Re: [PATCH] mm: reduce lock contention of pcp buffer refill

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  1 Feb 2023 17:25:49 +0100 Alexander Halbuer <halbuer@sra.uni-hannover.de> wrote:

> The `rmqueue_bulk` function batches the allocation of multiple elements to
> refill the per-CPU buffers into a single hold of the zone lock. Each
> element is allocated and checked using the `check_pcp_refill` function.
> The check touches every related struct page which is especially expensive
> for higher order allocations (huge pages). This patch reduces the time
> holding the lock by moving the check out of the critical section similar
> to the `rmqueue_buddy` function which allocates a single element.
> Measurements of parallel allocation-heavy workloads show a reduction of
> the average huge page allocation latency of 50 percent for two cores and
> nearly 90 percent for 24 cores.

Sounds nice.

Were you able to test how much benefit we get by simply removing the
check_new_pages() call from rmqueue_bulk()?

Vlastimil, I find this quite confusing:

#ifdef CONFIG_DEBUG_VM
/*
 * With DEBUG_VM enabled, order-0 pages are checked for expected state when
 * being allocated from pcp lists. With debug_pagealloc also enabled, they are
 * also checked when pcp lists are refilled from the free lists.
 */
static inline bool check_pcp_refill(struct page *page, unsigned int order)
{
	if (debug_pagealloc_enabled_static())
		return check_new_pages(page, order);
	else
		return false;
}

static inline bool check_new_pcp(struct page *page, unsigned int order)
{
	return check_new_pages(page, order);
}
#else
/*
 * With DEBUG_VM disabled, free order-0 pages are checked for expected state
 * when pcp lists are being refilled from the free lists. With debug_pagealloc
 * enabled, they are also checked when being allocated from the pcp lists.
 */
static inline bool check_pcp_refill(struct page *page, unsigned int order)
{
	return check_new_pages(page, order);
}
static inline bool check_new_pcp(struct page *page, unsigned int order)
{
	if (debug_pagealloc_enabled_static())
		return check_new_pages(page, order);
	else
		return false;
}
#endif /* CONFIG_DEBUG_VM */

and the 4462b32c9285b5 changelog is a struggle to follow.

Why are we performing *any* checks when CONFIG_DEBUG_VM=n and when
debug_pagealloc_enabled is false?

Anyway, these checks sounds quite costly so let's revisit their
desirability?