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Message-ID: <CAB=+i9RR-n4q5NU6LFqmhM8ys4kM0SPqwj0zYtr4twu=yYmPPA@mail.gmail.com>
Date: Sun, 3 Dec 2023 18:23:57 +0900
From: Hyeonggon Yoo <42.hyeyoo@...il.com>
To: chengming.zhou@...ux.dev
Cc: vbabka@...e.cz, cl@...ux.com, penberg@...nel.org,
rientjes@...gle.com, iamjoonsoo.kim@....com,
akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH v5 7/9] slub: Optimize deactivate_slab()
On Thu, Nov 2, 2023 at 12:25 PM <chengming.zhou@...ux.dev> wrote:
>
> From: Chengming Zhou <zhouchengming@...edance.com>
>
> Since the introduce of unfrozen slabs on cpu partial list, we don't
> need to synchronize the slab frozen state under the node list_lock.
>
> The caller of deactivate_slab() and the caller of __slab_free() won't
> manipulate the slab list concurrently.
>
> So we can get node list_lock in the last stage if we really need to
> manipulate the slab list in this path.
>
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> Tested-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
> ---
> mm/slub.c | 79 ++++++++++++++++++-------------------------------------
> 1 file changed, 26 insertions(+), 53 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index bcb5b2c4e213..d137468fe4b9 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2468,10 +2468,8 @@ 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_FREE, M_FULL_NOLIST };
> struct kmem_cache_node *n = get_node(s, slab_nid(slab));
> 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;
> @@ -2509,65 +2507,40 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
> /*
> * Stage two: Unfreeze the slab while splicing the per-cpu
> * freelist to the head of slab's freelist.
> - *
> - * Ensure that the slab is unfrozen while the list presence
> - * reflects the actual number of objects during unfreeze.
> - *
> - * 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:
> -
> - old.freelist = READ_ONCE(slab->freelist);
> - old.counters = READ_ONCE(slab->counters);
> - VM_BUG_ON(!old.frozen);
> -
> - /* Determine target state of the slab */
> - new.counters = old.counters;
> - if (freelist_tail) {
> - new.inuse -= free_delta;
> - set_freepointer(s, freelist_tail, old.freelist);
> - new.freelist = freelist;
> - } else
> - new.freelist = old.freelist;
> -
> - new.frozen = 0;
> + do {
> + old.freelist = READ_ONCE(slab->freelist);
> + old.counters = READ_ONCE(slab->counters);
> + VM_BUG_ON(!old.frozen);
> +
> + /* Determine target state of the slab */
> + new.counters = old.counters;
> + new.frozen = 0;
> + if (freelist_tail) {
> + new.inuse -= free_delta;
> + set_freepointer(s, freelist_tail, old.freelist);
> + new.freelist = freelist;
> + } else {
> + new.freelist = old.freelist;
> + }
> + } while (!slab_update_freelist(s, slab,
> + old.freelist, old.counters,
> + new.freelist, new.counters,
> + "unfreezing slab"));
>
> + /*
> + * Stage three: Manipulate the slab list based on the updated state.
> + */
deactivate_slab() might unconsciously put empty slabs into partial list, like:
deactivate_slab() __slab_free()
cmpxchg(), slab's not empty
cmpxchg(), slab's empty
and unfrozen
spin_lock(&n->list_lock)
(slab's empty but not
on partial list,
spin_unlock(&n->list_lock) and return)
spin_lock(&n->list_lock)
put slab into partial list
spin_unlock(&n->list_lock)
IMHO it should be fine in the real world, but just wanted to
mention as it doesn't seem to be intentional.
Otherwise it looks good to me!
> if (!new.inuse && n->nr_partial >= s->min_partial) {
> - mode = M_FREE;
> + stat(s, DEACTIVATE_EMPTY);
> + discard_slab(s, slab);
> + stat(s, FREE_SLAB);
> } else if (new.freelist) {
> - 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 {
> - mode = M_FULL_NOLIST;
> - }
> -
> -
> - if (!slab_update_freelist(s, slab,
> - old.freelist, old.counters,
> - new.freelist, new.counters,
> - "unfreezing slab")) {
> - if (mode == M_PARTIAL)
> - spin_unlock_irqrestore(&n->list_lock, flags);
> - goto redo;
> - }
> -
> -
> - if (mode == M_PARTIAL) {
> add_partial(n, slab, tail);
> spin_unlock_irqrestore(&n->list_lock, flags);
> stat(s, tail);
> - } else if (mode == M_FREE) {
> - stat(s, DEACTIVATE_EMPTY);
> - discard_slab(s, slab);
> - stat(s, FREE_SLAB);
> - } else if (mode == M_FULL_NOLIST) {
> + } else {
> stat(s, DEACTIVATE_FULL);
> }
> }
> --
> 2.20.1
>
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