Re: [RFC PATCH 47/86] rcu: select PREEMPT_RCU if PREEMPT

[Steven Rostedt]

On Tue, 5 Dec 2023 11:38:42 -0800
"Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:

> > 
> > Note that the new preemption model is a new paradigm and we need to start
> > thinking a bit differently if we go to it.  
> 
> We can of course think differently, but existing hardware and software
> will probably be a bit more stubborn.

Not at all. I don't see how hardware plays a role here, but how software is
designed does sometimes require thinking differently.

> 
> > One thing I would like to look into with the new work is to have holding a
> > mutex ignore the NEED_RESCHED_LAZY (similar to what is done with spinlock
> > converted to mutex in the RT kernel). That way you are less likely to be
> > preempted while holding a mutex.  
> 
> I like the concept, but those with mutex_lock() of rarely-held mutexes
> in their fastpaths might have workloads that have a contrary opinion.

I don't understand your above statement. Maybe I wasn't clear with my
statement? The above is more about PREEMPT_FULL, as it currently will
preempt immediately. My above comment is that we can have an option for
PREEMPT_FULL where if the scheduler decided to preempt even in a fast path,
it would at least hold off until there's no mutex held. Who cares if it's a
fast path when a task needs to give up the CPU for another task? What I
worry about is scheduling out while holding a mutex which increases the
chance of that mutex being contended upon. Which does have drastic impact
on performance.

> 
> > > Another is the aforementioned situations where removing the cond_resched()
> > > increases latency.  Yes, capping the preemption latency is a wonderful
> > > thing, and the people I chatted with are all for that, but it is only
> > > natural that there would be a corresponding level of concern about the
> > > cases where removing the cond_resched() calls increases latency.  
> > 
> > With the "capped preemption" I'm not sure that would still be the case.
> > cond_resched() currently only preempts if NEED_RESCHED is set. That means
> > the system had to already be in a situation that a schedule needs to
> > happen. There's lots of places in the kernel that run for over a tick
> > without any cond_resched(). The cond_resched() is usually added for
> > locations that show tremendous latency (where either a watchdog triggered,
> > or showed up in some analysis that had a latency that was much greater than
> > a tick).  
> 
> For non-real-time workloads, the average case is important, not just the
> worst case.  In the new lazily preemptible mode of thought, a preemption
> by a non-real-time task will wait a tick.  Earlier, it would have waited
> for the next cond_resched().  Which, in the average case, might have
> arrived much sooner than one tick.

Or much later. It's random. And what's nice about this model, we can add
more models than just "NONE", "VOLUNTARY", "FULL". We could have a way to
say "this task needs to preempt immediately" and not just for RT tasks.

This allows the user to decide which task preempts more and which does not
(defined by the scheduler), instead of some random cond_resched() that can
also preempt a higher priority task that just finished its quota to run a
low priority task causing latency for the higher priority task.

This is what I mean by "think differently".

> 
> > The point is, if/when we switch to the new preemption model, we would need
> > to re-evaluate if any cond_resched() is needed. Yes, testing needs to be
> > done to prevent regressions. But the reasons I see cond_resched() being
> > added today, should no longer exist with this new model.  
> 
> This I agree with.  Also, with the new paradigm and new mode of thought
> in place, it should be safe to drop any cond_resched() that is in a loop
> that consumes more than a tick of CPU time per iteration.

Why does that matter? Is the loop not important? Why stop it from finishing
for some random task that may not be important, and cond_resched() has no
idea if it is or not.

> 
> > > There might be others as well.  These are the possibilities that have
> > > come up thus far.
> > >   
> > > > They all suck and keeping some of them is just counterproductive as
> > > > again people will sprinkle them all over the place for the very wrong
> > > > reasons.    
> > > 
> > > Yes, but do they suck enough and are they counterproductive enough to
> > > be useful and necessary?  ;-)  
> > 
> > They are only useful and necessary because of the way we handle preemption
> > today. With the new preemption model, they are all likely to be useless and
> > unnecessary ;-)  
> 
> The "all likely" needs some demonstration.  I agree that a great many
> of them would be useless and unnecessary.  Maybe even the vast majority.
> But that is different than "all".  ;-)

I'm betting it is "all" ;-) But I also agree that this "needs some
demonstration". We are not there yet, and likely will not be until the
second half of next year. So we have plenty of time to speak rhetorically
to each other!



