On Mon, Jun 13, 2016 at 09:43:13PM +0200, Geert Uytterhoeven wrote:

Hi Joonsoo,

Hello,

On Tue, Apr 12, 2016 at 6:51 AM,  [off-list ref] wrote:

quoted

From: Joonsoo Kim <redacted>

To check whther free objects exist or not precisely, we need to grab a
lock.  But, accuracy isn't that important because race window would be
even small and if there is too much free object, cache reaper would reap
it.  So, this patch makes the check for free object exisistence not to
hold a lock.  This will reduce lock contention in heavily allocation case.

Note that until now, n->shared can be freed during the processing by
writing slabinfo, but, with some trick in this patch, we can access it
freely within interrupt disabled period.

Below is the result of concurrent allocation/free in slab allocation
benchmark made by Christoph a long time ago.  I make the output simpler.
The number shows cycle count during alloc/free respectively so less is
better.

* Before
Kmalloc N*alloc N*free(32): Average=248/966
Kmalloc N*alloc N*free(64): Average=261/949
Kmalloc N*alloc N*free(128): Average=314/1016
Kmalloc N*alloc N*free(256): Average=741/1061
Kmalloc N*alloc N*free(512): Average=1246/1152
Kmalloc N*alloc N*free(1024): Average=2437/1259
Kmalloc N*alloc N*free(2048): Average=4980/1800
Kmalloc N*alloc N*free(4096): Average=9000/2078

* After
Kmalloc N*alloc N*free(32): Average=344/792
Kmalloc N*alloc N*free(64): Average=347/882
Kmalloc N*alloc N*free(128): Average=390/959
Kmalloc N*alloc N*free(256): Average=393/1067
Kmalloc N*alloc N*free(512): Average=683/1229
Kmalloc N*alloc N*free(1024): Average=1295/1325
Kmalloc N*alloc N*free(2048): Average=2513/1664
Kmalloc N*alloc N*free(4096): Average=4742/2172

It shows that allocation performance decreases for the object size up to
128 and it may be due to extra checks in cache_alloc_refill().  But, with
considering improvement of free performance, net result looks the same.
Result for other size class looks very promising, roughly, 50% performance
improvement.

v2: replace kick_all_cpus_sync() with synchronize_sched().

Signed-off-by: Joonsoo Kim <redacted>

I've bisected a boot failure (no output at all) in v4.7-rc2 on emev2/kzm9d
(Renesas dual Cortex A9) to this patch, which is upstream commit
801faf0db8947e01877920e848a4d338dd7a99e7.

I've attached my .config. I don't know if it also happens with
shmobile_defconfig, as something went wrong with my remote access to the board,
preventing further testing. I also couldn't verify if the issue persists in
v4.7-rc3.

Do you have a clue?

I don't have yet. Could you help me to narrow down the problem?
Following diff is half-revert change to check that synchronize_sched()
has no problem.

Thanks.

----->8-----

diff --git a/mm/slab.c b/mm/slab.c
index 763096a..257a0eb 100644
--- a/mm/slab.c
+++ b/mm/slab.c

@@ -3016,9 +3016,6 @@ static void *cache_alloc_refill(struct kmem_cache *cachep, gfp_t flags)
        n = get_node(cachep, node);
 
        BUG_ON(ac->avail > 0 || !n);
-       shared = READ_ONCE(n->shared);
-       if (!n->free_objects && (!shared || !shared->avail))
-               goto direct_grow;
 
        spin_lock(&n->list_lock);
        shared = READ_ONCE(n->shared);

@@ -3047,7 +3044,6 @@ alloc_done:
        spin_unlock(&n->list_lock);
        fixup_objfreelist_debug(cachep, &list);
 
-direct_grow:
        if (unlikely(!ac->avail)) {
                /* Check if we can use obj in pfmemalloc slab */
                if (sk_memalloc_socks()) {