Re: [v4 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned

[Yang Shi <yang.shi@linux.alibaba.com>]

On 5/24/19 12:15 PM, Hillf Danton wrote:
> On Thu, 23 May 2019 10:27:37 +0800 Yang Shi wrote:
>> The commit 9092c71bb724 ("mm: use sc->priority for slab shrink targets")
>> has broken up the relationship between sc->nr_scanned and slab pressure.
>> The sc->nr_scanned can't double slab pressure anymore.  So, it sounds no
>> sense to still keep sc->nr_scanned inc'ed.  Actually, it would prevent
>> from adding pressure on slab shrink since excessive sc->nr_scanned would
>> prevent from scan->priority raise.
>>
> The deleted code below wants to get more slab pages shrinked, and it can do
> that without raising scan priority first even after commit 9092c71bb724. Or
> we may face the risk that priority goes up too much faster than thought, per
> the following snippet.

The priority is raised if kswapd_shrink_node() returns false for kswapd 
(The direct reclaim would just raise the priority if sc->nr_reclaimed >= 
sc->nr_to_reclaim). The kswapd_shrink_node() returns "return 
sc->nr_scanned >= sc->nr_to_reclaim". So, the old "double pressure" 
doesn't work as it was designed anymore since it would prevent from make 
"sc->nr_scanned < sc->nr_to_reclaim".

And, the patch 2/2 would not make the priority go up too much since one 
THP would be accounted as 512 base page.

>
> 		/*
> 		 * If we're getting trouble reclaiming, start doing
> 		 * writepage even in laptop mode.
> 		 */
> 		if (sc->priority < DEF_PRIORITY - 2)
>
>> The bonnie test doesn't show this would change the behavior of
>> slab shrinkers.
>>
>> 				w/		w/o
>> 			  /sec    %CP      /sec      %CP
>> Sequential delete: 	3960.6    94.6    3997.6     96.2
>> Random delete: 		2518      63.8    2561.6     64.6
>>
>> The slight increase of "/sec" without the patch would be caused by the
>> slight increase of CPU usage.
>>
>> Cc: Josef Bacik <josef@toxicpanda.com>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>> ---
>> v4: Added Johannes's ack
>>
>>   mm/vmscan.c | 5 -----
>>   1 file changed, 5 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 7acd0af..b65bc50 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1137,11 +1137,6 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>   		if (!sc->may_unmap && page_mapped(page))
>>   			goto keep_locked;
>>   
>> -		/* Double the slab pressure for mapped and swapcache pages */
>> -		if ((page_mapped(page) || PageSwapCache(page)) &&
>> -		    !(PageAnon(page) && !PageSwapBacked(page)))
>> -			sc->nr_scanned++;
>> -
>>   		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
>>   			(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
>>   
>> -- 
>> 1.8.3.1
>>
> Best Regards
> Hillf