Re: [PATCH v5 1/2] mm: vmscan: ignore non-LRU-based reclaim in memcg reclaim

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Apr 6, 2023 at 10:50 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 06.04.23 16:07, Yosry Ahmed wrote:
> > Thanks for taking a look, David!
> >
> > On Thu, Apr 6, 2023 at 3:31 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 05.04.23 20:54, Yosry Ahmed wrote:
> >>> We keep track of different types of reclaimed pages through
> >>> reclaim_state->reclaimed_slab, and we add them to the reported number
> >>> of reclaimed pages.  For non-memcg reclaim, this makes sense. For memcg
> >>> reclaim, we have no clue if those pages are charged to the memcg under
> >>> reclaim.
> >>>
> >>> Slab pages are shared by different memcgs, so a freed slab page may have
> >>> only been partially charged to the memcg under reclaim.  The same goes for
> >>> clean file pages from pruned inodes (on highmem systems) or xfs buffer
> >>> pages, there is no simple way to currently link them to the memcg under
> >>> reclaim.
> >>>
> >>> Stop reporting those freed pages as reclaimed pages during memcg reclaim.
> >>> This should make the return value of writing to memory.reclaim, and may
> >>> help reduce unnecessary reclaim retries during memcg charging.  Writing to
> >>> memory.reclaim on the root memcg is considered as cgroup_reclaim(), but
> >>> for this case we want to include any freed pages, so use the
> >>> global_reclaim() check instead of !cgroup_reclaim().
> >>>
> >>> Generally, this should make the return value of
> >>> try_to_free_mem_cgroup_pages() more accurate. In some limited cases (e.g.
> >>> freed a slab page that was mostly charged to the memcg under reclaim),
> >>> the return value of try_to_free_mem_cgroup_pages() can be underestimated,
> >>> but this should be fine. The freed pages will be uncharged anyway, and we
> >>
> >> Can't we end up in extreme situations where
> >> try_to_free_mem_cgroup_pages() returns close to 0 although a huge amount
> >> of memory for that cgroup was freed up.
> >>
> >> Can you extend on why "this should be fine" ?
> >>
> >> I suspect that overestimation might be worse than underestimation. (see
> >> my comment proposal below)
> >
> > In such extreme scenarios even though try_to_free_mem_cgroup_pages()
> > would return an underestimated value, the freed memory for the cgroup
> > will be uncharged. try_charge() (and most callers of
> > try_to_free_mem_cgroup_pages()) do so in a retry loop, so even if
> > try_to_free_mem_cgroup_pages() returns an underestimated value
> > charging will succeed the next time around.
> >
> > The only case where this might be a problem is if it happens in the
> > final retry, but I guess we need to be *really* unlucky for this
> > extreme scenario to happen. One could argue that if we reach such a
> > situation the cgroup will probably OOM soon anyway.
> >
> >>
> >>> can charge the memcg the next time around as we usually do memcg reclaim
> >>> in a retry loop.
> >>>
> >>> The next patch performs some cleanups around reclaim_state and adds an
> >>> elaborate comment explaining this to the code. This patch is kept
> >>> minimal for easy backporting.
> >>>
> >>> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> >>> Cc: stable@vger.kernel.org
> >>
> >> Fixes: ?
> >>
> >> Otherwise it's hard to judge how far to backport this.
> >
> > It's hard to judge. The issue has been there for a while, but
> > memory.reclaim just made it more user visible. I think we can
> > attribute it to per-object slab accounting, because before that any
> > freed slab pages in cgroup reclaim would be entirely charged to that
> > cgroup.
> >
> > Although in all fairness, other types of freed pages that use
> > reclaim_state->reclaimed_slab cannot be attributed to the cgroup under
> > reclaim have been there before that. I guess slab is the most
> > significant among them tho, so for the purposes of backporting I
> > guess:
> >
> > Fixes: f2fe7b09a52b ("mm: memcg/slab: charge individual slab objects
> > instead of pages")
> >
> >>
> >>> ---
> >>>
> >>> global_reclaim(sc) does not exist in kernels before 6.3. It can be
> >>> replaced with:
> >>> !cgroup_reclaim(sc) || mem_cgroup_is_root(sc->target_mem_cgroup)
> >>>
> >>> ---
> >>>    mm/vmscan.c | 8 +++++---
> >>>    1 file changed, 5 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >>> index 9c1c5e8b24b8f..c82bd89f90364 100644
> >>> --- a/mm/vmscan.c
> >>> +++ b/mm/vmscan.c
> >>> @@ -5346,8 +5346,10 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
> >>>                vmpressure(sc->gfp_mask, memcg, false, sc->nr_scanned - scanned,
> >>>                           sc->nr_reclaimed - reclaimed);
> >>>
> >>> -     sc->nr_reclaimed += current->reclaim_state->reclaimed_slab;
> >>> -     current->reclaim_state->reclaimed_slab = 0;
> >>
> >> Worth adding a comment like
> >>
> >> /*
> >>    * Slab pages cannot universally be linked to a single memcg. So only
> >>    * account them as reclaimed during global reclaim. Note that we might
> >>    * underestimate the amount of memory reclaimed (but won't overestimate
> >>    * it).
> >>    */
> >>
> >> but ...
> >>
> >>> +     if (global_reclaim(sc)) {
> >>> +             sc->nr_reclaimed += current->reclaim_state->reclaimed_slab;
> >>> +             current->reclaim_state->reclaimed_slab = 0;
> >>> +     }
> >>>
> >>>        return success ? MEMCG_LRU_YOUNG : 0;
> >>>    }
> >>> @@ -6472,7 +6474,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> >>>
> >>>        shrink_node_memcgs(pgdat, sc);
> >>>
> >>
> >> ... do we want to factor the add+clear into a simple helper such that we
> >> can have above comment there?
> >>
> >> static void cond_account_reclaimed_slab(reclaim_state, sc)
> >> {
> >>          /*
> >>           * Slab pages cannot universally be linked to a single memcg. So
> >>           * only account them as reclaimed during global reclaim. Note
> >>           * that we might underestimate the amount of memory reclaimed
> >>           * (but won't overestimate it).
> >>           */
> >>          if (global_reclaim(sc)) {
> >>                  sc->nr_reclaimed += reclaim_state->reclaimed_slab;
> >>                  reclaim_state->reclaimed_slab = 0;
> >>          }
> >> }
> >>
> >> Yes, effective a couple LOC more, but still straight-forward for a
> >> stable backport
> >
> > The next patch in the series performs some refactoring and cleanups,
> > among which we add a helper called flush_reclaim_state() that does
> > exactly that and contains a sizable comment. I left this outside of
> > this patch in v5 to make the effective change as small as possible for
> > backporting. Looks like it can be confusing tho without the comment.
> >
> > How about I pull this part to this patch as well for v6?
>
> As long as it's a helper similar to what I proposed, I think that makes
> a lot of sense (and doesn't particularly bloat this patch).

Sounds good to me, I will do that and respin.

Thanks David!

>
> --
> Thanks,
>
> David / dhildenb
>
>