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Subject: Re: [PATCH v3 26/44] SUNRPC: Improve latency for interactive tasks
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <f451e8ac200370c520f2fb3371f4b2ae637fd3a8.camel@hammerspace.com>
Date:   Wed, 2 Jan 2019 14:33:37 -0500
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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To:     Trond Myklebust <trondmy@hammerspace.com>
Sender: linux-nfs-owner@vger.kernel.org
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> On Jan 2, 2019, at 2:24 PM, Trond Myklebust <trondmy@hammerspace.com> =
wrote:
>=20
> On Wed, 2019-01-02 at 14:06 -0500, Trond Myklebust wrote:
>> On Wed, 2019-01-02 at 13:57 -0500, Trond Myklebust wrote:
>>> On Wed, 2019-01-02 at 13:51 -0500, Chuck Lever wrote:
>>>>> On Jan 2, 2019, at 1:45 PM, Trond Myklebust <
>>>>> trondmy@hammerspace.com> wrote:
>>>>>=20
>>>>> On Wed, 2019-01-02 at 13:17 -0500, Chuck Lever wrote:
>>>>>>> On Dec 31, 2018, at 2:21 PM, Trond Myklebust <
>>>>>>> trondmy@hammerspace.com> wrote:
>>>>>>>=20
>>>>>>> On Mon, 2018-12-31 at 19:18 +0000, Trond Myklebust wrote:
>>>>>>>> On Mon, 2018-12-31 at 14:09 -0500, Chuck Lever wrote:
>>>>>>>>>> On Dec 31, 2018, at 1:59 PM, Trond Myklebust <
>>>>>>>>>> trondmy@hammerspace.com> wrote:
>>>>>>>>>>=20
>>>>>>>>>>=20
>>>>>>>> The test for rpcauth_xmit_need_reencode() happens when we
>>>>>>>> call
>>>>>>>> xprt_request_transmit() to actually put the RPC call on
>>>>>>>> the
>>>>>>>> wire.
>>>>>>>> The
>>>>>>>> enqueue order should not be able to defeat that test.
>>>>>>>>=20
>>>>>>>> Hmm... Is it perhaps the test for req->rq_bytes_sent that
>>>>>>>> is
>>>>>>>> failing
>>>>>>>> because this is a retransmission after a
>>>>>>>> disconnect/reconnect
>>>>>>>> that
>>>>>>>> didn't trigger a re-encode?
>>>>>>>=20
>>>>>>> Actually, it might be worth a try to move the test for
>>>>>>> rpcauth_xmit_need_reencode() outside the enclosing test for
>>>>>>> req-
>>>>>>>> rq_bytes_sent as that is just a minor optimisation.
>>>>>>=20
>>>>>> Perhaps that's the case for TCP, but RPCs sent via xprtrdma
>>>>>> never
>>>>>> set
>>>>>> req->rq_bytes_sent to a non-zero value. The body of the "if"
>>>>>> statement
>>>>>> is always executed for those RPCs.
>>>>>>=20
>>>>>=20
>>>>> Then the question is what is defeating the call to
>>>>> rpcauth_xmit_need_reencode() in xprt_request_transmit() and
>>>>> causing
>>>>> it
>>>>> not to trigger in the misordered cases?
>>>>=20
>>>> Here's a sample RPC/RDMA case.
>>>>=20
>>>> My instrumented server reports this:
>>>>=20
>>>> Jan  2 13:29:00 klimt kernel: gss_check_seq_num: dropped:
>>>> seq_num=3D141220 sd->sd_max=3D141360
>>>>=20
>>>>=20
>>>> ftrace log on the client shows this:
>>>>=20
>>>>  kworker/u28:12-2191  [004]   194.048534: rpcgss_need_reencode:
>>>> task:1761@5 xid=3D0x88f4f47c rq_seqno=3D141220 seq_xmit=3D141336
>>>> reencode
>>>> unneeded
>>>>  kworker/u28:12-2191  [004]   194.048534:
>>>> xprt_transmit:        task:1761@5 xid=3D0x88f4f47c seqno=3D141220
>>>> status=3D-57
>>>>  kworker/u28:12-2191  [004]   194.048534:
>>>> rpc_task_run_action:  task:1779@5 flags=3DASYNC
>>>> runstate=3DRUNNING|ACTIVE|NEED_XMIT|NEED_RECV status=3D-57
>>>> action=3Dcall_transmit_status
>>>>  kworker/u28:12-2191  [004]   194.048535:
>>>> rpc_task_run_action:  task:1779@5 flags=3DASYNC
>>>> runstate=3DRUNNING|ACTIVE|NEED_XMIT|NEED_RECV status=3D0
>>>> action=3Dcall_transmit
>>>>  kworker/u28:12-2191  [004]   194.048535:
>>>> rpc_task_sleep:       task:1779@5 flags=3DASYNC
>>>> runstate=3DRUNNING|ACTIVE|NEED_XMIT|NEED_RECV status=3D-11 =
timeout=3D0
>>>> queue=3Dxprt_sending
>>>>=20
>>>>=20
>>>>  kworker/u28:12-2191  [004]   194.048552: rpcgss_need_reencode:
>>>> task:1761@5 xid=3D0x88f4f47c rq_seqno=3D141220 seq_xmit=3D141336
>>>> reencode
>>>> unneeded
>>>>  kworker/u28:12-2191  [004]   194.048557:
>>>> xprt_transmit:        task:1761@5 xid=3D0x88f4f47c seqno=3D141220
>>>> status=3D0
>>>>=20
>>>>=20
>>>>  kworker/u28:12-2191  [004]   194.048559: rpcgss_need_reencode:
>>>> task:1902@5 xid=3D0x14f5f47c rq_seqno=3D141360 seq_xmit=3D141336
>>>> reencode
>>>> unneeded
>>>>  kworker/u28:12-2191  [004]   194.048563:
>>>> xprt_transmit:        task:1902@5 xid=3D0x14f5f47c seqno=3D141360
>>>> status=3D0
>>>>=20
>>>>=20
>>>> Note that first need_reencode: the sequence numbers show that the
>>>> xmit
>>>> queue has been significantly re-ordered. The request being
>>>> transmitted is
>>>> already very close to the lower end of the GSS sequence number
>>>> window.
>>>>=20
>>>> The server then re-ordereds these two slightly because the first
>>>> one
>>>> had
>>>> some Read chunks that need to be pulled over, the second was pure
>>>> inline
>>>> and therefore could be processed immediately. That is enough to
>>>> force
>>>> the
>>>> first one outside the GSS sequence number window.
>>>>=20
>>>> I haven't looked closely at the pathology of the TCP case.
>>>=20
>>> Wait a minute... That's not OK. The client can't be expected to
>>> take
>>> into account reordering that happens on the server side.
>>=20
>> If that's the case, then we would need to halt transmission as soon
>> as
>> we hit the RPCSEC_GSS window edge. Off the cuff, I'm not sure how to
>> do
>> that, since those windows are per session (i.e. per user).
>=20
> So here is something we probably could do: modify
> xprt_request_enqueue_transmit() to order the list in req->rq_xmit2 by
> req->rq_seqno.

Why not add " && !req->rq_seq_no " to the third arm? Calls are already
enqueued in sequence number order.


> Since task->tk_owner is actually a pid, then that's not
> a perfect solution, but we could further mitigate by modifying
> gss_xmit_need_reencode() to only allow transmission of requests that
> are within 2/3 of the window.
> --=20
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com

--
Chuck Lever