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

[Gregory Price]

On Mon, Jul 27, 2026 at 06:50:19AM -0700, 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.
> 
> PS: This has also been discussed at [1]
> 
> Link: https://lore.kernel.org/all/amdWVTs0WKOxguxP@xxxxxxxxx/ [1]
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>

Reviewed-by: Gregory Price <gourry@xxxxxxxxxx>