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).