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

From: David Hildenbrand
Date: Thu Apr 06 2023 - 13:52:46 EST

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@xxxxxxxxxx> 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@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx

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).

--
Thanks,

David / dhildenb