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

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 27 Jul 2026 06:50:19 -0700 Breno Leitao <leitao@debian.org> 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

Well I doubt if users of 7.1 kernels and earlier want to see this.  So
a Fixes: and a cc:stable are needed.  The affected code is quite old
and might even predate the addition of cond_resched_tasks_rcu_qs().  So
I can't begin to suggest a Fixes: target.  Maybe omit it and let the
-stable team figure it out ;)

> --- 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();
>  

Totally off-topic but why the heck was that implemented as a macro. 
Which invokes another macro and another and another and turtles all the
way down.  End result:

   do { do { if (!((false)) && ({ do { __attribute__((__noreturn__)) extern void __compiletime_assert_606(void) __attribute__((__error__("Unsupported access size for {READ,WRITE}_ONCE()."))); if (!((sizeof(((current))->rcu_tasks_holdout) == sizeof(char) || sizeof(((current))->rcu_tasks_holdout) == sizeof(short) || sizeof(((current))->rcu_tasks_holdout) == sizeof(int) || sizeof(((current))->rcu_tasks_holdout) == sizeof(long)) || sizeof(((current))->rcu_tasks_holdout) == sizeof(long long))) __compiletime_assert_606(); } while (0); (*(const volatile __typeof_unqual__(((current))->rcu_tasks_holdout) *)&(((current))->rcu_tasks_holdout)); })) do { do { __attribute__((__noreturn__)) extern void __compiletime_assert_607(void) __attribute__((__error__("Unsupported access size for {READ,WRITE}_ONCE()."))); if (!((sizeof(((current))->rcu_tasks_holdout) == sizeof(char) || sizeof(((current))->rcu_tasks_holdout) == sizeof(short) || sizeof(((current))->rcu_tasks_holdout) == sizeof(int) || sizeof(((
 current))->rcu_tasks_holdout) == sizeof(long)) || sizeof(((current))->rcu_tasks_holdout) == sizeof(long long))) __compiletime_assert_607(); } while (0); do { *(volatile typeof(((current))->rcu_tasks_holdout) *)&(((current))->rcu_tasks_holdout) = (false); } while (0); } while (0); } while (0); ({ __might_resched("mm/migrate.c", 1846, 0); _cond_resched(); }); } while (0);


How much nicer would it be to have

static inline void cond_resched_tasks_rcu_qs(void)
{
	if (some brief and efficient test)
		some_slow_uninlined_thing();
}