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Date:   Mon, 20 Jun 2022 09:46:43 +0200
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Subject: Re: NFS regression between 5.17 and 5.18
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To:     Chuck Lever III <chuck.lever@oracle.com>,
        Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Cc:     Trond Myklebust <trondmy@hammerspace.com>,
        Olga Kornievskaia <aglo@umich.edu>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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From:   Thorsten Leemhuis <regressions@leemhuis.info>
In-Reply-To: <DA2DB426-6658-43CC-B331-C66B79BE8395@oracle.com>
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Dennis, Chuck, I have below issue on the list of tracked regressions.
What's the status? Has any progress been made? Or is this not really a
regression and can be ignored?

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)

P.S.: As the Linux kernel's regression tracker I deal with a lot of
reports and sometimes miss something important when writing mails like
this. If that's the case here, don't hesitate to tell me in a public
reply, it's in everyone's interest to set the public record straight.

#regzbot poke
##regzbot unlink: https://bugzilla.kernel.org/show_bug.cgi?id=215890

On 17.05.22 16:02, Chuck Lever III wrote:
>> On May 17, 2022, at 9:40 AM, Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com> wrote:
>>
>> On 5/13/22 10:59 AM, Chuck Lever III wrote:
>>>>>
>>>>> Ran a test with -rc6 and this time see a hung task trace on the
>>>>> console as well
>>>>> as an NFS RPC error.
>>>>>
>>>>> [32719.991175] nfs: RPC call returned error 512
>>>>> .
>>>>> .
>>>>> .
>>>>> [32933.285126] INFO: task kworker/u145:23:886141 blocked for more
>>>>> than 122 seconds.
>>>>> [32933.293543]       Tainted: G S                5.18.0-rc6 #1
>>>>> [32933.299869] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
>>>>> disables this
>>>>> message.
>>>>> [32933.308740] task:kworker/u145:23 state:D stack:    0 pid:886141
>>>>> ppid:     2
>>>>> flags:0x00004000
>>>>> [32933.318321] Workqueue: rpciod rpc_async_schedule [sunrpc]
>>>>> [32933.324524] Call Trace:
>>>>> [32933.327347]  <TASK>
>>>>> [32933.329785]  __schedule+0x3dd/0x970
>>>>> [32933.333783]  schedule+0x41/0xa0
>>>>> [32933.337388]  xprt_request_dequeue_xprt+0xd1/0x140 [sunrpc]
>>>>> [32933.343639]  ? prepare_to_wait+0xd0/0xd0
>>>>> [32933.348123]  ? rpc_destroy_wait_queue+0x10/0x10 [sunrpc]
>>>>> [32933.354183]  xprt_release+0x26/0x140 [sunrpc]
>>>>> [32933.359168]  ? rpc_destroy_wait_queue+0x10/0x10 [sunrpc]
>>>>> [32933.365225]  rpc_release_resources_task+0xe/0x50 [sunrpc]
>>>>> [32933.371381]  __rpc_execute+0x2c5/0x4e0 [sunrpc]
>>>>> [32933.376564]  ? __switch_to_asm+0x42/0x70
>>>>> [32933.381046]  ? finish_task_switch+0xb2/0x2c0
>>>>> [32933.385918]  rpc_async_schedule+0x29/0x40 [sunrpc]
>>>>> [32933.391391]  process_one_work+0x1c8/0x390
>>>>> [32933.395975]  worker_thread+0x30/0x360
>>>>> [32933.400162]  ? process_one_work+0x390/0x390
>>>>> [32933.404931]  kthread+0xd9/0x100
>>>>> [32933.408536]  ? kthread_complete_and_exit+0x20/0x20
>>>>> [32933.413984]  ret_from_fork+0x22/0x30
>>>>> [32933.418074]  </TASK>
>>>>>
>>>>> The call trace shows up again at 245, 368, and 491 seconds. Same
>>>>> task, same trace.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>> That's very helpful. The above trace suggests that the RDMA code is
>>>> leaking a call to xprt_unpin_rqst().
>>>
>>> IMHO this is unlikely to be related to the performance
>>> regression -- none of this code has changed in the past 5
>>> kernel releases. Could be a different issue, though.
>>>
>>> As is often the case in these situations, the INFO trace
>>> above happens long after the issue that caused the missing
>>> unpin. So... unless Dennis has a reproducer that can trigger
>>> the issue frequently, I don't think there's much that can
>>> be extracted from that.
>>
>> To be fair, I've only seen this one time and have had the performance regression
>> since -rc1.
>>
>>> Also "nfs: RPC call returned error 512" suggests someone
>>> hit ^C at some point. It's always possible that the
>>> xprt_rdma_free() path is missing an unpin. But again,
>>> that's not likely to be related to performance.
>>
>> I've checked our test code and after 10 minutes it does give up trying to do the
>> NFS copies and aborts (SIG_INT) the test.
> 
> After sleeping on it, I'm fairly certain the stack trace
> above is a result of a gap in how xprtrdma handles a
> signaled RPC.
> 
> Signal handling in that code is pretty hard to test, so not
> surprising that there's a lingering bug or two. One idea I
> had was to add a fault injector in the RPC scheduler to
> throw signals at random. I think it can be done without
> perturbing the hot path. Maybe I'll post an RFC patch.
> 
> 
>> So in all my tests and bisect attempts it seems the possibility to hit a slow
>> NFS operation that hangs for minutes has been possible for quite some time.
>> However in 5.18 it gets much worse.
>>
>> Any likely places I should add traces to try and find out what's stuck or taking
>> time?
> 
> There's been a lot of churn in that area in recent releases,
> so I'm not familiar with the existing tracepoints. Maybe
> Ben or Trond could provide some guidance.