Re: [RFC PATCH 1/2] mm, vmscan: account the number of isolated pages per zone

[Michal Hocko]

On Wed 18-01-17 17:00:10, Mel Gorman wrote:
> On Wed, Jan 18, 2017 at 05:17:31PM +0100, Michal Hocko wrote:
> > On Wed 18-01-17 15:54:30, Mel Gorman wrote:
> > > On Wed, Jan 18, 2017 at 04:15:31PM +0100, Michal Hocko wrote:
> > > > On Wed 18-01-17 14:46:55, Mel Gorman wrote:
> > > > > On Wed, Jan 18, 2017 at 02:44:52PM +0100, Michal Hocko wrote:
> > > > > > From: Michal Hocko <mhocko@xxxxxxxx>
> > > > > > 
> > > > > > 599d0c954f91 ("mm, vmscan: move LRU lists to node") has moved
> > > > > > NR_ISOLATED* counters from zones to nodes. This is not the best fit
> > > > > > especially for systems with high/lowmem because a heavy memory pressure
> > > > > > on the highmem zone might block lowmem requests from making progress. Or
> > > > > > we might allow to reclaim lowmem zone even though there are too many
> > > > > > pages already isolated from the eligible zones just because highmem
> > > > > > pages will easily bias too_many_isolated to say no.
> > > > > > 
> > > > > > Fix these potential issues by moving isolated stats back to zones and
> > > > > > teach too_many_isolated to consider only eligible zones. Per zone
> > > > > > isolation counters are a bit tricky with the node reclaim because
> > > > > > we have to track each page separatelly.
> > > > > > 
> > > > > 
> > > > > I'm quite unhappy with this. Each move back increases the cache footprint
> > > > > because of the counters
> > > > 
> > > > Why would per zone counters cause an increased cache footprint?
> > > > 
> > > 
> > > Because there are multiple counters, each of which need to be updated.
> > 
> > How does this differ from per node counter though.
> 
> A per-node counter is 2 * nr_online_nodes
> A per-zone counter is 2 * nr_populated_zones
> 
> > We would need to do
> > the accounting anyway. Moreover none of the accounting is done in a hot
> > path.
> > 
> > > > > but it's not clear at all this patch actually helps anything.
> > > > 
> > > > Yes, I cannot prove any real issue so far. The main motivation was the
> > > > patch 2 which needs per-zone accounting to use it in the retry logic
> > > > (should_reclaim_retry). I've spotted too_many_isoalated issues on the
> > > > way.
> > > > 
> > > 
> > > You don't appear to directly use that information in patch 2.
> > 
> > It is used via zone_reclaimable_pages in should_reclaim_retry
> > 
> 
> Which is still not directly required to avoid the infinite loop. There
> even is a small inherent risk if the too_isolated_condition no longer
> applies at the time should_reclaim_retry is attempted.

Not really because, if those pages are no longer isolated then they
either have been reclaimed - and NR_FREE_PAGES will increase - or they
have been put back to LRU in which case we will see them in regular LRU
counters. I need to catch the case where there are still too many pages
isolated which would skew should_reclaim_retry watermark check.
 
> > > The primary
> > > breakout is returning after stalling at least once. You could also avoid
> > > an infinite loop by using a waitqueue that sleeps on too many isolated.
> > 
> > That would be tricky on its own. Just consider the report form Tetsuo.
> > Basically all the direct reclamers are looping on too_many_isolated
> > while the kswapd is not making any progres because it is blocked on FS
> > locks which are held by flushers which are making dead slow progress.
> > Some of those direct reclaimers could have gone oom instead and release
> > some memory if we decide so, which we cannot because we are deep down in
> > the reclaim path. Waiting for on the reclaimer to increase the ISOLATED
> > counter wouldn't help in this situation.
> > 
> 
> If it's a waitqueue waking one process at a time, the progress may be
> slow but it'll still exit the loop, attempt the reclaim and then
> potentially OOM if no progress is made. The key is using the waitqueue
> to have a fair queue of processes making progress instead of a
> potentially infinite loop that never meets the exit conditions.

It is not clear to me who would wake waiters on the queue. You cannot
rely on kswapd to do that as already mentioned.

