linux-kernel - Re: [PATCH] mm/kmemleak: Avoid soft lockup in __kmemleak_do

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Message-ID: <aInt7Fa6N7Y3zW_K@arm.com>
Date: Wed, 30 Jul 2025 11:03:24 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Waiman Long <longman@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/kmemleak: Avoid soft lockup in __kmemleak_do_cleanup()

On Mon, Jul 28, 2025 at 03:02:48PM -0400, Waiman Long wrote:
> A soft lockup warning was observed on a relative small system x86-64
> system with 16 GB of memory when running a debug kernel with kmemleak
> enabled.
> 
>   watchdog: BUG: soft lockup - CPU#8 stuck for 33s! [kworker/8:1:134]
> 
> The test system was running a workload with hot unplug happening
> in parallel. Then kemleak decided to disable itself due to its
> inability to allocate more kmemleak objects. The debug kernel has its
> CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE set to 40,000.
> 
> The soft lockup happened in kmemleak_do_cleanup() when the existing
> kmemleak objects were being removed and deleted one-by-one in a loop
> via a workqueue. In this particular case, there are at least 40,000
> objects that need to be processed and given the slowness of a debug
> kernel and the fact that a raw_spinlock has to be acquired and released
> in __delete_object(), it could take a while to properly handle all
> these objects.
> 
> As kmemleak has been disabled in this case, the object removal and
> deletion process can be further optimized as locking isn't really
> needed. However, it is probably not worth the effort to optimize for
> such an edge case that should rarely happen. So the simple solution is
> to call cond_resched() at periodic interval in the iteration loop to
> avoid soft lockup.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>

I agree, it's not worth rewriting this path for an unlikely event. So
I'm fine with this approach. Thanks.

Acked-by: Catalin Marinas <catalin.marinas@....com>