Re: [RFC v2 PATCH] mm, sl[au]b: Introduce lockless cache
From: Hyeonggon Yoo <hidden>
Date: 2021-09-22 08:32:37
Also in:
linux-block, linux-mm, lkml
Subsystem:
memory management, slab allocator, the rest · Maintainers:
Andrew Morton, Vlastimil Babka, Harry Yoo, Linus Torvalds
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:quoted
quoted
quoted
+ /* 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);