linux-kernel - Re: [PATCH 2/5] mm/page_alloc: Track range of active PCP lists during bulk free

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Date:   Wed, 16 Feb 2022 13:02:01 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Aaron Lu <aaron.lu@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 2/5] mm/page_alloc: Track range of active PCP lists during
 bulk free

On 2/15/22 15:51, Mel Gorman wrote:
> free_pcppages_bulk() frees pages in a round-robin fashion. Originally,
> this was dealing only with migratetypes but storing high-order pages
> means that there can be many more empty lists that are uselessly
> checked. Track the minimum and maximum active pindex to reduce the
> search space.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> ---
>  mm/page_alloc.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 08de32cfd9bb..c5110fdeb115 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1450,6 +1450,8 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  					struct per_cpu_pages *pcp)
>  {
>  	int pindex = 0;
> +	int min_pindex = 0;
> +	int max_pindex = NR_PCP_LISTS - 1;
>  	int batch_free = 0;
>  	int nr_freed = 0;
>  	unsigned int order;
> @@ -1478,10 +1480,17 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  			if (++pindex == NR_PCP_LISTS)

Hmm, so in the very first iteration at this point pindex is already 1. This
looks odd even before the patch, as order 0 MIGRATE_UNMOVABLE list is only
processed after all the higher orders?

>  				pindex = 0;

Also shouldn't this wrap-around check also use min_index/max_index instead
of NR_PCP_LISTS and 0?

>  			list = &pcp->lists[pindex];
> -		} while (list_empty(list));
> +			if (!list_empty(list))
> +				break;
> +
> +			if (pindex == max_pindex)
> +				max_pindex--;
> +			if (pindex == min_pindex)

So with pindex 1 and min_pindex == 0 this will not trigger until
(eventually) the first pindex wrap around, which seems suboptimal. But I can
see the later patches change things substantially anyway so it may be moot...

> +				min_pindex++;
> +		} while (1);
>  
>  		/* This is the only non-empty list. Free them all. */
> -		if (batch_free == NR_PCP_LISTS)
> +		if (batch_free >= max_pindex - min_pindex)
>  			batch_free = count;
>  
>  		order = pindex_to_order(pindex);