Re: [PATCH for-next v3 6/9] mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT

[Harry Yoo]

On 6/18/26 5:40 AM, Paul E. McKenney wrote:
> On Wed, Jun 17, 2026 at 07:38:16AM +0200, Vlastimil Babka (SUSE) wrote:
>> On 6/17/26 07:14, Harry Yoo wrote:
>>> On 6/17/26 2:24 AM, Vlastimil Babka (SUSE) wrote:
>>>> On 6/15/26 13:06, Harry Yoo (Oracle) wrote:
>>>>> As suggested by Vlastimil Babka, kfree_rcu_sheaf() can be used
>>>>> on PREEMPT_RT if we always assume spinning is not allowed on PREEMPT_RT.
>>>>> This is because local_trylock and spinlock_t are safe to use with
>>>>> trylock variant as long as the kernel does not spin and the context is
>>>>> not NMI and not hardirq.
>>>>>
>>>>> Now that __kfree_rcu_sheaf() knows how to handle allow_spin = false,
>>>>> relax the limitation and try the sheaves path on PREEMPT_RT as well.
>>>>>
>>>>> Keep the lockdep map on non RT kernels. However, do not use the lockdep
>>>>> map on PREEMPT_RT to avoid suppressing valid lockdep warnings.
>>>>>
>>>>> Link: https://lore.kernel.org/linux-mm/6811cc17-8ee4-48c8-8cbf-6bf4d9f98162@xxxxxxxxxx
>>>>> Suggested-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
>>>>> Signed-off-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
>>>>
>>>> LGTM, but maybe unnecessary pessimistic wrt call_rcu() on PREEMPT_RT?
>>>> I thought (in the Link: above) we'd only need to downgrade allow_spin to
>>>> false on PREEMPT_RT for handling sheaves movement from/to barn and
>>>> alloc_empty_sheaf(), but call_rcu() would be safe from kfree_rcu() even on
>>>> RT?
>>>
>>> Indeed. Good point, thanks!
>>>
>>> Hmm, but I'm not sure that it's worth the complexity given that
>>> PREEMPT_RT tries very hard to avoid disabling IRQs...
>>>
>>>> Or is the irqs_disabled() condition rare enough so we don't care?
>>>
>>> Given that most users don't call kfree_rcu() under raw spinlock or
>>> IRQs-disabled section on PREEMPT_RT, I think it's okay to keep it as is
>>> (it's not making things worse, at least) and wait for call_rcu_nolock()?
>>
>> Sounds good.
>>
>>> On a side note, I don't have much idea on what needs to call kfree_rcu()
>>> under a raw spinlock, other than set_cpus_allowed_force(), which should
>>> really be using kfree_nolock() instead of kfree_rcu() once we support
>>> kmalloc() -> kfree_nolock():
>>
>> Looks like the case. Well if the fallback path of kfree_nolock() that is
>> irq_work_queue() is indeed safe here.
>>
>>>> 	/*
>>>> 	 * Because this is called with p->pi_lock held, it is not possible
>>>> 	 * to use kfree() here (when PREEMPT_RT=y), therefore punt to using
>>>> 	 * kfree_rcu().
>>>> 	 */
>>>> 	kfree_rcu((union cpumask_rcuhead *)ac.user_mask, rcu);
>>>
>>> Any thoughts, RCU/RT folks? 

Thanks for looking into it, Paul!

Perhaps I'm missing some context here... let me clarify.

> For the call_rcu*() counterparts, I am currently considering making the
> existing functions check for interrupts disabled, using irq_work_queue()
> in that case. 

Ack.

> I suppose that I could use raise_softirq() in the
> use_softirq=1 case when in_hardirq(). 

Ack.

> Either way, the check should be
> cheap compared to rest of the processing.

Agreed.

> The additional rcu_barrier() work required is of course way down in

Assuming "The additional rcu_barrier() work required" means
rcu_barrier() now needs to wait for all CPUs to complete irq_work or
softirq...

> the noise compared to acquiring a global mutex.

Are you referring to a specific mutex, or just in general?

> In your case, kfree_rcu() can be quite a bit lighter weight, though.
> So the extra checks might not be lost in the noise.

Assuming "the extra checks" means checking whether interrupts are
disabled in call_rcu()*...

It will probably be fine since we invoke call_rcu() only when sheaves
become full, not once for each object?

Perhaps slightly off-topic; at some point, though, I think it'd be
better to teach SLUB to use RCU polling API for RCU sheaves.

e.g.) put sheaves into barn->sheaves_pending instead of invoking
call_rcu(), and the SLUB alloc slowpath checks if there are sheaves
past grace period before allocating and refilling sheaves.

-- 
Cheers,
Harry / Hyeonggon

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