Re: [RFC PATCH 48/86] rcu: handle quiescent states for PREEMPT_RCU=n

[Thomas Gleixner]

Paul!

On Mon, Dec 04 2023 at 17:33, Paul E. McKenney wrote:
> On Tue, Nov 28, 2023 at 06:04:33PM +0100, Thomas Gleixner wrote:
>> So:
>> 
>>     loop()
>> 
>>       preempt_disable();
>> 
>>       --> tick interrupt
>>             rcu_flavor_sched_clock_irq()
>>                 sets NEED_RESCHED
>> 
>>       preempt_enable()
>>         preempt_schedule()
>>           schedule()
>>             report_QS()
>> 
>> See? No magic nonsense in preempt_enable(), no cond_resched(), nothing.
>
> Understood, but that does delay detection of that quiescent state by up
> to one tick.

Sure, but does that really matter in practice?

>> So if that turns out to matter in reality and not just by academic
>> inspection, then we are far better off to annotate such code with:
>> 
>>     do {
>>         preempt_lazy_disable();
>>         mutex_lock();
>>         do_stuff();
>>         mutex_unlock();
>>         preempt_lazy_enable();
>>     }
>> 
>> and let preempt_lazy_enable() evaluate the NEED_RESCHED_LAZY bit.
>
> I am not exactly sure what semantics you are proposing with this pairing
> as opposed to "this would be a good time to preempt in response to the
> pending lazy request".  But I do agree that something like this could
> replace at least a few more instance of cond_resched(), so that is good.
> Not necessarily all of them, though.

The main semantic difference is that such a mechanism is properly
nesting and can be eventually subsumed into the actual locking
constructs.

>> Just insisting that RCU_PREEMPT=n requires cond_resched() and whatsoever
>> is not really getting us anywhere.
>
> Except that this is not what is happening, Thomas.  ;-)
>
> You are asserting that all of the cond_resched() calls can safely be
> eliminated.  That might well be, but more than assertion is required.
> You have come up with some good ways of getting rid of some classes of
> them, which is a very good and very welcome thing.  But that is not the
> same as having proved that all of them may be safely removed.

Neither have you proven that any of them will be required with the new
PREEMPT_LAZY model. :)

Your experience and knowledge in this area is certainly appreciated, but
under the changed semantics of LAZY it's debatable whether observations
and assumptions which are based on PREEMPT_NONE behaviour still apply.

We'll see.

Thanks,

        tglx