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

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Message-ID: <alpine.DEB.2.10.1706071352100.38905@chino.kir.corp.google.com>
Date:   Wed, 7 Jun 2017 13:56:01 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     Larry Finger <Larry.Finger@...inger.net>,
        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 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.