在 2021/12/22 下午11:02, Trond Myklebust 写道:

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

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

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

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ping~

在 2021/12/14 下午9:53, Bixuan Cui 写道:
  

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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.

Hi，
I found the correct way to fix it, that is, do not free the request when 
there are tasks in the xprt->backlog :-)
And it has been fixed by e877a88d1f06 (SUNRPC in case of backlog, hand 
free slots directly to waiting task)
     commit e877a88d1f069edced4160792f42c2a8e2dba942
     Author: NeilBrown [off-list ref]
     Date:   Mon May 17 09:59:10 2021 +1000

     SUNRPC in case of backlog, hand free slots directly to waiting task

     If sunrpc.tcp_max_slot_table_entries is small and there are tasks
     on the backlog queue, then when a request completes it is freed and the
     first task on the queue is woken.  The expectation is that it will wake
     and claim that request.  However if it was a sync task and the waiting
     process was killed at just that moment, it will wake and NOT claim the
     request.
Thanks for your advice.

Thanks,
Bixuan Cui