linux-kernel - Re: [PATCH 2/2] mm: swap: use smp_mb__after

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Message-ID: <3c13c484-8fbf-3c3a-fbe1-a40434869e55@suse.cz>
Date:   Mon, 16 Mar 2020 18:40:01 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Yang Shi <yang.shi@...ux.alibaba.com>, shakeelb@...gle.com,
        akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: swap: use smp_mb__after_atomic() to order LRU bit
 set

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@...gle.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>

According to my understanding of Documentation/memory_barriers.txt this would be
correct (but it might not say much :)

Acked-by: Vlastimil Babka <vbabka@...e.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()...

>  	 *					// 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);
>