Re: [PATCH 6/6 v2] mm: Drain LRUs upon resume to userspace on nohz_full CPUs

[Hillf Danton <hdanton@sina.com>]

On Sun,  9 Feb 2025 23:30:04 +0100 Frederic Weisbecker <frederic@kernel.org>
> @@ -769,6 +772,9 @@ static bool cpu_needs_drain(unsigned int cpu)
>  {
>  	struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu);
>  
> +	if (!housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE))
> +		return false;
> +
>  	/* Check these in order of likelihood that they're not zero */
>  	return folio_batch_count(&fbatches->lru_add) ||
>  		folio_batch_count(&fbatches->lru_move_tail) ||
> -- 
> 2.46.0

Nit, I'd like to add a debug line to test your assumption that
isolated tasks are pinned to a single nohz_full CPU.

--- x/mm/swap.c
+++ y/mm/swap.c
@@ -767,9 +767,10 @@ static void lru_add_drain_per_cpu(struct
 static bool cpu_needs_drain(unsigned int cpu)
 {
 	struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu);
+	bool yes;
 
 	/* Check these in order of likelihood that they're not zero */
-	return folio_batch_count(&fbatches->lru_add) ||
+	yes = folio_batch_count(&fbatches->lru_add) ||
 		folio_batch_count(&fbatches->lru_move_tail) ||
 		folio_batch_count(&fbatches->lru_deactivate_file) ||
 		folio_batch_count(&fbatches->lru_deactivate) ||
@@ -777,6 +778,12 @@ static bool cpu_needs_drain(unsigned int
 		folio_batch_count(&fbatches->lru_activate) ||
 		need_mlock_drain(cpu) ||
 		has_bh_in_lru(cpu, NULL);
+
+	if (!housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE)) {
+		VM_BUG_ON(yes);
+		return false;
+	}
+	return yes;
 }
 
 /*