On Wed, 2021-12-22 at 10:55 +0800, Bixuan Cui wrote:

在 2021/12/21 上午2:22, Trond Myklebust 写道:
 

quoted

On Mon, 2021-12-20 at 11:39 +0800, Bixuan Cui wrote:
 

quoted

ping~

在 2021/12/14 下午9:53, Bixuan Cui 写道:
 

quoted

When the values of tcp_max_slot_table_entries and
sunrpc.tcp_slot_table_entries are lower than the number of rpc
tasks,
xprt_dynamic_alloc_slot() in xprt_alloc_slot() will return -
EAGAIN,
and
then set xprt->state to XPRT_CONGESTED:
   xprt_retry_reserve
     ->xprt_do_reserve
       ->xprt_alloc_slot
         ->xprt_dynamic_alloc_slot // return -EAGAIN and task-
 

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tk_rqstp is NULL

           ->xprt_add_backlog // set_bit(XPRT_CONGESTED, &xprt-
 

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state);

When rpc task is killed, XPRT_CONGESTED bit of xprt->state will
not
be
cleaned up and nfs hangs:
   rpc_exit_task
     ->xprt_release // if (req == NULL) is true, then
XPRT_CONGESTED
                   // bit not clean

Add xprt_wake_up_backlog(xprt) to clean XPRT_CONGESTED bit in
xprt_release().

I'm not seeing how this explanation makes sense. If the task
doesn't
hold a slot, then freeing that task isn't going to clear the
congestion
caused by all the slots being in use.

Hi，
If the rpc task is free, call xprt_release() :
void xprt_release(struct rpc_task *task)
 {
      if (req == NULL) {
                  if (task->tk_client) {
                          xprt = task->tk_xprt;
                          xprt_release_write(xprt, task); // 1.
release xprt
                  }
                  return;
          }
      ....
      if (likely(!bc_prealloc(req)))
                  xprt->ops->free_slot(xprt, req); // 2. release slot
and call xprt_wake_up_backlog(xprt, req) to wakeup next task(clear
XPRT_CONGESTED bit if next is NULL) in xprt_free_slot()
          else
                  xprt_free_bc_request(req);
 }
 I mean that in step 1, xprt was only released, but
xprt_wake_up_backlog was not called (I don’t know if it is necessary,
but xprt->state may still be XPRT_CONGESTED), which causes xprt to
hold up. I think it happens when the task that does not hold a slot
is the last released task，xprt_wake_up_backlog(clear XPRT_CONGESTED)
will not be executed. :-)
Thanks，
Bixuan Cui

 

My point is that in that case 1, there is no slot to free, so there is
no change to the congestion state.

IOW: your patch is incorrect because it is trying to assign a slot in a
case where there is no slot to assign.

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