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

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Date:   Fri, 21 May 2021 15:36:05 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Linux-MM <linux-mm@...ck.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] mm/page_alloc: Scale the number of pages that are
 batch freed

...
> +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@...ux.intel.com>