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

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?

For the call_rcu*() counterparts, I am currently considering making the
existing functions check for interrupts disabled, using irq_work_queue()
in that case.  I suppose that I could use raise_softirq() in the
use_softirq=1 case when in_hardirq().  Either way, the check should be
cheap compared to rest of the processing.  The additional rcu_barrier()
work required is of course way down in the noise compared to acquiring
a global mutex.

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

Thoughts?

								Thanx, Paul