Re: [PATCH 2/3] mm: vmscan: detect file thrashing at the reclaim root

[Johannes Weiner]

On Thu, Nov 14, 2019 at 03:47:59PM -0800, Shakeel Butt wrote:
> On Thu, Nov 7, 2019 at 12:53 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> >
> > We use refault information to determine whether the cache workingset
> > is stable or transitioning, and dynamically adjust the inactive:active
> > file LRU ratio so as to maximize protection from one-off cache during
> > stable periods, and minimize IO during transitions.
> >
> > With cgroups and their nested LRU lists, we currently don't do this
> > correctly. While recursive cgroup reclaim establishes a relative LRU
> > order among the pages of all involved cgroups, refaults only affect
> > the local LRU order in the cgroup in which they are occuring. As a
> > result, cache transitions can take longer in a cgrouped system as the
> > active pages of sibling cgroups aren't challenged when they should be.
> >
> > [ Right now, this is somewhat theoretical, because the siblings, under
> >   continued regular reclaim pressure, should eventually run out of
> >   inactive pages - and since inactive:active *size* balancing is also
> >   done on a cgroup-local level, we will challenge the active pages
> >   eventually in most cases. But the next patch will move that relative
> >   size enforcement to the reclaim root as well, and then this patch
> >   here will be necessary to propagate refault pressure to siblings. ]
> >
> > This patch moves refault detection to the root of reclaim. Instead of
> > remembering the cgroup owner of an evicted page, remember the cgroup
> > that caused the reclaim to happen. When refaults later occur, they'll
> > correctly influence the cross-cgroup LRU order that reclaim follows.
> 
> Can you please explain how "they'll correctly influence"? I see that
> if the refaulted page was evicted due to pressure in some ancestor,
> then that's ancestor's refault distance and active file size will be
> used to decide to activate the refaulted page but how that is
> influencing cross-cgroup LRUs?

I take it the next patch answered your question: Activating a page
inside a cgroup has an effect on how it's reclaimed relative to pages
in sibling cgroups. So the influence part isn't new with this change -
it's about recognizing that an activation has an effect on a wider
scope than just the local cgroup, and considering that scope when
making the decision whether to activate or not.

> > @@ -302,6 +330,17 @@ void workingset_refault(struct page *page, void *shadow)
> >          */
> >         refault_distance = (refault - eviction) & EVICTION_MASK;
> >
> > +       /*
> > +        * The activation decision for this page is made at the level
> > +        * where the eviction occurred, as that is where the LRU order
> > +        * during page reclaim is being determined.
> > +        *
> > +        * However, the cgroup that will own the page is the one that
> > +        * is actually experiencing the refault event.
> > +        */
> > +       memcg = get_mem_cgroup_from_mm(current->mm);
> 
> Why not page_memcg(page)? page is locked.

Nice catch! Indeed, the page is charged and locked at this point, so
we don't have to do another lookup and refcounting dance here.

Delta patch:

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