Re: [PATCH] mm: percpu: Increase PERCPU_DYNAMIC_SIZE_SHIFT on certain builds.

[Vlastimil Babka <vbabka@suse.cz>]

On 10/7/24 4:30 PM, Sebastian Andrzej Siewior wrote:
> Arnd reported a build failure due to the BUILD_BUG_ON() statement in
> alloc_kmem_cache_cpus(). The test
> 
>   PERCPU_DYNAMIC_EARLY_SIZE < NR_KMALLOC_TYPES * KMALLOC_SHIFT_HIGH * sizeof(struct kmem_cache_cpu)
> 
> The factors that increase the right side of the equation:
> - PAGE_SIZE > 4KiB increases KMALLOC_SHIFT_HIGH
> - For the local_lock_t in kmem_cache_cpu:
>   - PREEMPT_RT adds an actual lock.
>   - LOCKDEP increases the size of the lock.
>   - LOCK_STAT adds additional bytes plus padding to the lockdep
>     structure.
> 
> The net difference with and without PREEMPT_RT is 88 bytes for the
> lock_lock_t, 96 bytes for kmem_cache_cpu due to additional padding.
> This is enough to exceed the 80KiB limit with 16KiB page size - the 8KiB
> page size is fine.
> 
> Increase PERCPU_DYNAMIC_SIZE_SHIFT to 13 on configs with PAGE_SIZE larger
> than 4KiB and LOCKDEP enabled.
> 
> Fixes: d8fccd9ca5f9 ("arm64: Allow to enable PREEMPT_RT.")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202410020326.iaZIteIx-lkp@intel.com/
> Reported-by: Arnd Bergmann <arnd@kernel.org>
> Closes: https://lore.kernel.org/20241004095702.637528-1-arnd@kernel.org
> Acked-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

IIRC Dennis at some point mentioned this will eventually become more
robust as now it's kind of a whack-a-mole? :) But yeah, let's fix in a
way that we can.

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  include/linux/percpu.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> index b6321fc491598..52b5ea663b9f0 100644
> --- a/include/linux/percpu.h
> +++ b/include/linux/percpu.h
> @@ -41,7 +41,11 @@
>  					 PCPU_MIN_ALLOC_SHIFT)
>  
>  #ifdef CONFIG_RANDOM_KMALLOC_CACHES
> -#define PERCPU_DYNAMIC_SIZE_SHIFT      12
> +# if defined(CONFIG_LOCKDEP) && !defined(CONFIG_PAGE_SIZE_4KB)
> +# define PERCPU_DYNAMIC_SIZE_SHIFT      13
> +# else
> +# define PERCPU_DYNAMIC_SIZE_SHIFT      12
> +#endif /* LOCKDEP and PAGE_SIZE > 4KiB */
>  #else
>  #define PERCPU_DYNAMIC_SIZE_SHIFT      10
>  #endif