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

[vinayak menon <vinayakm.list@gmail.com>]

>
> Thanks for the explain. However, such case can happen with THP page
> as well as slab. In case of THP page, nr_scanned is 1 but nr_reclaimed
> could be 512 so I think vmpressure should have a logic to prevent undeflow
> regardless of slab shrinking.
>
I see. Going to send a vmpressure fix. But, wouldn't the THP case
result in incorrect
vmpressure reporting even if we fix the vmpressure underflow problem ?

>>
>> >
>> >> unsigned arithmetic results in the pressure value to be
>> >> huge, thus resulting in a critical event being sent to
>> >> root cgroup. Fix this by not passing the reclaimed slab
>> >> count to vmpressure, with the assumption that vmpressure
>> >> should show the actual pressure on LRU which is now
>> >> diluted by adding reclaimed slab without a corresponding
>> >> scanned value.
>> >
>> > I can't guess justfication of your assumption from the description.
>> > Why do we consider only LRU pages for vmpressure? Could you elaborate
>> > a bit?
>> >
>> When we encountered the false events from vmpressure, thought the problem
>> could be that slab scanned is not included in sc->nr_scanned, like it is done
>> for reclaimed. But later thought vmpressure works only on the scanned and
>> reclaimed from LRU. I can explain what I understand, let me know if this is
>> incorrect.
>> vmpressure is an index which tells the pressure on LRU, and thus an
>> indicator of thrashing. In shrink_node when we come out of the inner do-while
>> loop after shrinking the lruvec, the scanned and reclaimed corresponds to the
>> pressure felt on the LRUs which in turn indicates the pressure on VM. The
>> moment we add the slab reclaimed pages to the reclaimed, we dilute the
>> actual pressure felt on LRUs. When slab scanned/reclaimed is not included
>> in the vmpressure, the values will indicate the actual pressure and if there
>> were a lot of slab reclaimed pages it will result in lesser pressure
>> on LRUs in the next run which will again be indicated by vmpressure. i.e. the
>
> I think there is no intention to exclude slab by design of vmpressure.
> Beause slab is memory consumption so freeing of slab pages really helps
> the memory pressure. Also, there might be slab-intensive workload rather
> than LRU. It would be great if vmpressure works well with that case.
> But the problem with involving slab for vmpressure is it's not fair with
> LRU pages. LRU pages are 1:1 cost model for scan:free but slab shriking
> depends the each slab's object population. It means it's impossible to
> get stable cost model with current slab shrinkg model, unfortunately.
> So I don't obejct this patch although I want to see slab shrink model's
> change which is heavy-handed work.
>
Looking at the code, the slab reclaimed pages started getting passed to
vmpressure after the commit ("mm: vmscan: invoke slab shrinkers from
shrink_zone()").
But as you said, this may be helpful for slab intensive workloads. But in its
current form I think it results in incorrect vmpressure reporting because of not
accounting the slab scanned pages. Resending the patch with a modified
commit msg
since the underflow issue is fixed separately. Thanks Minchan.

Vinayak

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