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

[Josh Poimboeuf]

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:
> 
> 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 4df2b3e76b30..fbcd3acca25c 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -36,6 +36,7 @@
> >  #include <linux/seqlock.h>
> >  #include <linux/kcsan.h>
> >  #include <linux/rv.h>
> > +#include <linux/livepatch_sched.h>
> >  #include <asm/kmap_size.h>
> >  
> >  /* task_struct member predeclarations (sorted alphabetically): */
> > @@ -2074,6 +2075,9 @@ DECLARE_STATIC_CALL(cond_resched, __cond_resched);
> >  
> >  static __always_inline int _cond_resched(void)
> >  {
> > +	//FIXME this is a bit redundant with preemption disabled
> > +	klp_sched_try_switch();
> > +
> >  	return static_call_mod(cond_resched)();
> >  }
> 
> Right, I was thinking you'd do something like:
> 
> 	static_call_update(cond_resched, klp_cond_resched);
> 
> With:
> 
> static int klp_cond_resched(void)
> {
> 	klp_try_switch_task(current);
> 	return __cond_resched();
> }
> 
> That would force cond_resched() into doing the transition thing,
> irrespective of the preemption mode at hand. And then, when KLP be done,
> re-run sched_dynamic_update() to reset it to whatever it ought to be.

Ok, makes sense.

> 
> > @@ -401,8 +421,10 @@ void klp_try_complete_transition(void)
> >  	 */
> >  	read_lock(&tasklist_lock);
> >  	for_each_process_thread(g, task)
> > -		if (!klp_try_switch_task(task))
> > +		if (!klp_try_switch_task(task)) {
> > +			set_tsk_need_resched(task);
> >  			complete = false;
> > +		}
> 
> Yeah, no, that's broken -- preemption state live in more than just the
> TIF bit.

Oops.

> >  	read_unlock(&tasklist_lock);
> >  
> >  	/*
> > @@ -413,6 +435,7 @@ void klp_try_complete_transition(void)
> >  		task = idle_task(cpu);
> >  		if (cpu_online(cpu)) {
> >  			if (!klp_try_switch_task(task)) {
> > +				set_tsk_need_resched(task);
> >  				complete = false;
> >  				/* Make idle task go through the main loop. */
> >  				wake_up_if_idle(cpu);
> 
> Idem.
> 
> Also, I don't see the point of this and the __schedule() hook here:

The (poorly executed) idea was to catch kthreads which do

	if (need_resched())
		schedule();

but I guess we can just replace those usages with cond_resched()?

> > @@ -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.
> 
> Both ways this seems to make KLP 'depend' (or at least work lots better)
> when PREEMPT_DYNAMIC=y. Do we want a PREEMPT_DYNAMIC=n fallback for
> _cond_resched() too?

That was the intent but I obviously failed.  Let me go rework it a bit.

-- 
Josh