> > The conflict is with the new paradigm (I love that word! It's so "buzzy").
> > As I mentioned above, cond_resched() is usually added when a problem was
> > seen. I really believe that those problems would never had been seen if
> > the new paradigm had already been in place.  
> 
> Indeed, that sort of wording does quite the opposite of raising my
> confidence levels.  ;-)

Yes, I admit the "manager speak" isn't something to brag about here. But I
really do like that word. It's just fun to say (and spell)! Paradigm,
paradigm, paradigm! It's that silent 'g'. Although, I wonder if we should
be like gnu, and pronounce it when speaking about free software? Although,
that makes the word sound worse. :-p

> 
> You know, the ancient Romans would have had no problem dealing with the
> dot-com boom, cryptocurrency, some of the shadier areas of artificial
> intelligence and machine learning, and who knows what all else.  As the
> Romans used to say, "Beware of geeks bearing grifts."
> 
> > > >   3) Looking at the initial problem Ankur was trying to solve there is
> > > >      absolutely no acceptable solution to solve that unless you think
> > > >      that the semantically invers 'allow_preempt()/disallow_preempt()'
> > > >      is anywhere near acceptable.    
> > > 
> > > I am not arguing for allow_preempt()/disallow_preempt(), so for that
> > > argument, you need to find someone else to argue with.  ;-)  
> > 
> > Anyway, there's still a long path before cond_resched() can be removed. It
> > was a mistake by Ankur to add those removals this early (and he has
> > acknowledged that mistake).  
> 
> OK, that I can live with.  But that seems to be a bit different of a
> take than that of some earlier emails in this thread.  ;-)

Well, we are also stating the final goal as well. I think there's some
confusion to what's going to happen immediately and what's going to happen
in the long run.

> 
> > First we need to get the new preemption modeled implemented. When it is, it
> > can be just a config option at first. Then when that config option is set,
> > you can enable the NONE, VOLUNTARY or FULL preemption modes, even switch
> > between them at run time as they are just a way to tell the scheduler when
> > to set NEED_RESCHED_LAZY vs NEED_RSECHED.  
> 
> Assuming CONFIG_PREEMPT_RCU=y, agreed.  With CONFIG_PREEMPT_RCU=n,
> the runtime switching needs to be limited to NONE and VOLUNTARY.
> Which is fine.

But why? Because the run time switches of NONE and VOLUNTARY are no
different than FULL.

Why I say that? Because:

For all modes, NEED_RESCHED_LAZY is set, the kernel has one tick to get out
or NEED_RESCHED will be set (of course that one tick may be configurable).
Once the NEED_RESCHED is set, then the kernel is converted to PREEMPT_FULL.

Even if the user sets the mode to "NONE", after the above scenario (one tick
after NEED_RESCHED_LAZY is set) the kernel will be behaving no differently
than PREEMPT_FULL.

So why make the difference between CONFIG_PREEMPT_RCU=n and limit to only
NONE and VOLUNTARY. It must work with FULL or it will be broken for NONE
and VOLUNTARY after one tick from NEED_RESCHED_LAZY being set.

> 
> > At that moment, when that config is set, the cond_resched() can turn into a
> > nop. This will allow for testing to make sure there are no regressions in
> > latency, even with the NONE mode enabled.  
> 
> And once it appears to be reasonably stable (in concept as well as
> implementation), heavy testing should get underway.

Agreed.

> 
> > The real test is implementing the code and seeing how it affects things in
> > the real world. Us arguing about it isn't going to get anywhere.  
> 
> Indeed, the opinion of the objective universe always wins.  It all too
> often takes longer than necessary for the people arguing with each other
> to realize this, but such is life.
> 
> >                                                                  I just
> > don't want blind NACK. A NACK to a removal of a cond_resched() needs to
> > show that there was a real regression with that removal.  
> 
> Fair enough, although a single commit bulk removing a large number of
> cond_resched() calls will likely get a bulk NAK.

We'll see. I now have a goal to hit!

-- Steve