Re: [PATCH 02/11] mm/page_alloc: Convert per-cpu list protection to local_lock

[Peter Zijlstra]

On Wed, Apr 07, 2021 at 09:24:14PM +0100, Mel Gorman wrote:
> There is a lack of clarity of what exactly local_irq_save/local_irq_restore
> protects in page_alloc.c . It conflates the protection of per-cpu page
> allocation structures with per-cpu vmstat deltas.
> 
> This patch protects the PCP structure using local_lock which for most
> configurations is identical to IRQ enabling/disabling.  The scope of the
> lock is still wider than it should be but this is decreased laster.

> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index a4393ac27336..106da8fbc72a 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h

> @@ -337,6 +338,7 @@ enum zone_watermarks {
>  #define high_wmark_pages(z) (z->_watermark[WMARK_HIGH] + z->watermark_boost)
>  #define wmark_pages(z, i) (z->_watermark[i] + z->watermark_boost)
>  
> +/* Fields and list protected by pagesets local_lock in page_alloc.c */
>  struct per_cpu_pages {
>  	int count;		/* number of pages in the list */
>  	int high;		/* high watermark, emptying needed */

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a68bacddcae0..e9e60d1a85d4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -112,6 +112,13 @@ typedef int __bitwise fpi_t;
>  static DEFINE_MUTEX(pcp_batch_high_lock);
>  #define MIN_PERCPU_PAGELIST_FRACTION	(8)
>  
> +struct pagesets {
> +	local_lock_t lock;
> +};
> +static DEFINE_PER_CPU(struct pagesets, pagesets) = {
> +	.lock = INIT_LOCAL_LOCK(lock),
> +};

So why isn't the local_lock_t in struct per_cpu_pages ? That seems to be
the actual object that is protected by it and is already per-cpu.

Is that because you want to avoid the duplication across zones? Is that
worth the effort?