Re: kvm deadlock

[Vivek Goyal]

On Wed, Dec 14, 2011 at 05:03:54PM +0100, Jens Axboe wrote:
> On 2011-12-14 14:43, Avi Kivity wrote:
> > On 12/14/2011 02:25 PM, Marcelo Tosatti wrote:
> >> On Mon, Dec 05, 2011 at 04:48:16PM -0600, Nate Custer wrote:
> >>> Hello,
> >>>
> >>> I am struggling with repeatable full hardware locks when running 8-12 KVM vms. At some point before the hard lock I get a inconsistent lock state warning. An example of this can be found here:
> >>>
> >>> http://pastebin.com/8wKhgE2C
> >>>
> >>> After that the server continues to run for a while and then starts its death spiral. When it reaches that point it fails to log anything further to the disk, but by attaching a console I have been able to get a stack trace documenting the final implosion:
> >>>
> >>> http://pastebin.com/PbcN76bd
> >>>
> >>> All of the cores end up hung and the server stops responding to all input, including SysRq commands. 
> >>>
> >>> I have seen this behavior on two machines (dual E5606 running Fedora 16) both passed cpuburnin testing and memtest86 scans without error. 
> >>>
> >>> I have reproduced the crash and stack traces from a Fedora debugging kernel - 3.1.2-1 and with a vanilla 3.1.4 kernel.
> >>
> >> Busted hardware, apparently. Can you reproduce these issues with the
> >> same workload on different hardware?
> > 
> > I don't think it's hardware related.  The second trace (in the first
> > paste) is called during swap, so GFP_FS is set.  The first one is not,
> > so GFP_FS is clear.  Lockdep is worried about the following scenario:
> > 
> >   acpi_early_init() is called
> >   calls pcpu_alloc(), which takes pcpu_alloc_mutex
> >   eventually, calls kmalloc(), or some other allocation function
> >   no memory, so swap
> >   call try_to_free_pages()
> >   submit_bio()
> >   blk_throtl_bio()
> >   blkio_alloc_blkg_stats()
> >   alloc_percpu()
> >   pcpu_alloc(), which takes pcpu_alloc_mutex
> >   deadlock
> > 
> > It's a little unlikely that acpi_early_init() will OOM, but lockdep
> > doesn't know that.  Other callers of pcpu_alloc() could trigger the same
> > thing.
> > 
> > When lockdep says
> > 
> > [ 5839.924953] other info that might help us debug this:
> > [ 5839.925396]  Possible unsafe locking scenario:
> > [ 5839.925397]
> > [ 5839.925840]        CPU0
> > [ 5839.926063]        ----
> > [ 5839.926287]   lock(pcpu_alloc_mutex);
> > [ 5839.926533]   <Interrupt>
> > [ 5839.926756]     lock(pcpu_alloc_mutex);
> > [ 5839.926986]
> > 
> > It really means
> > 
> >    <swap, set GFP_FS>
> > 
> > GFP_FS simply marks the beginning of a nested, unrelated context that
> > uses the same thread, just like an interrupt.  Kudos to lockdep for
> > catching that.
> > 
> > I think the allocation in blkio_alloc_blkg_stats() should be moved out
> > of the I/O path into some init function. Copying Jens.
> 
> That's completely buggy, basically you end up with a GFP_KERNEL
> allocation from the IO submit path. Vivek, per_cpu data needs to be set
> up at init time. You can't allocate it dynamically off the IO path.

Hi Jens,

I am wondering how does CFQ get away with blocking cfqq allocation in
IO submit path. I see that blk_queue_bio() will do following.

blk_queue_bio()
 get_request_wait()
  get_request(..,..,GFP_NOIO)
   blk_alloc_request()
    elv_set_request()
     cfq_set_request()
      ---> Can sleep and do memory allocation in IO submit path as
           GFP_NOIO has __GFP_WAIT.

So that means sleeping allocation from IO submit path is not necessarily
a problem?

But in case of per cpu data allocation, we might be holding pcpu_alloc_mutex()
already at the time of calling pcpu allocation again and that might lead
to deadlock.  (As Avi mentioned). If yes, then it is a problem.

Right now allocation of root group and associated stats happens at queue
initialization time. For non-root cgroups, group allocation and associated
per cpu stats allocation happens dynamically when the IO is submitted. So
in this case may be we are creating a new blkio cgroup and then doing IO
which leads to this warning.

I am not sure how to move this allocation to init path. These stats are
per group and groups are created dynamically as IO happens in them. Only
init path seems to be cgroup creation time. blkg is an object which is
contained in a parent object and at that time parent object is not
available. It is created dynamically at the IO time (cfq_group,
blkio_group etc).

Though it is little hackish but can we just delay the allocation of stats
if pcpu_alloc_mutex is held. We shall have to make pcpu_alloc_mutex non
static though. Delaying will just not capture the stats for some time
and sooner or later we will get regular IO with pcpu_alloc_mutex not
held and we can do per cpu allocation at that time. I will write a
a test patch.

Or may be there is a safer version of pcpu alloc which will return
without allocation if pcpu_alloc_mutex is already locked.

CCing Tejun too.

Thanks
Vivek
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