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

[Mark Rutland <mark.rutland@arm.com>]

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).

Thanks,
Mark.
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