Re: [PATCH 2/2 v5] mm: vmscan: do not pass reclaimed slab to vmpressure

From: Michal Hocko
Date: Thu Feb 09 2017 - 07:55:00 EST

On Thu 09-02-17 17:29:37, Vinayak Menon wrote:
> During global reclaim, the nr_reclaimed passed to vmpressure includes the
> pages reclaimed from slab. But the corresponding scanned slab pages is
> not passed. There is an impact to the vmpressure values because of this.
> While moving from kernel version 3.18 to 4.4, a difference is seen
> in the vmpressure values for the same workload resulting in a different
> behaviour of the vmpressure consumer. One such case is of a vmpressure
> based lowmemorykiller. It is observed that the vmpressure events are
> received late and less in number resulting in tasks not being killed at
> the right time. The following numbers show the impact on reclaim activity
> due to the change in behaviour of lowmemorykiller on a 4GB device. The test
> launches a number of apps in sequence and repeats it multiple times.

this is really vague description of your workload and doesn't really
explain why getting critical events later is a bad thing.

> v4.4 v3.18
> pgpgin 163016456 145617236
> pgpgout 4366220 4188004
> workingset_refault 29857868 26781854
> workingset_activate 6293946 5634625
> pswpin 1327601 1133912
> pswpout 3593842 3229602
> pgalloc_dma 99520618 94402970
> pgalloc_normal 104046854 98124798
> pgfree 203772640 192600737
> pgmajfault 2126962 1851836
> pgsteal_kswapd_dma 19732899 18039462
> pgsteal_kswapd_normal 19945336 17977706
> pgsteal_direct_dma 206757 131376
> pgsteal_direct_normal 236783 138247
> pageoutrun 116622 108370
> allocstall 7220 4684
> compact_stall 931 856

this is missing any vmpressure events data and so it is not very useful
on its own

> This is a regression introduced by commit 6b4f7799c6a5 ("mm: vmscan:
> invoke slab shrinkers from shrink_zone()").
> So do not consider reclaimed slab pages for vmpressure calculation. The
> reclaimed pages from slab can be excluded because the freeing of a page
> by slab shrinking depends on each slab's object population, making the
> cost model (i.e. scan:free) different from that of LRU. Also, not every
> shrinker accounts the pages it reclaims. But ideally the pages reclaimed
> from slab should be passed to vmpressure, otherwise higher vmpressure
> levels can be triggered even when there is a reclaim progress. But
> accounting only the reclaimed slab pages without the scanned, and adding
> something which does not fit into the cost model just adds noise to the
> vmpressure values.
> Fixes: 6b4f7799c6a5 ("mm: vmscan: invoke slab shrinkers from shrink_zone()")
> Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Vladimir Davydov <>
> Cc: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>
> Cc: Shiraz Hashim <shashim@xxxxxxxxxxxxxx>
> Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>

I have already said I will _not_ NAK the patch but we need a much better
description and justification why the older behavior was better to
consider this a regression before this can be merged. It is hard to
expect that the underlying implementation of the vmpressure will stay
carved in stone and there might be changes in this area in the future. I
want to hear why we believe that the tested workload is sufficiently
universal and we won't see another report in few months because somebody
else will see higher vmpressure levels even though we make reclaim
progress. I have asked those questions already but it seems those were
Michal Hocko