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

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Message-ID: <CAPhsuW437A5SHnWyAmZh_RTTxbxvvp-swy0JBJQnwJ2jxubX2Q@mail.gmail.com>
Date:   Mon, 30 Jan 2023 17:53:14 -0800
From:   Song Liu <song@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>, kvm@...r.kernel.org,
        "Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org,
        Jiri Kosina <jikos@...nel.org>, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        "Seth Forshee (DigitalOcean)" <sforshee@...italocean.com>,
        live-patching@...r.kernel.org, Miroslav Benes <mbenes@...e.cz>
Subject: Re: [PATCH 0/2] vhost: improve livepatch switching for heavily loaded
 vhost worker kthreads

On Mon, Jan 30, 2023 at 11:48 AM Josh Poimboeuf <jpoimboe@...nel.org> 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