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

[Geert Uytterhoeven]

Hi Joonsoo,

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?

Thanks!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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

Attachment: kzm9d_config
Description: Binary data