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

[Yang Shi]

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@xxxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > ---
> > 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