Re: [PATCH 15/30] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y

[Joel Fernandes <joel@joelfernandes.org>]

On 3/10/2024 11:56 PM, Paul E. McKenney wrote:
> On Sun, Mar 10, 2024 at 08:48:28PM -0400, Joel Fernandes wrote:
>> On 3/10/2024 2:56 PM, Paul E. McKenney wrote:
>>> On Sun, Mar 10, 2024 at 06:03:30AM -0400, Joel Fernandes wrote:
>>>> Hello Ankur and Paul,
>>>>
>>>> On Mon, Feb 12, 2024 at 09:55:39PM -0800, Ankur Arora wrote:
>>>>> With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent
>>>>> states for read-side critical sections via rcu_all_qs().
>>>>> One reason why this was necessary: lacking preempt-count, the tick
>>>>> handler has no way of knowing whether it is executing in a read-side
>>>>> critical section or not.
>>>>>
>>>>> With PREEMPT_AUTO=y, there can be configurations with (PREEMPT_COUNT=y,
>>>>> PREEMPT_RCU=n). This means that cond_resched() is a stub which does
>>>>> not provide for quiescent states via rcu_all_qs().
>>>>>
>>>>> So, use the availability of preempt_count() to report quiescent states
>>>>> in rcu_flavor_sched_clock_irq().
>>>>>
>>>>> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
>>>>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>>>>> ---
>>>>>  kernel/rcu/tree_plugin.h | 11 +++++++----
>>>>>  1 file changed, 7 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>>>>> index 26c79246873a..9b72e9d2b6fe 100644
>>>>> --- a/kernel/rcu/tree_plugin.h
>>>>> +++ b/kernel/rcu/tree_plugin.h
>>>>> @@ -963,13 +963,16 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
>>>>>   */
>>>>>  static void rcu_flavor_sched_clock_irq(int user)
>>>>>  {
>>>>> -	if (user || rcu_is_cpu_rrupt_from_idle()) {
>>>>> +	if (user || rcu_is_cpu_rrupt_from_idle() ||
>>>>> +	    (IS_ENABLED(CONFIG_PREEMPT_COUNT) &&
>>>>> +	     !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)))) {
>>>>
>>>> I was wondering if it makes sense to even support !PREEMPT_RCU under
>>>> CONFIG_PREEMPT_AUTO.
>>>>
>>>> AFAIU, this CONFIG_PREEMPT_AUTO series preempts the kernel on
>>>> the next tick boundary in the worst case, with all preempt modes including
>>>> the preempt=none mode.
>>>>
>>>> Considering this, does it makes sense for RCU to be non-preemptible in
>>>> CONFIG_PREEMPT_AUTO=y? Because if that were the case, and a read-side critical
>>>> section extended beyond the tick, then it prevents the PREEMPT_AUTO preemption
>>>> from happening, because rcu_read_lock() would preempt_disable().
>>>
>>> Yes, it does make sense for RCU to be non-preemptible in kernels
>>> built with CONFIG_PREEMPT_AUTO=y and either CONFIG_PREEMPT_NONE=y or
>>> CONFIG_PREEMPT_VOLUNTARY=y.
>>> As noted in earlier discussions, there are
>>
>> Sorry if I missed a discussion, appreciate a link.
> 
> It is part of the discussion of the first version of this patch series,
> if I recall correctly.
> 
>>> systems that are adequately but not abundantly endowed with memory.
>>> Such systems need non-preemptible RCU to avoid preempted-reader OOMs.
>>
>> Then why don't such systems have a problem with CONFIG_PREEMPT_DYNAMIC=y and
>> preempt=none mode? CONFIG_PREEMPT_DYNAMIC forces CONFIG_PREEMPT_RCU=y. There's
>> no way to set CONFIG_PREEMPT_RCU=n with CONFIG_PREEMPT_DYNAMIC=y and
>> preempt=none boot parameter.  IMHO, if this feature is inconsistent with
>> CONFIG_PREEMPT_DYNAMIC, that makes it super confusing.  In fact, I feel
>> CONFIG_PREEMPT_AUTO should instead just be another "preempt=auto" boot parameter
>> mode added to CONFIG_PREEMPT_DYNAMIC feature, otherwise the proliferation of
>> CONFIG_PREEMPT config options is getting a bit insane. And likely going to be
>> burden to the users configuring the PREEMPT Kconfig option IMHO.
> 
> Because such systems are built with CONFIG_PREEMPT_DYNAMIC=n.
> 
> You could argue that we should just build with CONFIG_PREEMPT_AUTO=n,
> but the long-term goal of eliminating cond_resched() will make that
> ineffective.

I see what you mean. We/I could also highlight some of the differences in RCU
between DYNAMIC vs AUTO.

> 
>>> Note well that non-preemptible RCU explicitly disables preemption across
>>> all RCU readers.
>>
>> Yes, I mentioned this 'disabling preemption' aspect in my last email. My point
>> being, unlike CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_AUTO allows for kernel
>> preemption in preempt=none. So the "Don't preempt the kernel" behavior has
>> changed. That is, preempt=none under CONFIG_PREEMPT_AUTO is different from
>> CONFIG_PREEMPT_NONE=y already. Here we *are* preempting. And RCU is getting on
>> the way. It is like saying, you want an option for CONFIG_PREEMPT_RCU to be set
>> to =n for CONFIG_PREEMPT=y kernels, sighting users who want a fully-preemptible
>> kernel but are worried about reader preemptions.
> 
> Such users can simply avoid building with either CONFIG_PREEMPT_NONE=y
> or with CONFIG_PREEMPT_VOLUNTARY=y.  They might also experiment with
> CONFIG_RCU_BOOST=y, and also with short timeouts until boosting.
> If that doesn't do what they need, we talk with them and either help
> them configure their kernels, make RCU do what they need, or help work
> out some other way for them to get their jobs done.

Makes sense.

>> That aside, as such, I do agree your point of view, that preemptible readers
>> presents a problem to folks using preempt=none in this series and we could
>> decide to keep CONFIG_PREEMPT_RCU optional for whoever wants it that way. I was
>> just saying that I want CONFIG_PREEMPT_AUTO's preempt=none mode to be somewhat
>> consistent with CONFIG_PREEMPT_DYNAMIC's preempt=none. Because I'm pretty sure a
>> week from now, no one will likely be able to tell the difference ;-). So IMHO
>> either CONFIG_PREEMPT_DYNAMIC should be changed to make CONFIG_PREEMPT_RCU
>> optional, or this series should be altered to force CONFIG_PREEMPT_RCU=y.
>>
>> Let me know if I missed something.
> 
> Why not key off of the value of CONFIG_PREEMPT_DYNAMIC?  That way,
> if both CONFIG_PREEMPT_AUTO=y and CONFIG_PREEMPT_DYNAMIC=y, RCU is
> always preemptible.  Then CONFIG_PREEMPT_DYNAMIC=y enables boot-time
> (and maybe even run-time) switching between preemption flavors, while
> CONFIG_PREEMPT_AUTO instead enables unconditional preemption of any
> region of code that has not explicitly disabled preemption (or irq or
> bh or whatever).

That could be done. But currently, these patches disable DYNAMIC if AUTO is
enabled in the config. I think the reason is the 2 features are incompatible.
i.e. DYNAMIC wants to override the preemption mode at boot time, where as AUTO
wants the scheduler to have a say in it using the need-resched LAZY bit.

> But one way or another, we really do need non-preemptible RCU in
> conjunction with CONFIG_PREEMPT_AUTO=y.

Ok, lets go with that. Thanks,

 - Joel