Re: [PATCH] mm: madvise: fix uneven accounting of psi

[Andrew Morton]

On Wed, 31 May 2023 16:39:34 +0530 Charan Teja Kalla <quic_charante@xxxxxxxxxxx> wrote:

> A folio turns into a Workingset during:
> 1) shrink_active_list() placing the folio from active to inactive list.
> 2) When a workingset transition is happening during the folio refault.
> 
> And when Workingset is set on a folio, PSI for memory can be accounted
> during a) That folio is being reclaimed and b) Refault of that folio.
> 
> There exists clients who can do the proactive reclaim using the system
> calls like madvise(), whose folios can be safely treated as inactive
> folios assuming the client knows that these folios are not needed in the
> near future thus wanted to reclaim them. For such folios psi is not
> accounted uniformly:
> a) A folio started at inactive and moved to active as part of accesses.
> Workingset is absent on the folio thus madvise(MADV_PAGEOUT) don't
> account such folios for PSI.
> 
> b) When the same folio transition from inactive->active and then to
> inactive through shrink_active_list(). Workingset is set on the folio
> thus madvise(MADV_PAGEOUT) account such folios for PSI.
> 
> c) When the same folio is part of active list directly as a result of
> folio refault and this was a workingset folio prior to eviction.
> Workingset is set on the folio thus madvise(MADV_PAGEOUT) account such
> folios for PSI.
> 
> As said about the MADV_PAGEOUT on a folio is accounted in b) and c) but
> not in a) which is inconsistent. Remove this inconsistency by always not
> considering the PSI for folios that are getting reclaimed through
> madvise(MADV_PAGEOUT) by clearing the Workingset on a folio. This
> consistency of clearing the workingset was chosen under the assumption
> that client knows these folios are not in active use thus reclaiming
> them hence not eligible as workingset folios. Probably it is the same
> reason why workingset is not set on a folio through MADV_COLD but during
> the shrink_active_list() though both the actions make the folio put onto
> the inactive list.
> 
> This patch is tested on Android, Snapdragon SoC with 8Gb RAM, 4GB swap
> mounted on zram which has 2GB of backingdev. The test case involved
> launching some memory hungry apps in an order and do the proactive
> reclaim for the app that went to background using madvise(MADV_PAGEOUT).
> We are seeing ~40% less total values of psi mem some and full when this
> patch is combined with [1].

Does this accounting inaccuracy have any perceptible runtime effects?