stable stack unwinding is the missing piece at the moment?

Thanks,
Song