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SubjectRe: [PATCH v3 2/2] mm/slub: refactor deactivate_slab()
On 3/7/22 08:40, Hyeonggon Yoo 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>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

adding both to slab-next. Fixed up some nits myself, see below:

>
> @@ -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) {

This was against kernel style even before the patch - we use { } in the
'else if' branch, thus all branches should use { } even if one-line.

> - 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;

Ditto here (this is new).

> - 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);
> goto redo;
> + }
>
> - 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);

And here.

> }
>
> #ifdef CONFIG_SLUB_CPU_PARTIAL

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