[PATCH] mm/migrate: report RCU-tasks quiescent states in migrate_pages_batch()

[Breno Leitao]

migrate_pages_batch() unmaps each folio before moving it, and every
unmap runs the mmu_notifier invalidate callbacks.  On KVM hosts
try_to_migrate() ends up in kvm_mmu_notifier_invalidate_range_start() ->
tdp_mmu_zap_leafs(), which is expensive, so unmapping a large batch keeps
the CPU busy for a long time.

The loop already calls cond_resched(), but on PREEMPTION kernels that is
a no-op, and involuntary preemption is not a Tasks-RCU quiescent state.

A long batch therefore never reports a quiescent state, and the
migrating task (e.g. kcompactd) becomes a Tasks-RCU holdout, stalling the
Tasks-RCU grace period for minutes, which is common at Meta fleet:

  INFO: rcu_tasks detected stalls on tasks:
  0000000055349ecc: .. nvcsw: 1157401/1157401 holdout: 1 idle_cpu: -1/56 task:kcompactd0      state:R  running task
  Call Trace:
   tdp_mmu_zap_leafs
   tdp_mmu_next_root
   gfn_to_pfn_cache_invalidate_start
   kvm_mmu_notifier_invalidate_range_start
   __mmu_notifier_invalidate_range_start
   try_to_migrate_one
   try_to_migrate
   migrate_pages_batch
   migrate_pages
   compact_zone
   compact_node
   kcompactd
   kthread

Use cond_resched_tasks_rcu_qs() so a quiescent state is reported even
when cond_resched() does nothing.

PS: This has also been discussed at [1]

Link: https://lore.kernel.org/all/amdWVTs0WKOxguxP@xxxxxxxxx/ [1]
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 mm/migrate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index ab15a4dddd047..b937cbd764808 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1843,7 +1843,7 @@ static int migrate_pages_batch(struct list_head *from,
 			is_thp = folio_test_pmd_mappable(folio);
 			nr_pages = folio_nr_pages(folio);
 
-			cond_resched();
+			cond_resched_tasks_rcu_qs();
 
 			/*
 			 * The rare folio on the deferred split list should

---
base-commit: c5e32e86ca02b003f86e095d379b38148999293d
change-id: 20260727-kcompact-ff0550fc0ebb

Best regards,
--  
Breno Leitao <leitao@xxxxxxxxxx>