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

[Geert Uytterhoeven]

Hi Joonsoo,

On Tue, Jun 14, 2016 at 10:11 AM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> On Tue, Jun 14, 2016 at 09:31:23AM +0200, Geert Uytterhoeven wrote:
>> On Tue, Jun 14, 2016 at 8:24 AM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
>> > On Mon, Jun 13, 2016 at 09:43:13PM +0200, Geert Uytterhoeven wrote:
>> >> 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.
>>
>> 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);

Works.

> ------->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);
>

Also works.

Note that I do not see this problem on any of the other boards I use, one
of which is also a dual Cortex A9.

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