linux-kernel - Re: [PATCH] mm/mm_init: Don't call cond_resched() in deferred_init_memmap_chunk() if rcu_preempt

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Message-Id: <20260121114330.6cd34b4732c7803f1720f0ba@linux-foundation.org>
Date: Wed, 21 Jan 2026 11:43:30 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Waiman Long <longman@...hat.com>
Cc: Mike Rapoport <rppt@...nel.org>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, Clark Williams <clrkwllms@...nel.org>, Steven
 Rostedt <rostedt@...dmis.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev, Wei Yang
 <richard.weiyang@...il.com>, David Hildenbrand <david@...nel.org>,
 "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] mm/mm_init: Don't call cond_resched() in
 deferred_init_memmap_chunk() if rcu_preempt_depth() set

On Wed, 21 Jan 2026 14:10:36 -0500 Waiman Long <longman@...hat.com> wrote:

> Commit 3acb913c9d5b ("mm/mm_init: use deferred_init_memmap_chunk()
> in deferred_grow_zone()") made deferred_grow_zone() call
> deferred_init_memmap_chunk() within a pgdat_resize_lock() critical
> section with irqs disabled. It did check for irqs_disabled() in
> deferred_init_memmap_chunk() to avoid calling cond_resched(). For a
> PREEMPT_RT kernel build, however, spin_lock_irqsave() does not disable
> interrupt but rcu_read_lock() is called. This leads to the following
> bug report.
> 
>   BUG: sleeping function called from invalid context at mm/mm_init.c:2091
>   in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0
>   preempt_count: 0, expected: 0
>   RCU nest depth: 1, expected: 0
>   3 locks held by swapper/0/1:
>    #0: ffff80008471b7a0 (sched_domains_mutex){+.+.}-{4:4}, at: sched_domains_mutex_lock+0x28/0x40
>    #1: ffff003bdfffef48 (&pgdat->node_size_lock){+.+.}-{3:3}, at: deferred_grow_zone+0x140/0x278
>    #2: ffff800084acf600 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1b4/0x408
>   CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Tainted: G        W           6.19.0-rc6-test #1 PREEMPT_{RT,(full)
> }
>   Tainted: [W]=WARN
>   Call trace:
>    show_stack+0x20/0x38 (C)
>    dump_stack_lvl+0xdc/0xf8
>    dump_stack+0x1c/0x28
>    __might_resched+0x384/0x530
>    deferred_init_memmap_chunk+0x560/0x688
>    deferred_grow_zone+0x190/0x278
>    _deferred_grow_zone+0x18/0x30
>    get_page_from_freelist+0x780/0xf78
>    __alloc_frozen_pages_noprof+0x1dc/0x348
>    alloc_slab_page+0x30/0x110
>    allocate_slab+0x98/0x2a0
>    new_slab+0x4c/0x80
>    ___slab_alloc+0x5a4/0x770
>    __slab_alloc.constprop.0+0x88/0x1e0
>    __kmalloc_node_noprof+0x2c0/0x598
>    __sdt_alloc+0x3b8/0x728
>    build_sched_domains+0xe0/0x1260
>    sched_init_domains+0x14c/0x1c8
>    sched_init_smp+0x9c/0x1d0
>    kernel_init_freeable+0x218/0x358
>    kernel_init+0x28/0x208
>    ret_from_fork+0x10/0x20
> 
> Fix it by checking rcu_preempt_depth() as well to prevent calling
> cond_resched(). Note that CONFIG_PREEMPT_RCU should always be enabled
> in a PREEMPT_RT kernel.
>
> ...
> 
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -2085,7 +2085,12 @@ 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 calls rcu_read_lock() in a PREEMPT_RT
> +			 * kernel.
> +			 */
> +			if (irqs_disabled() || rcu_preempt_depth())
>  				touch_nmi_watchdog();

rcu_preempt_depth() seems a fairly internal low-level thing - it's
rarely used.

Is there a more official way of detecting this condition?  Maybe even
#ifdef CONFIG_PREEMPT_RCU?