linux-kernel - Re: [PATCH 07/11] mm/page_alloc: Remove duplicate checks if migratetype should be isolated

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Date:   Wed, 14 Apr 2021 19:21:42 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Linux-MM <linux-mm@...ck.org>,
        Linux-RT-Users <linux-rt-users@...r.kernel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH 07/11] mm/page_alloc: Remove duplicate checks if
 migratetype should be isolated

On 4/14/21 3:39 PM, Mel Gorman wrote:
> Both free_pcppages_bulk() and free_one_page() have very similar
> checks about whether a page's migratetype has changed under the
> zone lock. Use a common helper.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Seems like for free_pcppages_bulk() this patch makes it check for each page on
the pcplist
- zone->nr_isolate_pageblock != 0 instead of local bool (the performance might
be the same I guess on modern cpu though)
- is_migrate_isolate(migratetype) for a migratetype obtained by
get_pcppage_migratetype() which cannot be migrate_isolate so the check is useless.

As such it doesn't seem a worthwhile cleanup to me considering all the other
microoptimisations?

> ---
>  mm/page_alloc.c | 32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 295624fe293b..1ed370668e7f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1354,6 +1354,23 @@ static inline void prefetch_buddy(struct page *page)
>  	prefetch(buddy);
>  }
>  
> +/*
> + * The migratetype of a page may have changed due to isolation so check.
> + * Assumes the caller holds the zone->lock to serialise against page
> + * isolation.
> + */
> +static inline int
> +check_migratetype_isolated(struct zone *zone, struct page *page, unsigned long pfn, int migratetype)
> +{
> +	/* If isolating, check if the migratetype has changed */
> +	if (unlikely(has_isolate_pageblock(zone) ||
> +		is_migrate_isolate(migratetype))) {
> +		migratetype = get_pfnblock_migratetype(page, pfn);
> +	}
> +
> +	return migratetype;
> +}
> +
>  /*
>   * Frees a number of pages from the PCP lists
>   * Assumes all pages on list are in same zone, and of same order.
> @@ -1371,7 +1388,6 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  	int migratetype = 0;
>  	int batch_free = 0;
>  	int prefetch_nr = READ_ONCE(pcp->batch);
> -	bool isolated_pageblocks;
>  	struct page *page, *tmp;
>  	LIST_HEAD(head);
>  
> @@ -1433,21 +1449,20 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  	 * both PREEMPT_RT and non-PREEMPT_RT configurations.
>  	 */
>  	spin_lock(&zone->lock);
> -	isolated_pageblocks = has_isolate_pageblock(zone);
>  
>  	/*
>  	 * Use safe version since after __free_one_page(),
>  	 * page->lru.next will not point to original list.
>  	 */
>  	list_for_each_entry_safe(page, tmp, &head, lru) {
> +		unsigned long pfn = page_to_pfn(page);
>  		int mt = get_pcppage_migratetype(page);
> +
>  		/* MIGRATE_ISOLATE page should not go to pcplists */
>  		VM_BUG_ON_PAGE(is_migrate_isolate(mt), page);
> -		/* Pageblock could have been isolated meanwhile */
> -		if (unlikely(isolated_pageblocks))
> -			mt = get_pageblock_migratetype(page);
>  
> -		__free_one_page(page, page_to_pfn(page), zone, 0, mt, FPI_NONE);
> +		mt = check_migratetype_isolated(zone, page, pfn, mt);
> +		__free_one_page(page, pfn, zone, 0, mt, FPI_NONE);
>  		trace_mm_page_pcpu_drain(page, 0, mt);
>  	}
>  	spin_unlock(&zone->lock);
> @@ -1459,10 +1474,7 @@ static void free_one_page(struct zone *zone,
>  				int migratetype, fpi_t fpi_flags)
>  {
>  	spin_lock(&zone->lock);
> -	if (unlikely(has_isolate_pageblock(zone) ||
> -		is_migrate_isolate(migratetype))) {
> -		migratetype = get_pfnblock_migratetype(page, pfn);
> -	}
> +	migratetype = check_migratetype_isolated(zone, page, pfn, migratetype);
>  	__free_one_page(page, pfn, zone, order, migratetype, fpi_flags);
>  	spin_unlock(&zone->lock);
>  }
>