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

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* nishimura@mxp.nes.nec.co.jp <nishimura@mxp.nes.nec.co.jp> [2011-03-28 16:33:11]:

> Hi,
> 
> This patch looks good to me, except for one nitpick.
> 
> On Sun, 27 Mar 2011 23:12:54 -0700
> Ying Han <yinghan@google.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>
> > ---
> >  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
> 
> You should remove this comment too.
> 
> But, Balbir-san, do you remember why did you ignore the return value here ?
>

We do that since soft limit reclaim cannot help us make a decision
from the return value. balance_gap is recomputed following this
routine. May be it might make sense to increment sc.nr_reclaimed based
on the return value? 

-- 
	Three Cheers,
	Balbir

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