linux-kernel - Re: Sleeping BUG in khugepaged for i586

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Message-ID: <a94e1315-aded-0984-327e-e091e76b4a66@lwfinger.net>
Date:   Thu, 8 Jun 2017 10:29:38 -0500
From:   Larry Finger <Larry.Finger@...inger.net>
To:     David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: Sleeping BUG in khugepaged for i586

On 06/07/2017 03:56 PM, David Rientjes wrote:
> On Wed, 7 Jun 2017, Vlastimil Babka wrote:
> 
>>>> Hmm I'd expect such spin lock to be reported together with mmap_sem in
>>>> the debugging "locks held" message?
>>>
>>> My bisection of the problem is about half done. My latest good version is commit
>>> 7b8cd33 and the latest bad one is 2ea659a. Only about 7 steps to go.
>>
>> Hmm, your bisection will most likely just find commit 338a16ba15495
>> which added the cond_resched() at mm/khugepaged.c:655. CCing David who
>> added it.
>>
> 
> I agree it's probably going to bisect to 338a16ba15495 since it's the
> cond_resched() at the line number reported, but I think there must be
> something else going on.  I think the list of locks held by khugepaged is
> correct because it matches with the implementation.  The preempt_count(),
> as suggested by Andrew, does not.  If this is reproducible, I'd like to
> know what preempt_count() is.
> 

The BUG output is reproducible. By the time the box finishes booting, there are 
at least 2 of them logged. My bisection shows that commit 338a16ba15495 is the 
bad one. I added a pr_info() to output the value of preempt_count() just before 
the cond_resched() statement. The count was always 1 whether the BUG was 
triggered or not.

If there are other things you would like logged at that point, or any other 
diagnostics, please let me know.

Larry