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

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Joonsoo,

On Tue, Jun 14, 2016 at 8:24 AM, Joonsoo Kim <iamjoonsoo.kim@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@gmail.com> wrote:
>> > From: Joonsoo Kim <iamjoonsoo.kim@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@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.

> ----->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()) {

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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