On Fri 03-04-15 10:43:57, Nishanth Aravamudan wrote:
> On 31.03.2015 [11:48:29 +0200], Michal Hocko wrote:
[...]
> > I would expect kswapd would be looping endlessly because the zone
> > wouldn't be balanced obviously. But I would be wrong... because
> > pgdat_balanced is doing this:
> > /*
> > * A special case here:
> > *
> > * balance_pgdat() skips over all_unreclaimable after
> > * DEF_PRIORITY. Effectively, it considers them balanced so
> > * they must be considered balanced here as well!
> > */
> > if (!zone_reclaimable(zone)) {
> > balanced_pages += zone->managed_pages;
> > continue;
> > }
> >
> > and zone_reclaimable is false for you as you didn't have any
> > zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
> > would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
> > zones right?) and so the kswapd would be woken up easily. So it looks
> > like a mess.
>
> My understanding, and I could easily be wrong, is that kswapd2 (node 2
> is the exhausted one) spins endlessly, because the reclaim logic sees
> that we are reclaiming from somewhere but the allocation request for
> node 2 (which is __GFP_THISNODE for hugepages, not GFP_THISNODE) will
> never complete, so we just continue to reclaim.
__GFP_THISNODE would be waking up kswapd2 again and again, that is true.
I am just wondering whether we will have any __GFP_THISNODE allocations
for a node without CPUs (numa_node_id() shouldn't return such a node
AFAICS). Maybe if somebody is bound to Node2 explicitly but I would
consider this as a misconfiguration.
--
Michal Hocko
SUSE Labs
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