Re: [patch 2/4] mm: PageLRU no testset

[Nikita Danilov]

Nick Piggin writes:
 > PG_lru is protected by zone->lru_lock. It does not need TestSet/TestClear
 > operations.
 > 
 > Signed-off-by: Nick Piggin <npiggin@xxxxxxx>
 > 
 > Index: linux-2.6/mm/vmscan.c
 > ===================================================================
 > --- linux-2.6.orig/mm/vmscan.c
 > +++ linux-2.6/mm/vmscan.c
 > @@ -775,9 +775,10 @@ int isolate_lru_page(struct page *page)
 >  	if (PageLRU(page)) {
 >  		struct zone *zone = page_zone(page);
 >  		spin_lock_irq(&zone->lru_lock);
 > -		if (TestClearPageLRU(page)) {
 > +		if (PageLRU(page)) {
 >  			ret = 1;
 >  			get_page(page);
 > +			ClearPageLRU(page);

Why is that better? ClearPageLRU() is also "atomic" operation (in the
sense of using LOCK_PREFIX on x86), so it seems this change strictly
adds cycles to the hot-path when page is on LRU.

Nikita.
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