Messages in this thread | | | Subject | Re: [PATCH 2/2] mm: swap: use smp_mb__after_atomic() to order LRU bit set | From | Yang Shi <> | Date | Mon, 16 Mar 2020 15:18:27 -0700 |
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On 3/16/20 10:49 AM, Yang Shi wrote: > > > On 3/16/20 10:40 AM, Vlastimil Babka wrote: >> On 3/13/20 7:34 PM, Yang Shi wrote: >>> Memory barrier is needed after setting LRU bit, but smp_mb() is too >>> strong. Some architectures, i.e. x86, imply memory barrier with atomic >>> operations, so replacing it with smp_mb__after_atomic() sounds better, >>> which is nop on strong ordered machines, and full memory barriers on >>> others. With this change the vm-calability cases would perform better >>> on x86, I saw total 6% improvement with this patch and previous inline >>> fix. >>> >>> The test data (lru-file-readtwice throughput) against v5.6-rc4: >>> mainline w/ inline fix w/ both (adding this) >>> 150MB 154MB 159MB >>> >>> Fixes: 9c4e6b1a7027 ("mm, mlock, vmscan: no more skipping pagevecs") >>> Cc: Shakeel Butt <shakeelb@google.com> >>> Cc: Vlastimil Babka <vbabka@suse.cz> >>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com> >> According to my understanding of Documentation/memory_barriers.txt >> this would be >> correct (but it might not say much :) > > This is my understanding too. > >> >> Acked-by: Vlastimil Babka <vbabka@suse.cz> >> >> But i have some suggestions... >> >>> --- >>> mm/swap.c | 6 +++--- >>> 1 file changed, 3 insertions(+), 3 deletions(-) >>> >>> diff --git a/mm/swap.c b/mm/swap.c >>> index cf39d24..118bac4 100644 >>> --- a/mm/swap.c >>> +++ b/mm/swap.c >>> @@ -945,20 +945,20 @@ static void __pagevec_lru_add_fn(struct page >>> *page, struct lruvec *lruvec, >>> * #0: __pagevec_lru_add_fn #1: clear_page_mlock >>> * >>> * SetPageLRU() TestClearPageMlocked() >>> - * smp_mb() // explicit ordering // above provides strict >>> + * MB() // explicit ordering // above provides strict >> Why MB()? That would be the first appareance of 'MB()' in the whole >> tree. I >> think it's fine keeping smp_mb()... > > I would like to use a more general name, maybe just use "memory barrier"?
Keeping smp_mb() should be just fine...
> >> >>> * // ordering >>> * PageMlocked() PageLRU() >>> * >>> * >>> * if '#1' does not observe setting of PG_lru by '#0' and fails >>> * isolation, the explicit barrier will make sure that >>> page_evictable >>> - * check will put the page in correct LRU. Without smp_mb(), >>> SetPageLRU >>> + * check will put the page in correct LRU. Without MB(), >>> SetPageLRU >> ... same here ... >> >>> * can be reordered after PageMlocked check and can make '#1' >>> to fail >>> * the isolation of the page whose Mlocked bit is cleared (#0 >>> is also >>> * looking at the same page) and the evictable page will be >>> stranded >>> * in an unevictable LRU. >> Only here I would note that SetPageLRU() is an atomic bitop so we can >> use the >> __after_atomic() variant. And I would move the actual SetPageLRU() >> call from >> above the comment here right before the barrier. > > Sure. Thanks. > >> >>> */ >>> - smp_mb(); >>> + smp_mb__after_atomic(); >> Thanks. >> >>> if (page_evictable(page)) { >>> lru = page_lru(page); >>> >
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