Gregory Price gourry@gourry.net writes:
On Mon, Oct 28, 2024 at 01:45:48PM -0700, Yang Shi wrote:
On Fri, Oct 25, 2024 at 7:17 AM Gregory Price gourry@gourry.net wrote:
When numa balancing is enabled with demotion, vmscan will call migrate_pages when shrinking LRUs. Successful demotions will cause node vmstat numbers to double-decrement, leading to an imbalanced page count. The result is dmesg output like such:
$ cat /proc/sys/vm/stat_refresh
[77383.088417] vmstat_refresh: nr_isolated_anon -103212 [77383.088417] vmstat_refresh: nr_isolated_file -899642
This negative value may impact compaction and reclaim throttling.
The double-decrement occurs in the migrate_pages path:
caller to shrink_folio_list decrements the count shrink_folio_list demote_folio_list migrate_pages migrate_pages_batch migrate_folio_move migrate_folio_done mod_node_page_state(-ve) <- second decrement
This path happens for SUCCESSFUL migrations, not failures. Typically callers to migrate_pages are required to handle putback/accounting for failures, but this is already handled in the shrink code.
AFAIK, MGLRU doesn't dec/inc this counter, so it is not double-decrement for MGLRU. Maybe "imbalance update" is better? Anyway, it is just a nit. I'd suggest capturing the MGLRU case in the commit log too.
Gotcha, so yeah saying it's an imbalance fix is more accurate.
So more accurate changelog is:
[PATCH] vmscan,migrate: fix page count imbalance on node stats when demoting pages
When numa balancing is enabled with demotion, vmscan will call migrate_pages when shrinking LRUs. migrate_pages will decrement the the node's isolated page count, leading to an imbalanced count when invoked from (MG)LRU code.
The result is dmesg output like such:
$ cat /proc/sys/vm/stat_refresh
[77383.088417] vmstat_refresh: nr_isolated_anon -103212 [77383.088417] vmstat_refresh: nr_isolated_file -899642
This negative value may impact compaction and reclaim throttling.
The following path produces the decrement:
shrink_folio_list demote_folio_list migrate_pages migrate_pages_batch migrate_folio_move migrate_folio_done mod_node_page_state(-ve) <- decrement
I think that it may be better to mention the different behavior of LRU and MGLRU. But that's not a big deal, change it again only if you think it's necessary.
-- Best Regards, Huang, Ying
This path happens for SUCCESSFUL migrations, not failures. Typically callers to migrate_pages are required to handle putback/accounting for failures, but this is already handled in the shrink code.
When accounting for migrations, instead do not decrement the count when the migration reason is MR_DEMOTION. As of v6.11, this demotion logic is the only source of MR_DEMOTION.
Signed-off-by: Gregory Price gourry@gourry.net Fixes: 26aa2d199d6f2 ("mm/migrate: demote pages during reclaim") Cc: stable@vger.kernel.org
Thanks for catching this. Reviewed-by: Yang Shi shy828301@gmail.com
mm/migrate.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/migrate.c b/mm/migrate.c index 923ea80ba744..e3aac274cf16 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -1099,7 +1099,7 @@ static void migrate_folio_done(struct folio *src, * not accounted to NR_ISOLATED_*. They can be recognized * as __folio_test_movable */
if (likely(!__folio_test_movable(src)))
if (likely(!__folio_test_movable(src)) && reason != MR_DEMOTION) mod_node_page_state(folio_pgdat(src), NR_ISOLATED_ANON + folio_is_file_lru(src), -folio_nr_pages(src));-- 2.43.0