Re: deadlock on vmap_area_lock

[Shawn Bohrer <sbohrer@rgmadvisors.com>]

On Thu, May 02, 2013 at 08:03:04AM +1000, Dave Chinner wrote:
> On Wed, May 01, 2013 at 08:57:38AM -0700, David Rientjes wrote:
> > On Wed, 1 May 2013, Shawn Bohrer wrote:
> > 
> > > I've got two compute clusters with around 350 machines each which are
> > > running kernels based off of 3.1.9 (Yes I realize this is ancient by
> > > todays standards).
> 
> xfs_info output of one of those filesystems? What platform are you
> running (32 or 64 bit)?

# cat /proc/mounts | grep data-cache
/dev/sdb1 /data-cache xfs rw,nodiratime,relatime,attr2,delaylog,noquota 0 0
# xfs_info /data-cache 
meta-data=/dev/sdb1              isize=256    agcount=4, agsize=66705344 blks
         =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=266821376, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal               bsize=4096   blocks=130283, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

These are 64-bit systems.  The ones that hit the issue more frequently
have 96 GB of RAM.

> > > All of the machines run a 'find' command once an
> > > hour on one of the mounted XFS filesystems.  Occasionally these find
> > > commands get stuck requiring a reboot of the system.  I took a peek
> > > today and see this with perf:
> > > 
> > >     72.22%          find  [kernel.kallsyms]          [k] _raw_spin_lock
> > >                     |
> > >                     --- _raw_spin_lock
> > >                        |          
> > >                        |--98.84%-- vm_map_ram
> > >                        |          _xfs_buf_map_pages
> > >                        |          xfs_buf_get
> > >                        |          xfs_buf_read
> > >                        |          xfs_trans_read_buf
> > >                        |          xfs_da_do_buf
> > >                        |          xfs_da_read_buf
> > >                        |          xfs_dir2_block_getdents
> > >                        |          xfs_readdir
> > >                        |          xfs_file_readdir
> > >                        |          vfs_readdir
> > >                        |          sys_getdents
> > >                        |          system_call_fastpath
> > >                        |          __getdents64
> > >                        |          
> > >                        |--1.12%-- _xfs_buf_map_pages
> > >                        |          xfs_buf_get
> > >                        |          xfs_buf_read
> > >                        |          xfs_trans_read_buf
> > >                        |          xfs_da_do_buf
> > >                        |          xfs_da_read_buf
> > >                        |          xfs_dir2_block_getdents
> > >                        |          xfs_readdir
> > >                        |          xfs_file_readdir
> > >                        |          vfs_readdir
> > >                        |          sys_getdents
> > >                        |          system_call_fastpath
> > >                        |          __getdents64
> > >                         --0.04%-- [...]
> > > 
> > > Looking at the code my best guess is that we are spinning on
> > > vmap_area_lock, but I could be wrong.  This is the only process
> > > spinning on the machine so I'm assuming either another process has
> > > blocked while holding the lock, or perhaps this find process has tried
> > > to acquire the vmap_area_lock twice?
> > > 
> > 
> > Significant spinlock contention doesn't necessarily mean that there's a 
> > deadlock, but it also doesn't mean the opposite.  Depending on your 
> > definition of "occassionally", would it be possible to run with 
> > CONFIG_PROVE_LOCKING and CONFIG_LOCKDEP to see if it uncovers any real 
> > deadlock potential?
> 
> It sure will. We've been reporting that vm_map_ram is doing
> GFP_KERNEL allocations from GFP_NOFS context for years, and have
> reported plenty of lockdep dumps as a result of it.
> 
> But that's not the problem that is occurring above - lockstat is
> probably a good thing to look at here to determine exactly what
> locks are being contended on....

I've built a kernel with lock_stat, CONFIG_PROVE_LOCKING,
CONFIG_LOCKDEP and have one machine running with that kernel.  We'll
probably put machines on this debug kernel when we reboot them and
hopefully one will trigger the issue.

Thanks,
Shawn

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