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

[Waiman Long <llong@redhat.com>]

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On 1/21/26 2:43 PM, Andrew Morton wrote:
> On Wed, 21 Jan 2026 14:10:36 -0500 Waiman Long<longman@redhat.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.
That is true. Beside the scheduler, workqueue also use 
rcu_preempt_depth(). This API is included in "include/linux/rcupdate.h" 
which is included directly or indirectly by many kernel files. So even 
though it is rarely used, but it is still a public API.

>
> Is there a more official way of detecting this condition?  Maybe even
> #ifdef CONFIG_PREEMPT_RCU?
>
I am not aware of a more official way of detecting this. Maybe Sebastian 
has some ideas. rcu_preempt_count() is defined 
whether CONFIG_PREEMPT_RCU is defined or not. So we don't need a "#ifdef 
CONFIG_PREEMPT_RCU". Maybe I should explicitly include 
"include/linux/rcupdate.h" in mm/mm_init.c just to be sure.

CONFIG_PREEMPT_RCU defaults to on if PREMPT_RT is set. With 
!CONFIG_PREEMPT_RCU, rcu_preempt_depth() is hard-coded to 0 and will be 
optimized out.

Cheers,
Longman

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