> > > That would both avoid the clunky congestion_wait() and guarantee forward
> > > progress. If the primary motivation is to avoid an infinite loop with
> > > too_many_isolated then there are ways of handling that without reintroducing
> > > zone-based counters.
> > > 
> > > > > Heavy memory pressure on highmem should be spread across the whole node as
> > > > > we no longer are applying the fair zone allocation policy. The processes
> > > > > with highmem requirements will be reclaiming from all zones and when it
> > > > > finishes, it's possible that a lowmem-specific request will be clear to make
> > > > > progress. It's all the same LRU so if there are too many pages isolated,
> > > > > it makes sense to wait regardless of the allocation request.
> > > > 
> > > > This is true but I am not sure how it is realated to the patch.
> > > 
> > > Because heavy pressure that is enough to trigger too many isolated pages
> > > is unlikely to be specifically targetting a lower zone.
> > 
> > Why? Basically any GFP_KERNEL allocation will make lowmem pressure and
> > going OOM on lowmem is not all that unrealistic scenario on 32b systems.
> > 
> 
> If the sole source of pressure is from GFP_KERNEL allocations then the
> isolated counter will also be specific to the lower zones and there is no
> benefit from the patch.

I believe you are wrong here. Just consider that you have isolated
basically all lowmem pages. too_many_isolated will still happily tell
you to not throttle or back off because NR_INACTIVE_* are way too bigger
than all low mem pages altogether. Or am I still missing your point?

> If there is a combination of highmem and lowmem pressure then the highmem
> reclaimers will also reclaim lowmem memory.
> 
> > > There is general
> > > pressure with multiple direct reclaimers being applied. If the system is
> > > under enough pressure with parallel reclaimers to trigger too_many_isolated
> > > checks then the system is grinding already and making little progress. Adding
> > > multiple counters to allow a lowmem reclaimer to potentially make faster
> > > progress is going to be marginal at best.
> > 
> > OK, I agree that the situation where highmem blocks lowmem from making
> > progress is much less likely than the other situation described in the
> > changelog when lowmem doesn't get throttled ever. Which is the one I am
> > interested more about.
> > 
> 
> That is of some concern but could be handled by having too_may_isolated
> take into account if it's a zone-restricted allocation and if so, then
> decrement the LRU counts from the higher zones. Counters already exist
> there. It would not be as strict but it should be sufficient.

Well, this is what this patch tries to do. Which other counters I can
use to consider only eligible zones when evaluating the number of
isolated pages?

> > > > Also consider that lowmem throttling in too_many_isolated has only small
> > > > chance to ever work with the node counters because highmem >> lowmem in
> > > > many/most configurations.
> > > > 
> > > 
> > > While true, it's also not that important.
> > > 
> > > > > More importantly, this patch may make things worse and delay reclaim. If
> > > > > this patch allowed a lowmem request to make progress that would have
> > > > > previously stalled, it's going to spend time skipping pages in the LRU
> > > > > instead of letting kswapd and the highmem pressured processes make progress.
> > > > 
> > > > I am not sure I understand this part. Say that we have highmem pressure
> > > > which would isolated too many pages from the LRU.
> > > 
> > > Which requires multiple direct reclaimers or tiny inactive lists. In the
> > > event there is such highmem pressure, it also means the lower zones are
> > > depleted.
> > 
> > But consider a lowmem without highmem pressure. E.g. a heavy parallel
> > fork or any other GFP_KERNEL intensive workload.
> >  
> 
> Lowmem without highmem pressure means all isolated pages are in the lowmem
> nodes and the per-zone counters are unnecessary.

But most configurations will have highmem and lowmem zones in the same
node...
 
> > > > lowmem request would
> > > > stall previously regardless of where those pages came from. With this
> > > > patch it would stall only when we isolated too many pages from the
> > > > eligible zones.
> > > 
> > > And when it makes progress, it's goign to compete with the other direct
> > > reclaimers except the lowmem reclaim is skipping some pages and
> > > recycling them through the LRU. It chews up CPU that would probably have
> > > been better spent letting kswapd and the other direct reclaimers do
> > > their work.
> > 
> > OK, I guess we are talking past each other. What I meant to say is that
> > it doesn't really make any difference who is chewing through the LRU to
> > find last few lowmem pages to reclaim. So I do not see much of a
> > difference sleeping and postponing that to the kswapd.
> > 
> > That being said, I _believe_ I will need per zone ISOLATED counters in
> > order to make the other patch work reliably and do not declare oom
> > prematurely. Maybe there is some other way around that (hence this RFC).
> > Would you be strongly opposed to the patch which would make counters per
> > zone without touching too_many_isolated?
> 
> I'm resistent to the per-zone counters in general but it's unfortunate to
> add them just to avoid a potentially infinite loop from isolated pages.

I am really open to any alternative solutions, of course. This is
the best I could come up with. I will keep thinking but removing
too_many_isolated without considering isolated pages during the oom
detection is just too risky. We can isolate many pages to ignore them.
-- 
Michal Hocko
SUSE Labs