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From:   SeongJae Park <sjpark@amazon.com>
To:     Shakeel Butt <shakeelb@google.com>
CC:     SeongJae Park <sjpark@amazon.com>,
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Subject: Re: Re: Re: [PATCH v18 06/14] mm/damon: Implement callbacks for the virtual memory address spaces
Date:   Wed, 29 Jul 2020 08:20:42 +0200
Message-ID: <20200729062042.30136-1-sjpark@amazon.com>
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On Tue, 28 Jul 2020 10:42:11 -0700 Shakeel Butt <shakeelb@google.com> wrote:

> 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.

Agreed, I don't see the benefit, neither.  I already selected the mutual
exclusive way :)


Thanks,
SeongJae Park