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

[Joel Fernandes <joel@joelfernandes.org>]

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.

> 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.

> 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.

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.

Thanks!

 - Joel