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SubjectRe: [PATCH -mm v2 3/3] slub: make dead caches discard free slabs immediately
On Wed, Jan 28, 2015 at 07:22:51PM +0300, Vladimir Davydov wrote:
> +++ b/mm/slub.c
> @@ -2007,6 +2007,7 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
> int pages;
> int pobjects;
>
> + preempt_disable();
> do {
> pages = 0;
> pobjects = 0;
> @@ -2040,6 +2041,14 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
>
> } while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
> != oldpage);
> + if (unlikely(!s->cpu_partial)) {
> + unsigned long flags;
> +
> + local_irq_save(flags);
> + unfreeze_partials(s, this_cpu_ptr(s->cpu_slab));
> + local_irq_restore(flags);
> + }
> + preempt_enable();
> #endif
> }
>
> @@ -3369,7 +3378,7 @@ EXPORT_SYMBOL(kfree);
> * being allocated from last increasing the chance that the last objects
> * are freed in them.
> */
> -int __kmem_cache_shrink(struct kmem_cache *s)
> +int __kmem_cache_shrink(struct kmem_cache *s, bool deactivate)
> {
> int node;
> int i;
> @@ -3381,14 +3390,26 @@ int __kmem_cache_shrink(struct kmem_cache *s)
> unsigned long flags;
> int ret = 0;
>
> + if (deactivate) {
> + /*
> + * Disable empty slabs caching. Used to avoid pinning offline
> + * memory cgroups by kmem pages that can be freed.
> + */
> + s->cpu_partial = 0;
> + s->min_partial = 0;
> +
> + /*
> + * s->cpu_partial is checked locklessly (see put_cpu_partial),
> + * so we have to make sure the change is visible.
> + */
> + kick_all_cpus_sync();
> + }

Argh! what the heck! and without a single mention in the changelog.

Why are you spraying IPIs across the entire machine? Why isn't
synchronize_sched() good enough, that would allow you to get rid of the
local_irq_save/restore as well.

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