Re: [PATCH 1/9] slub: Make PREEMPT_RT support less convoluted

[Vlastimil Babka <vbabka@suse.cz>]

On 8/18/22 11:42, Christoph Lameter wrote:
> On Wed, 17 Aug 2022, Sebastian Andrzej Siewior wrote:
> 
>> + *   On PREEMPT_RT, the local lock neither disables interrupts nor preemption
>> + *   which means the lockless fastpath cannot be used as it might interfere with
>> + *   an in-progress slow path operations. In this case the local lock is always
>> + *   taken but it still utilizes the freelist for the common operations.
> 
> The slub fastpath does not interfere with slow path operations and the

That's true on !PREEMPT_RT because a slowpath operation under
local_lock_irqsave() will disable interrupts, so there can't be a
fastpath operation in an interrupt handler appearing in the middle of a
slowpath operation.

On PREEMPT_RT local_lock_irqsave() doesn't actually disable interrupts,
so that can happen. IIRC we learned that the hard way when Mike
Galbraith was testing early versions of my PREEMPT_RT changes for SLUB.

> fastpath does not require disabling preemption or interrupts if the
> processor supports local rmv operations. So you should be able to use the
> fastpath on PREEMPT_RT.
> 
> If the fastpath is not possible then you need to disable preemption and
> eventually take locks etc and then things may get a bit more complicated.

Yeah that's basically the solution for PREEMPT_RT, take the local_lock.