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@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202410020326.iaZIteIx-lkp@xxxxxxxxx/
> Reported-by: Arnd Bergmann <arnd@xxxxxxxxxx>
> Closes: https://lore.kernel.org/20241004095702.637528-1-arnd@xxxxxxxxxx
> Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
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@xxxxxxx>
> ---
> 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
