Re: [PATCH 4/6] mm/page_alloc: Scale the number of pages that are batch freed

Dave Hansen
2021-05-21T22:36:11Z

...
> +static int nr_pcp_free(struct per_cpu_pages *pcp, int high, int batch)
> +{
> + int min_nr_free, max_nr_free;
> +
> + /* Check for PCP disabled or boot pageset */
> + if (unlikely(high < batch))
> + return 1;
> +
> + min_nr_free = batch;
> + max_nr_free = high - batch;

I puzzled over this for a minute. I *think* it means to say: "Leave at
least one batch worth of pages in the pcp at all times so that the next
allocation can still be satisfied from this pcp."

> + batch <<= pcp->free_factor;
> + if (batch < max_nr_free)
> + pcp->free_factor++;
> + batch = clamp(batch, min_nr_free, max_nr_free);
> +
> + return batch;
> +}
> +
> static void free_unref_page_commit(struct page *page, unsigned long pfn,
> int migratetype)
> {
> struct zone *zone = page_zone(page);
> struct per_cpu_pages *pcp;
> + int high;
>
> __count_vm_event(PGFREE);
> pcp = this_cpu_ptr(zone->per_cpu_pageset);
> list_add(&page->lru, &pcp->lists[migratetype]);
> pcp->count++;
> - if (pcp->count >= READ_ONCE(pcp->high))
> - free_pcppages_bulk(zone, READ_ONCE(pcp->batch), pcp);
> + high = READ_ONCE(pcp->high);
> + if (pcp->count >= high) {
> + int batch = READ_ONCE(pcp->batch);
> +
> + free_pcppages_bulk(zone, nr_pcp_free(pcp, high, batch), pcp);
> + }
> }
>
> /*
> @@ -3531,6 +3555,7 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
>
> local_lock_irqsave(&pagesets.lock, flags);
> pcp = this_cpu_ptr(zone->per_cpu_pageset);
> + pcp->free_factor >>= 1;
> list = &pcp->lists[migratetype];
> page = __rmqueue_pcplist(zone, migratetype, alloc_flags, pcp, list);
> local_unlock_irqrestore(&pagesets.lock, flags);

A high-level description of the algorithm in the changelog would also be
nice. I *think* it's basically:

After hitting the high pcp mark, free one pcp->batch at a time. But, as
subsequent pcp free operations occur, keep doubling the size of the
freed batches. Cap them so that they always leave at least one
pcp->batch worth of pages. Scale the size back down by half whenever an
allocation that consumes a page from the pcp occurs.

While I'd appreciate another comment or two, I do think this is worth
doing, and the approach seems sound:

Acked-by: Dave Hansen <dave.hansen@linux.intel.com>