Re: CPU stall, eventual host hang with BTRFS + NFS under heavy load

[Scott Mayhew <smayhew@redhat.com>]

On Fri, 15 Oct 2021, Trond Myklebust wrote:

> On Fri, 2021-10-15 at 08:03 +0000, Trond Myklebust wrote:
> > On Fri, 2021-10-15 at 09:51 +1100, NeilBrown wrote:
> > > On Fri, 15 Oct 2021, Trond Myklebust wrote:
> > > > On Tue, 2021-10-12 at 08:57 +1100, NeilBrown wrote:
> > > > > On Tue, 12 Oct 2021, Chuck Lever III wrote:
> > > > > > 
> > > > > > Scott seems well positioned to identify a reproducer. Maybe
> > > > > > we
> > > > > > can give him some likely candidates for possible bugs to
> > > > > > explore
> > > > > > first.
> > > > > 
> > > > > Has this patch been tried?
> > > > > 
> > > > > NeilBrown
> > > > > 
> > > > > 
> > > > > diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
> > > > > index c045f63d11fa..308f5961cb78 100644
> > > > > --- a/net/sunrpc/sched.c
> > > > > +++ b/net/sunrpc/sched.c
> > > > > @@ -814,6 +814,7 @@ rpc_reset_task_statistics(struct rpc_task
> > > > > *task)
> > > > >  {
> > > > >         task->tk_timeouts = 0;
> > > > >         task->tk_flags &= ~(RPC_CALL_MAJORSEEN|RPC_TASK_SENT);
> > > > > +       clear_bit(RPC_TASK_SIGNALLED, &task->tk_runstate);
> > > > >         rpc_init_task_st
> > > > 
> > > > We shouldn't automatically "unsignal" a task once it has been
> > > > told
> > > > to
> > > > die. The correct thing to do here should rather be to change
> > > > rpc_restart_call() to exit early if the task was signalled.
> > > > 
> > > 
> > > Maybe.  It depends on exactly what the signal meant
> > > (rpc_killall_tasks()
> > > is a bit different from getting a SIGKILL), and exactly what the
> > > task
> > > is
> > > trying to achieve.
> > > 
> > > Before Commit ae67bd3821bb ("SUNRPC: Fix up task signalling")
> > > that is exactly what we did.
> > > If we want to change the behaviour of a task responding to
> > > rpc_killall_tasks(), we should clearly justify it in a patch doing
> > > exactly that.
> > > 
> > 
> > The intention behind rpc_killall_tasks() never changed, which is why
> > it
> 
> ("it" being the error ERESTARTSYS)
> 
> > is listed in nfs_error_is_fatal(). I'm not aware of any case where we
> > deliberately override in order to restart the RPC call on an
> > ERESTARTSYS error.
> > 
Update: I'm not able to reproduce this with an upstream kernel.  I
bisected it down to commit 2ba5acfb3495 "SUNRPC: fix sign error causing
rpcsec_gss drops" as the commit that "fixed" the issue (but really just
makes the issue less likely to occur, I think).

I also tested commit 10b9d99a3dbb "SUNRPC: Augment server-side rpcgss
tracepoints" (the commit in the Fixes: tag of 2ba5acfb3495) as well as
commit 0e885e846d96 "nfsd: add fattr support for user extended attributes"
(the parent of commit 10b9d99a3dbb) and verified that commit
10b9d99a3dbb is where the issue started occurring.

I think what is happening is that the NFS server gets a request that it
thinks is outside of the GSS sequence window and drops the request,
closes the connection and calls nfsd4_conn_lost(), which calls
nfsd4_probe_callback() which sets NFSD4_CLIENT_CB_UPDATE in
clp->cl_flags.  Then the client reestablishes the connection on that
port, sends another request which receives
NFS4ERR_CONN_NOT_BOUND_TO_SESSION.  The client runs the state manager
which calls nfs4_bind_conn_to_session(), which calls
nfs4_begin_drain_session(), which sets NFS4_SLOT_TBL_DRAINING in
tbl->slot_tbl_state.  Meanwhile a conflicting request comes in that
causes the server to recall the delegation.  Since
NFS4_SLOT_TBL_DRAINING is set, the client responds to the CB_SEQUENCE
with NFS4ERR_DELAY.  At the same time, the BIND_CONN_TO_SESSION requests
from the client are causing the server to call
nfsd4_process_cb_update(), since NFSD4_CLIENT_CB_UPDATE flag is set.
nfsd4_process_cb_update() calls rpc_shutdown_client() which signals the
CB_RECALL task, which the server is trying re-send due to the
NFS4ERR_DELAY, and we get into the soft-lockup.

I tried this patch

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index 20db98679d6b..187f7f1cc02a 100644
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -803,6 +803,7 @@ rpc_reset_task_statistics(struct rpc_task *task)
 {
        task->tk_timeouts = 0;
        task->tk_flags &= ~(RPC_CALL_MAJORSEEN|RPC_TASK_SENT);
+       clear_bit(RPC_TASK_SIGNALLED, &task->tk_runstate);
        rpc_init_task_statistics(task);
 }
 
but instead of fixing the soft-lockup I just wind up with a hung task:

INFO: task nfsd:1367 blocked for more than 120 seconds.
[ 3195.902559]       Not tainted 4.18.0-353.el8.jsm.test.1.x86_64 #1
[ 3195.905411] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 3195.908076] task:nfsd            state:D stack:    0 pid: 1367 ppid:     2 flags:0x80004080
[ 3195.910906] Call Trace:
[ 3195.911915]  __schedule+0x2d1/0x830
[ 3195.913211]  schedule+0x35/0xa0
[ 3195.914377]  schedule_timeout+0x274/0x300
[ 3195.915919]  ? check_preempt_wakeup+0x113/0x230
[ 3195.916907]  wait_for_completion+0x96/0x100
[ 3195.917629]  flush_workqueue+0x14d/0x440
[ 3195.918342]  nfsd4_destroy_session+0x198/0x230 [nfsd]
[ 3195.919277]  nfsd4_proc_compound+0x388/0x6d0 [nfsd]
[ 3195.920144]  nfsd_dispatch+0x108/0x210 [nfsd]
[ 3195.920922]  svc_process_common+0x2b3/0x700 [sunrpc]
[ 3195.921871]  ? svc_xprt_received+0x45/0x80 [sunrpc]
[ 3195.922722]  ? nfsd_svc+0x2e0/0x2e0 [nfsd]
[ 3195.923441]  ? nfsd_destroy+0x50/0x50 [nfsd]
[ 3195.924199]  svc_process+0xb7/0xf0 [sunrpc]
[ 3195.924971]  nfsd+0xe3/0x140 [nfsd]
[ 3195.925596]  kthread+0x10a/0x120
[ 3195.926383]  ? set_kthread_struct+0x40/0x40
[ 3195.927100]  ret_from_fork+0x35/0x40

I then tried this patch:

diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
index 0e212ac0fe44..5667fd15f157 100644
--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -1573,6 +1573,8 @@ __rpc_restart_call(struct rpc_task *task, void (*action)(struct rpc_task *))
 int
 rpc_restart_call(struct rpc_task *task)
 {
+       if (RPC_SIGNALLED(task))
+               return 0;
        return __rpc_restart_call(task, call_start);
 }
 EXPORT_SYMBOL_GPL(rpc_restart_call);

and that seems to work.

-Scott
> > 
> 
> -- 
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
> 
>