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SubjectRe: [PATCH v3 2/2] mm/slub: refactor deactivate_slab()
Hello,

On Mon, Mar 7, 2022 at 3:41 PM Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
>
> Simplify deactivate_slab() by unlocking n->list_lock and retrying
> cmpxchg_double() when cmpxchg_double() fails, and perform
> add_{partial,full} only when it succeed.
>
> Releasing and taking n->list_lock again here is not harmful as SLUB
> avoids deactivating slabs as much as possible.
>
> [ vbabka@suse.cz: perform add_{partial,full} when cmpxchg_double()
> succeed.
>
> count deactivating full slabs even if debugging flag is not set. ]
>
> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> ---
> mm/slub.c | 91 +++++++++++++++++++++++--------------------------------
> 1 file changed, 38 insertions(+), 53 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 1ce09b0347ad..f0cb9d0443ac 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2348,10 +2348,10 @@ static void init_kmem_cache_cpus(struct kmem_cache *s)
> static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
> void *freelist)
> {
> - enum slab_modes { M_NONE, M_PARTIAL, M_FULL, M_FREE };
> + enum slab_modes { M_NONE, M_PARTIAL, M_FULL, M_FREE, M_FULL_NOLIST };
> struct kmem_cache_node *n = get_node(s, slab_nid(slab));
> - int lock = 0, free_delta = 0;
> - enum slab_modes l = M_NONE, m = M_NONE;
> + int free_delta = 0;
> + enum slab_modes mode = M_NONE;
> void *nextfree, *freelist_iter, *freelist_tail;
> int tail = DEACTIVATE_TO_HEAD;
> unsigned long flags = 0;
> @@ -2393,14 +2393,10 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
> * Ensure that the slab is unfrozen while the list presence
> * reflects the actual number of objects during unfreeze.
> *
> - * We setup the list membership and then perform a cmpxchg
> - * with the count. If there is a mismatch then the slab
> - * is not unfrozen but the slab is on the wrong list.
> - *
> - * Then we restart the process which may have to remove
> - * the slab from the list that we just put it on again
> - * because the number of objects in the slab may have
> - * changed.
> + * We first perform cmpxchg holding lock and insert to list
> + * when it succeed. If there is mismatch then the slab is not
> + * unfrozen and number of objects in the slab may have changed.
> + * Then release lock and retry cmpxchg again.
> */
> redo:
>
> @@ -2420,61 +2416,50 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
> new.frozen = 0;
>
> if (!new.inuse && n->nr_partial >= s->min_partial)
> - m = M_FREE;
> + mode = M_FREE;
> else if (new.freelist) {
> - m = M_PARTIAL;
> - if (!lock) {
> - lock = 1;
> - /*
> - * Taking the spinlock removes the possibility that
> - * acquire_slab() will see a slab that is frozen
> - */
> - spin_lock_irqsave(&n->list_lock, flags);
> - }
> - } else {
> - m = M_FULL;
> - if (kmem_cache_debug_flags(s, SLAB_STORE_USER) && !lock) {
> - lock = 1;
> - /*
> - * This also ensures that the scanning of full
> - * slabs from diagnostic functions will not see
> - * any frozen slabs.
> - */
> - spin_lock_irqsave(&n->list_lock, flags);
> - }
> - }
> -
> - if (l != m) {
> - if (l == M_PARTIAL)
> - remove_partial(n, slab);
> - else if (l == M_FULL)
> - remove_full(s, n, slab);
> + mode = M_PARTIAL;
> + /*
> + * Taking the spinlock removes the possibility that
> + * acquire_slab() will see a slab that is frozen
> + */
> + spin_lock_irqsave(&n->list_lock, flags);
> + } else if (kmem_cache_debug_flags(s, SLAB_STORE_USER)) {
> + mode = M_FULL;
> + /*
> + * This also ensures that the scanning of full
> + * slabs from diagnostic functions will not see
> + * any frozen slabs.
> + */
> + spin_lock_irqsave(&n->list_lock, flags);
> + } else
> + mode = M_FULL_NOLIST;
>
> - if (m == M_PARTIAL)
> - add_partial(n, slab, tail);
> - else if (m == M_FULL)
> - add_full(s, n, slab);
> - }
>
> - l = m;
> if (!cmpxchg_double_slab(s, slab,
> old.freelist, old.counters,
> new.freelist, new.counters,
> - "unfreezing slab"))
> + "unfreezing slab")) {
> + if (mode == M_PARTIAL || mode == M_FULL)
> + spin_unlock_irqrestore(&n->list_lock, flags);

The slab doesn't belong to any node here, should we remove locking/unlocking
spin for cmpxchg_double_slab() call? Just calling spin_lock_irqsave() before
add_partial()/add_full call is fine?

> goto redo;

How about do {...} while(!cmpxchg_double_slab())? The readability looks better?

Regards,
Xiongwei

> + }
>
> - if (lock)
> - spin_unlock_irqrestore(&n->list_lock, flags);
>
> - if (m == M_PARTIAL)
> + if (mode == M_PARTIAL) {
> + add_partial(n, slab, tail);
> + spin_unlock_irqrestore(&n->list_lock, flags);
> stat(s, tail);
> - else if (m == M_FULL)
> - stat(s, DEACTIVATE_FULL);
> - else if (m == M_FREE) {
> + } else if (mode == M_FREE) {
> stat(s, DEACTIVATE_EMPTY);
> discard_slab(s, slab);
> stat(s, FREE_SLAB);
> - }
> + } else if (mode == M_FULL) {
> + add_full(s, n, slab);
> + spin_unlock_irqrestore(&n->list_lock, flags);
> + stat(s, DEACTIVATE_FULL);
> + } else if (mode == M_FULL_NOLIST)
> + stat(s, DEACTIVATE_FULL);
> }
>
> #ifdef CONFIG_SLUB_CPU_PARTIAL
> --
> 2.33.1
>
>

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