Re: [PATCH 4/7] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y

[Ankur Arora]

Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> writes:

> On 2024-10-09 09:54:08 [-0700], 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 needed, was 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_LAZY=y, there can be configurations with PREEMPT_COUNT=y,
>> PREEMPT_RCU=n, where cond_resched() is a stub that does not provide
>> quiescent states via rcu_all_qs().
>
> PREEMPT_LAZY selects PREEMPT_BUILD which selects PREEMPTION which in
> turn selects PREEMPT_RCU. So this is not a valid combination. Do you
> have a different tree than I do? Because maybe I am missing something.

The second patch in the series?

>> So, use the availability of preempt_count() to report quiescent states
>> in rcu_flavor_sched_clock_irq().
>>
>> Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>> Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
>> ---
>>  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 1c7cbd145d5e..da324d66034b 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -974,13 +974,16 @@ 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)))) {
>
> couldn't you use a helper preemptible()?

Alas no. This check isn't trying to establish preemptibility (this is
called in irq context so we already know that we aren't preemptible.)
The check is using the preempt count to see if it can infer the state
of RCU read side critical section on this CPU.

>>  		/*
>>  		 * Get here if this CPU took its interrupt from user
>> -		 * mode or from the idle loop, and if this is not a
>> -		 * nested interrupt.  In this case, the CPU is in
>> -		 * a quiescent state, so note it.
>> +		 * mode, from the idle loop without this being a nested
>> +		 * interrupt, or while not holding a preempt count (but
>> +		 * with PREEMPT_COUNT=y. In this case, the CPU is in a
>> +		 * quiescent state, so note it.
>>  		 *
>>  		 * No memory barrier is required here because rcu_qs()
>>  		 * references only CPU-local variables that other CPUs
>
> Sebastian


--
ankur