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

[Joonsoo Kim]

On Tue, Jun 14, 2016 at 09:31:23AM +0200, Geert Uytterhoeven wrote:
> Hi Joonsoo,
> 
> On Tue, Jun 14, 2016 at 8:24 AM, Joonsoo Kim <iamjoonsoo.kim at lge.com> wrote:
> > On Mon, Jun 13, 2016 at 09:43:13PM +0200, Geert Uytterhoeven wrote:
> >> On Tue, Apr 12, 2016 at 6:51 AM,  <js1304 at gmail.com> wrote:
> >> > From: Joonsoo Kim <iamjoonsoo.kim at lge.com>
> >> > 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 at lge.com>
> >>
> >> 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.
> 
> In the mean time, I've verified it also happens with shmobile_defconfig.
> 
> >>
> >> 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!
> 
> Unfortunately the half revert is not sufficient. The full revert is.

Thanks for quick testing!

Could I ask one more time to check that synchronize_sched() is root
cause of the problem? Testing following two diffs will be helpful to me.

Thanks.

------->8--------
diff --git a/mm/slab.c b/mm/slab.c
index 763096a..d892364 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -965,7 +965,7 @@ static int setup_kmem_cache_node(struct kmem_cache *cachep,
         * freed after synchronize_sched().
         */
        if (force_change)
-               synchronize_sched();
+               kick_all_cpus_sync();
 
 fail:
        kfree(old_shared);

------->8------
diff --git a/mm/slab.c b/mm/slab.c
index 763096a..38d99c2 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -964,8 +964,6 @@ static int setup_kmem_cache_node(struct kmem_cache *cachep,
         * guaranteed to be valid until irq is re-enabled, because it will be
         * freed after synchronize_sched().
         */
-       if (force_change)
-               synchronize_sched();
 
 fail:
        kfree(old_shared);