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

[Minchan Kim]

Hi Vinayak,
Sorry for late response. It was Lunar New Year holidays.

On Fri, Jan 27, 2017 at 01:43:23PM +0530, vinayak menon wrote:
> >
> > 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 ?

If a THP page is reclaimed, it reports lower pressure due to bigger
reclaim ratio(ie, reclaimed/scanned) compared to normal pages but
it's not a problem, is it? Because VM reclaimed more memory than
expected so memory pressure isn't severe now.

> 
> >>
> >> >
> >> >> 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.

Make sense.

Thanks, Vinayak!