Re: [PATCH 11/19] mm: page_alloc: Lookup pageblock migratetype with IRQs enabled during free

[Mel Gorman <mgorman@suse.de>]

On Tue, May 13, 2014 at 03:36:25PM +0200, Vlastimil Babka wrote:
> On 05/13/2014 11:45 AM, Mel Gorman wrote:
> >get_pageblock_migratetype() is called during free with IRQs disabled. This
> >is unnecessary and disables IRQs for longer than necessary.
> >
> >Signed-off-by: Mel Gorman <mgorman@suse.de>
> >Acked-by: Rik van Riel <riel@redhat.com>
> 
> With a comment below,
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> 

Thanks

> >---
> >  mm/page_alloc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >index 3948f0a..fcbf637 100644
> >--- a/mm/page_alloc.c
> >+++ b/mm/page_alloc.c
> >@@ -773,9 +773,9 @@ static void __free_pages_ok(struct page *page, unsigned int order)
> >  	if (!free_pages_prepare(page, order))
> >  		return;
> >
> >+	migratetype = get_pfnblock_migratetype(page, pfn);
> >  	local_irq_save(flags);
> >  	__count_vm_events(PGFREE, 1 << order);
> >-	migratetype = get_pfnblock_migratetype(page, pfn);
> >  	set_freepage_migratetype(page, migratetype);
> 
> The line above could be also outside disabled IRQ, no?
> 

I guess it could but the difference would be marginal at
best. get_pfnblock_migratetype is a lookup of the pageblock bitfield and is
an expensive operation. set_freepage_migratetype() on the other hand is just

static inline void set_freepage_migratetype(struct page *page, int migratetype)
{
        page->index = migratetype;
}

If anything the line could be just removed as right now nothing below
that level is actually using the information (it's primarily of interest
in the per-cpu allocator) but that would be outside the scope of this
patch as move_freepages would also need addressing. I feel the gain is
too marginal to justify the churn.

> >  	free_one_page(page_zone(page), page, pfn, order, migratetype);
> >  	local_irq_restore(flags);
> >
> 

-- 
Mel Gorman
SUSE Labs

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