On Mon, Mar 28, 2011 at 1:44 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Mon, 28 Mar 2011 15:39:59 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> 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@xxxxxxxxxx>
>>
>> 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..
The change make sense to me. I will make the next patch to update
total_scanned and sc.nr_reclaimed.
Also, we might not want to skip shrink_slab() in this case, so i will add that.
>
> 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);
> ==
> This condition, you should update mem_cgroup_soft_limit_relcaim() to satisfy this,
> rather than continue here.
>
> I guess this is not easy...So, how about starting from updating 'sc' passed to
> mem_cgroup_soft_limit_reclaim() ? Then, we can think of algorithm.
The original patch introducing the "gap" was doing memory pressure
balancing across physical zones. Eventually we should get rid of
global per-zone reclaim in memcg(due to isolation), and maybe we need
something similar on per-memcg-per-zone. I will think about that.
So i will make the change on updating the two counters in scan_control
in next patch.
Thanks
--Ying
>
> Thanks,
> -Kame
>
>
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