Re: Sleeping BUG in khugepaged for i586

[Larry Finger]

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