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 10:49:26 -0700 |
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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"?
> >> * // 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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