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([2601:188:c102:b180:1f8b:71d0:77b1:1f6e]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-502f7f724d2sm2586161cf.20.2026.01.22.12.56.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 22 Jan 2026 12:56:40 -0800 (PST) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: <87d0eae3-e16e-4820-adde-afb519c5dcfc@redhat.com> Date: Thu, 22 Jan 2026 15:56:39 -0500 Precedence: bulk X-Mailing-List: linux-rt-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/mm_init: Don't cond_resched() in deferred_init_memmap_chunk() if called from deferred_grow_zone() To: Andrew Morton Cc: Mike Rapoport , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, Wei Yang , David Hildenbrand , "Paul E . McKenney" References: <20260122184343.546627-1-longman@redhat.com> <20260122112920.2b435873a0cc5f396df5d1a7@linux-foundation.org> In-Reply-To: <20260122112920.2b435873a0cc5f396df5d1a7@linux-foundation.org> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 9YvvLhu7NQWxglDE0ZI7OpQXW97CyktpZCpmq-BqliI_1769115402 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 1/22/26 2:29 PM, Andrew Morton wrote: > On Thu, 22 Jan 2026 13:43:43 -0500 Waiman Long 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 >> >> @@ -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 (can_resched) >> cond_resched(); >> + else >> + touch_nmi_watchdog(); >> } >> } > Disables the cond_resched() in some situations. Can this reintroduce > the watchdog warnings which that cond_resched() was intended to > prevent? cond_resched() is disabled only when it is called from deferred_grow_zone() where a spinlock was acquired with irqs disabled in the case of non-RT kernel and in a rcu_read_lock() acquired with RT kernel. In either case, scheduling out should not be allowed or something bad may happen. I suppose that iterating of pfn's in deferred_grow_zone() requires pgdat_resize_lock() protection. > > The cond_resched() was added by da97f2d56bbd ("mm: call > cond_resched() from deferred_init_memmap()"). > > Pasha's 2020 patch replaced touch_nmi_watchdog() with cond_resched() to > prevent RCU stall warnings. So I think the answer to my question is > yes, going back to touch_nmi_watchdog() could reintroduce those RCU > warnings. deferred_init_memmap() will  still have cond_resched() called in the iteration loop. It had RCU stall problem before without cond_resched() because it needs to iterate all the available memory which can takes a long time if we are talking about TBs of memory. For deferred_grow_zone(), as long as the number of pfn's that are iterated are not huge, RCU stall warning shouldn't happen. Cheers, Longman