Re: Re: [PATCH v18 06/14] mm/damon: Implement callbacks for the virtual memory address spaces

[Shakeel Butt <shakeelb@google.com>]

On Mon, Jul 27, 2020 at 2:03 AM SeongJae Park <sjpark@amazon.com> wrote:
>
> On Mon, 27 Jul 2020 00:34:54 -0700 Greg Thelen <gthelen@google.com> wrote:
>
> > SeongJae Park <sjpark@amazon.com> wrote:
> >
> > > From: SeongJae Park <sjpark@amazon.de>
> > >
> > > This commit introduces a reference implementation of the address space
> > > specific low level primitives for the virtual address space, so that
> > > users of DAMON can easily monitor the data accesses on virtual address
> > > spaces of specific processes by simply configuring the implementation to
> > > be used by DAMON.
> [...]
> > > diff --git a/mm/damon.c b/mm/damon.c
> > > index b844924b9fdb..386780739007 100644
> > > --- a/mm/damon.c
> > > +++ b/mm/damon.c
> > > @@ -9,6 +9,9 @@
> [...]
> > > +/*
> > > + * Functions for the access checking of the regions
> > > + */
> > > +
> > > +static void damon_mkold(struct mm_struct *mm, unsigned long addr)
> > > +{
> > > +   pte_t *pte = NULL;
> > > +   pmd_t *pmd = NULL;
> > > +   spinlock_t *ptl;
> > > +
> > > +   if (follow_pte_pmd(mm, addr, NULL, &pte, &pmd, &ptl))
> > > +           return;
> > > +
> > > +   if (pte) {
> > > +           if (pte_young(*pte)) {
> > > +                   clear_page_idle(pte_page(*pte));
> > > +                   set_page_young(pte_page(*pte));
> >
> > While this compiles without support for PG_young and PG_idle, I assume
> > it won't work well because it'd clear pte.young without setting
> > PG_young.  And this would mess with vmscan.
>
> You're right, thanks for catching this up!  This definitely need to be fixed in
> the next spin.
>
> >
> > So this code appears to depend on PG_young and PG_idle, which are
> > currently only available via CONFIG_IDLE_PAGE_TRACKING.  DAMON could
> > depend on CONFIG_IDLE_PAGE_TRACKING via Kconfig.  But I assume that
> > CONFIG_IDLE_PAGE_TRACKING and CONFIG_DAMON cannot be concurrently used
> > because they'll stomp on each other's use of pte.young, PG_young,
> > PG_idle.
> > So I suspect we want:
> > 1. CONFIG_DAMON to depend on !CONFIG_IDLE_PAGE_TRACKING and vise-versa.
> > 2. PG_young,PG_idle and related helpers to depend on
> >    CONFIG_DAMON||CONFIG_IDLE_PAGE_TRACKING.
>
> Awesome insights and suggestions, thanks!
>
> I would like to note that DAMON could be interfered by IDLE_PAGE_TRACKING and
> vmscan, but not vice versa, as DAMON respects PG_idle and PG_young.  This
> design came from the weak goal of DAMON.  DAMON aims to provide not perfect
> monitoring but only best effort accuracy that would be sufficient for
> performance-centric DRAM level memory management.  So, at that time, I thought
> being interfered by IDLE_PAGE_TRACKING and the reclaim logic would not be a
> real problem but letting IDLE_PAGE_TRACKING coexist is somehow beneficial.
> That said, I couldn't find a real benefit of the coexistance yet, and the
> problem of being interference now seems bigger as we will support more cases
> including the page granularity.
>
> Maybe we could make IDLE_PAGE_TRACKING and DAMON coexist but mutual exclusive
> in runtime, if the beneficial of coexistance turns out big.  However, I would
> like to make it simple first and optimize the case later if real requirement
> found.

If you are planning to have support for tracking at page granularity
and physical memory monitoring in DAMON then I don't see any benefit
of coexistence of DAMON with IDLE_PAGE_TRACKING. Though I will not
push you to go that route if the code with coexistence is simple
enough.