Re: [RFC v2 33/34] mm, slub: use migrate_disable() on PREEMPT_RT

[Vlastimil Babka]

On 6/9/21 1:39 PM, Vlastimil Babka wrote:
> We currently use preempt_disable() (directly or via get_cpu_ptr()) to stabilize
> the pointer to kmem_cache_cpu. On PREEMPT_RT this would be incompatible with
> the list_lock spinlock. We can use migrate_disable() instead, but that
> increases overhead on !PREEMPT_RT as it's an unconditional function call even
> though it's ultimately a migrate_disable() there.
> 
> In order to get the best available mechanism on both PREEMPT_RT and
> !PREEMPT_RT, introduce private slub_get_cpu_ptr() and slub_put_cpu_ptr()
> wrappers and use them.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/slub.c | 41 +++++++++++++++++++++++++++++++----------
>  1 file changed, 31 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 12e966f07f19..caa206213e72 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -115,6 +115,26 @@
>   * 			the fast path and disables lockless freelists.
>   */
>  
> +/*
> + * We could simply use migrate_disable()/enable() but as long as it's a
> + * function call even on !PREEMPT_RT, use inline preempt_disable() there.
> + */
> +#ifdef CONFIG_PREEMPT_RT
> +#define slub_get_cpu_ptr(var)	get_cpu_ptr(var)
> +#define slub_put_cpu_ptr(var)	put_cpu_ptr(var)

After Mel's report and bisect pointing to this patch, I realized I got the
#ifdef wrong and it should be #ifnded

Fixed patch follows:

----8<----