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

[Ankur Arora <ankur.a.arora@oracle.com>]

Joel Fernandes <joel@joelfernandes.org> writes:

> 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).

Currently CONFIG_PREEMPT_DYNAMIC does a few things:

1. dynamic selection of preemption model
2. dynamically toggling explicit preemption points
3. PREEMPT_RCU=y (though maybe this should be fixed to also
   also allow PREEMPT_RCU=n)

Of these 3, PREEMPT_AUTO only really needs (1).

Maybe combining gives us the option of switching between the old and the
new models:
  preempt=none | voluntary | full | auto-none | auto-voluntary

Where the last two provide the new auto semantics. But, the mixture
seems too rich.
This just complicates all the CONFIG_PREEMPT_* configurations more than
they were before when the end goal is to actually reduce and simplify
the number of options.

> 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.

Yeah exactly. That's why I originally made PREEMPT_AUTO and
PREEMPT_DYNAMIC exclusive of each other.

Thanks

--
ankur