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SubjectRe: [PATCH 2/2] mm: swap: use smp_mb__after_atomic() to order LRU bit set
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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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