Re: [PATCH 1/2] check the return value of soft_limit reclaim

[Minchan Kim <minchan.kim@gmail.com>]

On Mon, Mar 28, 2011 at 05:44:21PM +0900, KAMEZAWA Hiroyuki wrote:
> On Mon, 28 Mar 2011 15:39:59 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
> 
> > > In the global background reclaim, we do soft reclaim before scanning the
> > > per-zone LRU. However, the return value is ignored. This patch adds the logic
> > > where no per-zone reclaim happens if the soft reclaim raise the free pages
> > > above the zone's high_wmark.
> > > 
> > > I did notice a similar check exists but instead leaving a "gap" above the
> > > high_wmark(the code right after my change in vmscan.c). There are discussions
> > > on whether or not removing the "gap" which intends to balance pressures across
> > > zones over time. Without fully understand the logic behind, I didn't try to
> > > merge them into one, but instead adding the condition only for memcg users
> > > who care a lot on memory isolation.
> > > 
> > > Signed-off-by: Ying Han <yinghan@google.com>
> > 
> > Looks good to me. But this depend on "memcg soft limit" spec. To be honest,
> > I don't know this return value ignorance is intentional or not. So I think 
> > you need to get ack from memcg folks.
> > 
> > 
> Hi,
> 
> 
> > > ---
> > >  mm/vmscan.c |   16 +++++++++++++++-
> > >  1 files changed, 15 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index 060e4c1..e4601c5 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -2320,6 +2320,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
> > >  	int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
> > >  	unsigned long total_scanned;
> > >  	struct reclaim_state *reclaim_state = current->reclaim_state;
> > > +	unsigned long nr_soft_reclaimed;
> > >  	struct scan_control sc = {
> > >  		.gfp_mask = GFP_KERNEL,
> > >  		.may_unmap = 1,
> > > @@ -2413,7 +2414,20 @@ loop_again:
> > >  			 * Call soft limit reclaim before calling shrink_zone.
> > >  			 * For now we ignore the return value
> > >  			 */
> > > -			mem_cgroup_soft_limit_reclaim(zone, order, sc.gfp_mask);
> > > +			nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
> > > +							order, sc.gfp_mask);
> > > +
> > > +			/*
> > > +			 * Check the watermark after the soft limit reclaim. If
> > > +			 * the free pages is above the watermark, no need to
> > > +			 * proceed to the zone reclaim.
> > > +			 */
> > > +			if (nr_soft_reclaimed && zone_watermark_ok_safe(zone,
> > > +					order, high_wmark_pages(zone),
> > > +					end_zone, 0)) {
> > > +				__inc_zone_state(zone, NR_SKIP_RECLAIM_GLOBAL);
> > 
> > NR_SKIP_RECLAIM_GLOBAL is defined by patch 2/2. please don't break bisectability.
> > 
> > 
> > 
> > > +				continue;
> > > +			}
> 
> Hmm, this "continue" seems not good to me. And, IIUC, this was a reason
> we ignore the result. But yes, ignore the result is bad.
> I think you should just do sc.nr_reclaimed += nr_soft_reclaimed.
> Or mem_cgroup_soft_limit_reclaim() should update sc.
> 
> 
> And allow kswapd to do some jobs as
>  - call shrink_slab()
>  - update total_scanned
>  - update other flags.. etc...etc..
> 
> If extra shink_zone() seems bad, please skip it, if mem_cgroup_soft_limit_reclaim()
> did enough jobs.
> 
> IOW, mem_cgroup_soft_limit_reclaim() can't do enough jobs to satisfy
> ==
>    2426 			balance_gap = min(low_wmark_pages(zone),
>    2427 				(zone->present_pages +
>    2428 					KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
>    2429 				KSWAPD_ZONE_BALANCE_GAP_RATIO);
>    2430 			if (!zone_watermark_ok_safe(zone, order,
>    2431 					high_wmark_pages(zone) + balance_gap,
>    2432 					end_zone, 0))
>    2433 				shrink_zone(priority, zone, &sc);
> ==

Good point. 
We should consider balancing the pressure on every zones.


> Thanks,
> -Kame
> 

-- 
Kind regards,
Minchan Kim

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