Re: [PATCH V2] mm: madvise: fix uneven accounting of psi

[Pavan Kondeti]

On Wed, Jun 28, 2023 at 04:19:01PM +0530, Charan Teja Kalla wrote:
> Hi Pavan,
> 
> On 6/27/2023 7:26 PM, Pavan Kondeti 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.
> >>
> > Please help me understand why PSI for memory (I understood it as the 
> > time spent in psi_memstall_enter() to psi_memstall_leave()) would be
> > accounted in (a) i.e during reclaim. I understand that when a working
> > 
> > The (b) part is very clear.
> > 
> I meant to say, for usual reclaim, PSI is accounted on a folio for both
> reclaim and as well during the refault operation when Workingset is set
> on a folio i.e., both a) and b) cases above.
> 

Got it.

> >> This accounting of PSI for memory is not consistent in the cases where
> >> clients use madvise(COLD/PAGEOUT) to deactivate or proactively reclaim a
> >> folio:
> 
> Seems I need to be explicit here. How about the below?
> 
> This accounting of PSI for memory is not consistent for reclaim +
> refault operation between usual reclaim and madvise(COLD/PAGEOUT) which
> deactivate or proactively reclaim a folio:
> 

Looks good.

> lmk for any better rephrasing?
> >> 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 both the operations of MADV_PAGEOUT
> >> and reclaim of the MADV_COLD operated folio account for PSI.
> >>
> >> d) madvise(MADV_COLD) transfers the folio from active list to inactive
> >> list. Such folios may not have the Workingset thus reclaim operation
> >> on such folio doesn't account for PSI.
> > This is not limited to madvise(PAGEOUT) right, anywhere an active page
> > is reclaimed we have the same problem. For ex: damon_pa_pageout() and
> > __alloc_contig_migrate_range()->reclaim_clean_pages_from_list().
> >> If that is the case, can we set mark a folio as a workingset when it is
> > activated? That way, we don't have make madvise() as a special case?
> I think marking the folio as a workingset when it sits on the active is
> not a correct thing. For the same example you mentioned, a simple CMA
> allocation will be dropping the clean pages instead of migration. PSI
> accounting on refault of those pages don't reveal anything to the user.
> 

Agreed. Thanks for the clarification.

> Where as in the madvise() cases, this PSI tells the user about the type
> of pages that he is working on.[1]
> 
> BTW, damon_pa_pageout() seems a valid case above. let me fix it in the
> next patch.

Looks good.

Thanks,
Pavan