Re: NFS Freezer and stuck tasks

[Shawn Bohrer]

On Fri, May 01, 2015 at 05:10:34PM -0400, Benjamin Coddington wrote:
> On Fri, 1 May 2015, Benjamin Coddington wrote:
> 
> > On Wed, 4 Mar 2015, Shawn Bohrer wrote:
> >
> > > Hello,
> > >
> > > We're using the Linux cgroup Freezer on some machines that use NFS and
> > > have run into what appears to be a bug where frozen tasks are blocking
> > > running tasks and preventing them from completing.  On one of our
> > > machines which happens to be running an older 3.10.46 kernel we have
> > > frozen some of the tasks on the system using the cgroup Freezer.  We
> > > also have a separate set of tasks which are NOT frozen which are stuck
> > > trying to open some files on NFS.
> > >
> > > Looking at the frozen tasks there are several that have the following
> > > stack:
> > >
> > > [<ffffffff814fd055>] rpc_wait_bit_killable+0x35/0x80
> > > [<ffffffff814fd01d>] __rpc_wait_for_completion_task+0x2d/0x30
> > > [<ffffffff811dce5d>] nfs4_run_open_task+0x11d/0x170
> > > [<ffffffff811de7a3>] _nfs4_open_and_get_state+0x53/0x260
> > > [<ffffffff811e12d1>] nfs4_do_open+0x121/0x400
> > > [<ffffffff811e15e1>] nfs4_atomic_open+0x31/0x50
> > > [<ffffffff811f02dc>] nfs4_file_open+0xac/0x180
> > > [<ffffffff811479be>] do_dentry_open.isra.19+0x1ee/0x280
> > > [<ffffffff81147b3e>] finish_open+0x1e/0x30
> > > [<ffffffff811578d2>] do_last.isra.64+0x2c2/0xc40
> > > [<ffffffff81158519>] path_openat.isra.65+0x2c9/0x490
> > > [<ffffffff81158c38>] do_filp_open+0x38/0x80
> > > [<ffffffff81148cd4>] do_sys_open+0xe4/0x1c0
> > > [<ffffffff81148dce>] SyS_open+0x1e/0x20
> > > [<ffffffff8153e719>] system_call_fastpath+0x16/0x1b
> > > [<ffffffffffffffff>] 0xffffffffffffffff
> > >
> > > Here it looks like we are waiting in a wait queue inside
> > > rpc_wait_bit_killable() for RPC_TASK_ACTIVE.
> > >
> > > And there is a single task with a stack that looks like the following:
> > >
> > > [<ffffffff8107dc05>] __refrigerator+0x55/0x150
> > > [<ffffffff814fd086>] rpc_wait_bit_killable+0x66/0x80
> > > [<ffffffff814fd01d>] __rpc_wait_for_completion_task+0x2d/0x30
> > > [<ffffffff811dce5d>] nfs4_run_open_task+0x11d/0x170
> > > [<ffffffff811de7a3>] _nfs4_open_and_get_state+0x53/0x260
> > > [<ffffffff811e12d1>] nfs4_do_open+0x121/0x400
> > > [<ffffffff811e15e1>] nfs4_atomic_open+0x31/0x50
> > > [<ffffffff811f02dc>] nfs4_file_open+0xac/0x180
> > > [<ffffffff811479be>] do_dentry_open.isra.19+0x1ee/0x280
> > > [<ffffffff81147b3e>] finish_open+0x1e/0x30
> > > [<ffffffff811578d2>] do_last.isra.64+0x2c2/0xc40
> > > [<ffffffff81158519>] path_openat.isra.65+0x2c9/0x490
> > > [<ffffffff81158c38>] do_filp_open+0x38/0x80
> > > [<ffffffff81148cd4>] do_sys_open+0xe4/0x1c0
> > > [<ffffffff81148dce>] SyS_open+0x1e/0x20
> > > [<ffffffff8153e719>] system_call_fastpath+0x16/0x1b
> > > [<ffffffffffffffff>] 0xffffffffffffffff
> > >
> > > This looks similar but the different offset into
> > > rpc_wait_bit_killable() shows that we have returned from the
> > > schedule() call in freezable_schedule() and are now blocked in
> > > __refrigerator() inside freezer_count()
> > >
> > > Similarly if you look at the tasks that are NOT frozen but are stuck
> > > opening a NFS file, they also have the following stack showing they are
> > > waiting in the wait queue for RPC_TASK_ACTIVE.
> > >
> > > [<ffffffff814fd055>] rpc_wait_bit_killable+0x35/0x80
> > > [<ffffffff814fd01d>] __rpc_wait_for_completion_task+0x2d/0x30
> > > [<ffffffff811dce5d>] nfs4_run_open_task+0x11d/0x170
> > > [<ffffffff811de7a3>] _nfs4_open_and_get_state+0x53/0x260
> > > [<ffffffff811e12d1>] nfs4_do_open+0x121/0x400
> > > [<ffffffff811e15e1>] nfs4_atomic_open+0x31/0x50
> > > [<ffffffff811f02dc>] nfs4_file_open+0xac/0x180
> > > [<ffffffff811479be>] do_dentry_open.isra.19+0x1ee/0x280
> > > [<ffffffff81147b3e>] finish_open+0x1e/0x30
> > > [<ffffffff811578d2>] do_last.isra.64+0x2c2/0xc40
> > > [<ffffffff81158519>] path_openat.isra.65+0x2c9/0x490
> > > [<ffffffff81158c38>] do_filp_open+0x38/0x80
> > > [<ffffffff81148cd4>] do_sys_open+0xe4/0x1c0
> > > [<ffffffff81148dce>] SyS_open+0x1e/0x20
> > > [<ffffffff8153e719>] system_call_fastpath+0x16/0x1b
> > > [<ffffffffffffffff>] 0xffffffffffffffff
> > >
> > > We have hit this a couple of times now and know that if we THAW all of
> > > the frozen tasks that running tasks will unwedge and finish.
> > >
> > > Additionally we have also tried thawing the single task that is frozen
> > > in __refrigerator() inside rpc_wait_bit_killable().  This usually
> > > results in different frozen task entering the __refrigerator() state
> > > inside rpc_wait_bit_killable().  It looks like each one of those tasks
> > > must wake up another letting it progress.  Again if you thaw enough of
> > > the frozen tasks eventually everything unwedges and everything
> > > completes.
> > >
> > > I've looked through the 3.10 stable patches since 3.10.46 and don't
> > > see anything that looks like it addresses this.  Does anyone have any
> > > idea what might be going on here, and what the fix might be?
> > >
> > > Thanks,
> > > Shawn
> >
> > Hi Shawn, just started looking at this myself, and as Frank Sorensen points
> > out in https://bugzilla.redhat.com/show_bug.cgi?id=1209143 the problem is
> > that a task takes the xprt lock and then ends up in the refrigerator
> > effectively blocking other tasks from proceeding.
> >
> > Jeff, any suggestions on how to proceed here?
> 
> Sorry for the noise, and self-reply..  Looks like there's additional context
> here: http://marc.info/?t=136761512100007&r=1&w=2
> 
> Due to a number of locking problems the answer to this problem is likely to
> be "don't do that" for now.

Sorry I found the NFS + Freezer is broken threads and probably should
have replied to myself.  We are now using SIGSTOP/SIGCONT with a brief
freeze to send the signals without race conditions.  With that said it
would be nice if these locking issues were eventually fixed because I
suspect it makes the freezer essentially useless for a large number of
enterprise users.

--
Shawn
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