On Tue, Aug 23, 2022 at 07:04:00PM +0200, Vlastimil Babka wrote:
> The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
> (through slab_[un]lock()) is unnecessary as bit_spin_lock() disables
> preemption and that's sufficient on RT where interrupts are threaded.
>
> That means we no longer need the slab_[un]lock() wrappers, so delete
> them and rename the current __slab_[un]lock() to slab_[un]lock().
>
I'm not familiar with PREEMPT_RT preemption model so not sure I'm following.
1) Does "interrupts are threaded on RT" mean processing _most_ (all handlers
that did not specified IRQF_NO_THREAD) of interrupts are delayed to irq threads
and processed later in process context, and the kernel *never* use
spinlock_t, local_lock_t that does not disable interrupts (and sleep) on RT
in hardware/software interrupt context?
2) Do we need disabling irq in cmpxchg_double_slab() on RT?
BTW Is there a good documentation/papers on PREEMPT_RT preemption model?
I tried to find but only found Documentation/locking/locktypes.rst :(
Thanks!
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> mm/slub.c | 39 ++++++++++++---------------------------
> 1 file changed, 12 insertions(+), 27 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 0444a2ba4f12..bb8c1292d7e8 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -446,7 +446,7 @@ slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
> /*
> * Per slab locking using the pagelock
> */
> -static __always_inline void __slab_lock(struct slab *slab)
> +static __always_inline void slab_lock(struct slab *slab)
> {
> struct page *page = slab_page(slab);
>
> @@ -454,7 +454,7 @@ static __always_inline void __slab_lock(struct slab *slab)
> bit_spin_lock(PG_locked, &page->flags);
> }
>
> -static __always_inline void __slab_unlock(struct slab *slab)
> +static __always_inline void slab_unlock(struct slab *slab)
> {
> struct page *page = slab_page(slab);
>
> @@ -462,24 +462,12 @@ static __always_inline void __slab_unlock(struct slab *slab)
> __bit_spin_unlock(PG_locked, &page->flags);
> }
>
> -static __always_inline void slab_lock(struct slab *slab, unsigned long *flags)
> -{
> - if (IS_ENABLED(CONFIG_PREEMPT_RT))
> - local_irq_save(*flags);
> - __slab_lock(slab);
> -}
> -
> -static __always_inline void slab_unlock(struct slab *slab, unsigned long *flags)
> -{
> - __slab_unlock(slab);
> - if (IS_ENABLED(CONFIG_PREEMPT_RT))
> - local_irq_restore(*flags);
> -}
> -
> /*
> * Interrupts must be disabled (for the fallback code to work right), typically
> - * by an _irqsave() lock variant. Except on PREEMPT_RT where locks are different
> - * so we disable interrupts as part of slab_[un]lock().
> + * by an _irqsave() lock variant. Except on PREEMPT_RT where these variants do
> + * not actually disable interrupts. On the other hand the migrate_disable()
> + * done by bit_spin_lock() is sufficient on PREEMPT_RT thanks to its threaded
> + * interrupts.
> */
> static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
> void *freelist_old, unsigned long counters_old,
> @@ -498,18 +486,15 @@ static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab
> } else
> #endif
> {
> - /* init to 0 to prevent spurious warnings */
> - unsigned long flags = 0;
> -
> - slab_lock(slab, &flags);
> + slab_lock(slab);
> if (slab->freelist == freelist_old &&
> slab->counters == counters_old) {
> slab->freelist = freelist_new;
> slab->counters = counters_new;
> - slab_unlock(slab, &flags);
> + slab_unlock(slab);
> return true;
> }
> - slab_unlock(slab, &flags);
> + slab_unlock(slab);
> }
>
> cpu_relax();
> @@ -540,16 +525,16 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
> unsigned long flags;
>
> local_irq_save(flags);
> - __slab_lock(slab);
> + slab_lock(slab);
> if (slab->freelist == freelist_old &&
> slab->counters == counters_old) {
> slab->freelist = freelist_new;
> slab->counters = counters_new;
> - __slab_unlock(slab);
> + slab_unlock(slab);
> local_irq_restore(flags);
> return true;
> }
> - __slab_unlock(slab);
> + slab_unlock(slab);
> local_irq_restore(flags);
> }
>
> --
> 2.37.2
>
--
Thanks,
Hyeonggon
