Re: Boot failure on emev2/kzm9d (was: Re: [PATCH v2 11/11] mm/slab: lockless decision to grow cache)

[Joonsoo Kim]

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,  <js1304@xxxxxxxxx> wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> >
> > 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 <iamjoonsoo.kim@xxxxxxx>
> 
> 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()) {