Re: [PATCH v2] mm/mm_init: Don't call cond_resched() in deferred_init_memmap_chunk() if rcu_preempt_depth() set

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-01-21 22:40:17 [-0500], Waiman Long wrote:
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -2085,7 +2086,12 @@ deferred_init_memmap_chunk(unsigned long start_pfn, unsigned long end_pfn,
>  
>  			spfn = chunk_end;
>  
> -			if (irqs_disabled())
> +			/*
> +			 * pgdat_resize_lock() only disables irqs in non-RT
> +			 * kernels but implies rcu_read_lock() in a PREEMPT_RT
> +			 * kernel.
> +			 */
> +			if (irqs_disabled() || rcu_preempt_depth())
>  				touch_nmi_watchdog();
>  			else
>  				cond_resched();

What about (as suggested in the previous mail)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index fc2a6f1e518f1..0b283fd48b282 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -2059,7 +2059,7 @@ static unsigned long __init deferred_init_pages(struct zone *zone,
  */
 static unsigned long __init
 deferred_init_memmap_chunk(unsigned long start_pfn, unsigned long end_pfn,
-			   struct zone *zone)
+			   struct zone *zone, bool may_schedule)
 {
 	int nid = zone_to_nid(zone);
 	unsigned long nr_pages = 0;
@@ -2085,10 +2085,10 @@ deferred_init_memmap_chunk(unsigned long start_pfn, unsigned long end_pfn,
 
 			spfn = chunk_end;
 
-			if (irqs_disabled())
-				touch_nmi_watchdog();
-			else
+			if (may_schedule)
 				cond_resched();
+			else
+				touch_nmi_watchdog();
 		}
 	}
 
@@ -2101,7 +2101,7 @@ deferred_init_memmap_job(unsigned long start_pfn, unsigned long end_pfn,
 {
 	struct zone *zone = arg;
 
-	deferred_init_memmap_chunk(start_pfn, end_pfn, zone);
+	deferred_init_memmap_chunk(start_pfn, end_pfn, zone, true);
 }
 
 static unsigned int __init
@@ -2216,7 +2216,7 @@ bool __init deferred_grow_zone(struct zone *zone, unsigned int order)
 	for (spfn = first_deferred_pfn, epfn = SECTION_ALIGN_UP(spfn + 1);
 	     nr_pages < nr_pages_needed && spfn < zone_end_pfn(zone);
 	     spfn = epfn, epfn += PAGES_PER_SECTION) {
-		nr_pages += deferred_init_memmap_chunk(spfn, epfn, zone);
+		nr_pages += deferred_init_memmap_chunk(spfn, epfn, zone, false);
 	}
 
 	/*


Sebastian