Re: [RFC v2 PATCH] mm, sl[au]b: Introduce lockless cache

[Hyeonggon Yoo]

Hello Matthew.
There's good news.

in v3 (work in progress now), I fixed some bugs (I hate kernel panics!)
And for test, made NAPI use it. it works pretty well.

On Tue, Sep 21, 2021 at 05:17:02PM +0100, Matthew Wilcox wrote:
> On Tue, Sep 21, 2021 at 03:42:39PM +0000, Hyeonggon Yoo wrote:
> > > > +	/* slowpath */
> > > > +	cache->size = kmem_cache_alloc_bulk(s, gfpflags,
> > > > +			KMEM_LOCKLESS_CACHE_QUEUE_SIZE, cache->queue);
> > > 
> > > Go back to the Bonwick paper and look at the magazine section again.
> > > You have to allocate _half_ the size of the queue, otherwise you get
> > > into pathological situations where you start to free and allocate
> > > every time.
> > 
> > I want to ask you where idea of allocating 'half' the size of queue came from.
> > the paper you sent does not work with single queue(magazine). Instead,
> > it manages pool of magazines.
> > 
> > And after reading the paper, I see managing pool of magazine (where M is
> > an boot parameter) is valid approach to reduce hitting slowpath.
> 
> Bonwick uses two magazines per cpu; if both are empty, one is replaced
> with a full one.  If both are full, one is replaced with an empty one.
> Our slab implementation doesn't provide magazine allocation, but it does
> provide bulk allocation.
> So translating the Bonwick implementation to
> our implementation, we need to bulk-allocate or bulk-free half of the
> array at any time.

Is there a reason that the number should be 'half'?

what about something like this:

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 884d3311cd8e..f32736302d53 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -455,12 +455,13 @@ void *kmem_cache_alloc_cached(struct kmem_cache *s, gfp_t gfpflags)
        }

        cache = get_cpu_ptr(s->cache);
-       if (cache->size) /* fastpath without lock */
+       if (cache->size) /* fastpath without lock */
                p = cache->queue[--cache->size];
        else {
                /* slowpath */
-               cache->size = kmem_cache_alloc_bulk(s, gfpflags,
-                               KMEM_LOCKLESS_CACHE_QUEUE_SIZE, cache->queue);
+               cache->size += kmem_cache_alloc_bulk(s, gfpflags,
+                               KMEM_LOCKLESS_CACHE_BATCHCOUNT,
+                               cache->queue);
                if (cache->size)
                        p = cache->queue[--cache->size];
                else
@@ -491,13 +492,13 @@ void kmem_cache_free_cached(struct kmem_cache *s, void *p)
        cache = get_cpu_ptr(s->cache);
        if (cache->size < KMEM_LOCKLESS_CACHE_QUEUE_SIZE) {
                cache->queue[cache->size++] = p;
-               put_cpu_ptr(s->cache);
-               return ;
+       } else {
+               kmem_cache_free_bulk(s,
+                               KMEM_LOCKLESS_CACHE_BATCHCOUNT,
+                               cache->queue - KMEM_LOCKLESS_CACHE_BATCHCOUNT);
+               cache->size -= KMEM_LOCKLESS_CACHE_BATCHCOUNT;
        }
        put_cpu_ptr(s->cache);
-
-       /* Is there better way to do this? */
-       kmem_cache_free(s, p);
 }
 EXPORT_SYMBOL(kmem_cache_free_cached);