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

[Zi Yan]

On 27 Jul 2026, at 9:50, Breno Leitao wrote:

> 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.

The explanation looks good to me.

Acked-by: Zi Yan <ziy@xxxxxxxxxx>

BTW, Sashiko complained about hugetlb path, but hugetlb does not batch
migration, so there should not be an issue like this one.

>
> 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>


Best Regards,
Yan, Zi