Re: [PATCH v4] page_alloc: consider highatomic reserve in watermark fast

[Michal Hocko]

On Sat 20-06-20 08:59:58, Jaewon Kim wrote:
[...]
> @@ -3502,19 +3525,12 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
>  	const bool alloc_harder = (alloc_flags & (ALLOC_HARDER|ALLOC_OOM));
>  
>  	/* free_pages may go negative - that's OK */
> -	free_pages -= (1 << order) - 1;
> +	free_pages -= __zone_watermark_unusable_free(z, order, alloc_flags);
>  
>  	if (alloc_flags & ALLOC_HIGH)
>  		min -= min / 2;
>  
> -	/*
> -	 * If the caller does not have rights to ALLOC_HARDER then subtract
> -	 * the high-atomic reserves. This will over-estimate the size of the
> -	 * atomic reserve but it avoids a search.
> -	 */
> -	if (likely(!alloc_harder)) {
> -		free_pages -= z->nr_reserved_highatomic;
> -	} else {
> +	if (unlikely(alloc_harder)) {
>  		/*
>  		 * OOM victims can try even harder than normal ALLOC_HARDER
>  		 * users on the grounds that it's definitely going to be in
[...]
> @@ -3582,25 +3591,22 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
>  				unsigned long mark, int highest_zoneidx,
>  				unsigned int alloc_flags)
>  {
> -	long free_pages = zone_page_state(z, NR_FREE_PAGES);
> -	long cma_pages = 0;
> +	long free_pages;
> +	long unusable_free;
>  
> -#ifdef CONFIG_CMA
> -	/* If allocation can't use CMA areas don't use free CMA pages */
> -	if (!(alloc_flags & ALLOC_CMA))
> -		cma_pages = zone_page_state(z, NR_FREE_CMA_PAGES);
> -#endif
> +	free_pages = zone_page_state(z, NR_FREE_PAGES);
> +	unusable_free = __zone_watermark_unusable_free(z, order, alloc_flags);
>  
>  	/*
>  	 * Fast check for order-0 only. If this fails then the reserves
> -	 * need to be calculated. There is a corner case where the check
> -	 * passes but only the high-order atomic reserve are free. If
> -	 * the caller is !atomic then it'll uselessly search the free
> -	 * list. That corner case is then slower but it is harmless.
> +	 * need to be calculated.
>  	 */
> -	if (!order && (free_pages - cma_pages) >
> -				mark + z->lowmem_reserve[highest_zoneidx])
> -		return true;
> +	if (!order) {
> +		long fast_free = free_pages - unusable_free;
> +
> +		if (fast_free > mark + z->lowmem_reserve[highest_zoneidx])
> +			return true;
> +	}

There is no user of unusable_free for order > 0. With you current code
__zone_watermark_unusable_free would be called twice for high order
allocations unless compiler tries to be clever..

But more importantly, I have hard time to follow why we need both
zone_watermark_fast and zone_watermark_ok now. They should be
essentially the same for anything but order == 0. For order 0 the
only difference between the two is that zone_watermark_ok checks for
ALLOC_HIGH resp ALLOC_HARDER, ALLOC_OOM. So what is exactly fast about
the former and why do we need it these days?

> 
>  	return __zone_watermark_ok(z, order, mark, highest_zoneidx, alloc_flags,
>  					free_pages);
> -- 
> 2.17.1
> 

-- 
Michal Hocko
SUSE Labs