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@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
According to my understanding of Documentation/memory_barriers.txt this would be
correct (but it might not say much :)
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
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()...
> * // 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.
> */
> - smp_mb();
> + smp_mb__after_atomic();
Thanks.
>
> if (page_evictable(page)) {
> lru = page_lru(page);
>
