Re: [PATCH]vmscan: fix a livelock in kswapd

[Minchan Kim]

On Tue, Jul 19, 2011 at 04:53:04PM +0800, Shaohua Li wrote:
> On Tue, 2011-07-19 at 16:45 +0800, Minchan Kim wrote:
> > On Tue, Jul 19, 2011 at 4:09 PM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote:
> > > I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> > > After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> > > kswapd2 are keeping running and I can't access filesystem, but most memory is
> > > free. This looks like a regression since commit 08951e545918c159.
> > 
> > Could you tell me what is 08951e545918c159?
> > You mean [ebd64e21ec5a,
> > mm-vmscan-only-read-new_classzone_idx-from-pgdat-when-reclaiming-successfully]
> > ?
> ha, sorry, I should copy the commit title.
> 08951e545918c159(mm: vmscan: correct check for kswapd sleeping in
> sleeping_prematurely)
> 

I don't mean it. In my bogus git tree, I can't find it but I can look at it in repaired git tree. :)
Anyway, I have a comment. Please look at below.

On Tue, Jul 19, 2011 at 03:09:27PM +0800, Shaohua Li wrote:
> I'm running a workload which triggers a lot of swap in a machine with 4 nodes.
> After I kill the workload, I found a kswapd livelock. Sometimes kswapd3 or
> kswapd2 are keeping running and I can't access filesystem, but most memory is
> free. This looks like a regression since commit 08951e545918c159.
> Node 2 and 3 have only ZONE_NORMAL, but balance_pgdat() will return 0 for
> classzone_idx. The reason is end_zone in balance_pgdat() is 0 by default, if
> all zones have watermark ok, end_zone will keep 0.
> Later sleeping_prematurely() always returns true. Because this is an order 3
> wakeup, and if classzone_idx is 0, both balanced_pages and present_pages
> in pgdat_balanced() are 0.

Sigh. Yes.

> We add a special case here. If a zone has no page, we think it's balanced. This
> fixes the livelock.

Yes. Your patch can fix it but I don't like that it adds handling special case.
(Although Andrew merged quickly).

The problem is to return 0-classzone_idx if all zones was okay.
So how about this?

This can change old behavior slightly.
For example, if balance_pgdat calls with order-3 and all zones are okay about order-3,
it will recheck order-0 as end_zone isn't 0 any more.
But I think it's desriable side effect we have missed.

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5ed24b9..cfef52b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2389,7 +2389,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
        unsigned long balanced;
        int priority;
        int i;
-       int end_zone = 0;       /* Inclusive.  0 = ZONE_DMA */
+       int end_zone = *classzone_idx;
        unsigned long total_scanned;
        struct reclaim_state *reclaim_state = current->reclaim_state;
        unsigned long nr_soft_reclaimed;

-- 
Kinds regards,
Minchan Kim
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