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

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