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/[email protected] >>>>> Suggested-by: Vlastimil Babka (SUSE) <[email protected]> >>>>> Signed-off-by: Harry Yoo (Oracle) <[email protected]> >>>> >>>> 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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