[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] xen/evtchn and forced threaded irq


  • To: Julien Grall <julien.grall@xxxxxxx>
  • From: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
  • Date: Wed, 20 Feb 2019 16:46:30 -0500
  • Autocrypt: addr=boris.ostrovsky@xxxxxxxxxx; prefer-encrypt=mutual; keydata= mQINBFH8CgsBEAC0KiOi9siOvlXatK2xX99e/J3OvApoYWjieVQ9232Eb7GzCWrItCzP8FUV PQg8rMsSd0OzIvvjbEAvaWLlbs8wa3MtVLysHY/DfqRK9Zvr/RgrsYC6ukOB7igy2PGqZd+M MDnSmVzik0sPvB6xPV7QyFsykEgpnHbvdZAUy/vyys8xgT0PVYR5hyvhyf6VIfGuvqIsvJw5 C8+P71CHI+U/IhsKrLrsiYHpAhQkw+Zvyeml6XSi5w4LXDbF+3oholKYCkPwxmGdK8MUIdkM d7iYdKqiP4W6FKQou/lC3jvOceGupEoDV9botSWEIIlKdtm6C4GfL45RD8V4B9iy24JHPlom woVWc0xBZboQguhauQqrBFooHO3roEeM1pxXjLUbDtH4t3SAI3gt4dpSyT3EvzhyNQVVIxj2 FXnIChrYxR6S0ijSqUKO0cAduenhBrpYbz9qFcB/GyxD+ZWY7OgQKHUZMWapx5bHGQ8bUZz2 SfjZwK+GETGhfkvNMf6zXbZkDq4kKB/ywaKvVPodS1Poa44+B9sxbUp1jMfFtlOJ3AYB0WDS Op3d7F2ry20CIf1Ifh0nIxkQPkTX7aX5rI92oZeu5u038dHUu/dO2EcuCjl1eDMGm5PLHDSP 0QUw5xzk1Y8MG1JQ56PtqReO33inBXG63yTIikJmUXFTw6lLJwARAQABtDNCb3JpcyBPc3Ry b3Zza3kgKFdvcmspIDxib3Jpcy5vc3Ryb3Zza3lAb3JhY2xlLmNvbT6JAjgEEwECACIFAlH8 CgsCGwMGCwkIBwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEIredpCGysGyasEP/j5xApopUf4g 9Fl3UxZuBx+oduuw3JHqgbGZ2siA3EA4bKwtKq8eT7ekpApn4c0HA8TWTDtgZtLSV5IdH+9z JimBDrhLkDI3Zsx2CafL4pMJvpUavhc5mEU8myp4dWCuIylHiWG65agvUeFZYK4P33fGqoaS VGx3tsQIAr7MsQxilMfRiTEoYH0WWthhE0YVQzV6kx4wj4yLGYPPBtFqnrapKKC8yFTpgjaK jImqWhU9CSUAXdNEs/oKVR1XlkDpMCFDl88vKAuJwugnixjbPFTVPyoC7+4Bm/FnL3iwlJVE qIGQRspt09r+datFzPqSbp5Fo/9m4JSvgtPp2X2+gIGgLPWp2ft1NXHHVWP19sPgEsEJXSr9 tskM8ScxEkqAUuDs6+x/ISX8wa5Pvmo65drN+JWA8EqKOHQG6LUsUdJolFM2i4Z0k40BnFU/ kjTARjrXW94LwokVy4x+ZYgImrnKWeKac6fMfMwH2aKpCQLlVxdO4qvJkv92SzZz4538az1T m+3ekJAimou89cXwXHCFb5WqJcyjDfdQF857vTn1z4qu7udYCuuV/4xDEhslUq1+GcNDjAhB nNYPzD+SvhWEsrjuXv+fDONdJtmLUpKs4Jtak3smGGhZsqpcNv8nQzUGDQZjuCSmDqW8vn2o hWwveNeRTkxh+2x1Qb3GT46uuQINBFH8CgsBEADGC/yx5ctcLQlB9hbq7KNqCDyZNoYu1HAB Hal3MuxPfoGKObEktawQPQaSTB5vNlDxKihezLnlT/PKjcXC2R1OjSDinlu5XNGc6mnky03q yymUPyiMtWhBBftezTRxWRslPaFWlg/h/Y1iDuOcklhpr7K1h1jRPCrf1yIoxbIpDbffnuyz kuto4AahRvBU4Js4sU7f/btU+h+e0AcLVzIhTVPIz7PM+Gk2LNzZ3/on4dnEc/qd+ZZFlOQ4 KDN/hPqlwA/YJsKzAPX51L6Vv344pqTm6Z0f9M7YALB/11FO2nBB7zw7HAUYqJeHutCwxm7i BDNt0g9fhviNcJzagqJ1R7aPjtjBoYvKkbwNu5sWDpQ4idnsnck4YT6ctzN4I+6lfkU8zMzC gM2R4qqUXmxFIS4Bee+gnJi0Pc3KcBYBZsDK44FtM//5Cp9DrxRQOh19kNHBlxkmEb8kL/pw XIDcEq8MXzPBbxwHKJ3QRWRe5jPNpf8HCjnZz0XyJV0/4M1JvOua7IZftOttQ6KnM4m6WNIZ 2ydg7dBhDa6iv1oKdL7wdp/rCulVWn8R7+3cRK95SnWiJ0qKDlMbIN8oGMhHdin8cSRYdmHK kTnvSGJNlkis5a+048o0C6jI3LozQYD/W9wq7MvgChgVQw1iEOB4u/3FXDEGulRVko6xCBU4 SQARAQABiQIfBBgBAgAJBQJR/AoLAhsMAAoJEIredpCGysGyfvMQAIywR6jTqix6/fL0Ip8G jpt3uk//QNxGJE3ZkUNLX6N786vnEJvc1beCu6EwqD1ezG9fJKMl7F3SEgpYaiKEcHfoKGdh 30B3Hsq44vOoxR6zxw2B/giADjhmWTP5tWQ9548N4VhIZMYQMQCkdqaueSL+8asp8tBNP+TJ PAIIANYvJaD8xA7sYUXGTzOXDh2THWSvmEWWmzok8er/u6ZKdS1YmZkUy8cfzrll/9hiGCTj u3qcaOM6i/m4hqtvsI1cOORMVwjJF4+IkC5ZBoeRs/xW5zIBdSUoC8L+OCyj5JETWTt40+lu qoqAF/AEGsNZTrwHJYu9rbHH260C0KYCNqmxDdcROUqIzJdzDKOrDmebkEVnxVeLJBIhYZUd t3Iq9hdjpU50TA6sQ3mZxzBdfRgg+vaj2DsJqI5Xla9QGKD+xNT6v14cZuIMZzO7w0DoojM4 ByrabFsOQxGvE0w9Dch2BDSI2Xyk1zjPKxG1VNBQVx3flH37QDWpL2zlJikW29Ws86PHdthh Fm5PY8YtX576DchSP6qJC57/eAAe/9ztZdVAdesQwGb9hZHJc75B+VNm4xrh/PJO6c1THqdQ 19WVJ+7rDx3PhVncGlbAOiiiE3NOFPJ1OQYxPKtpBUukAlOTnkKE6QcA4zckFepUkfmBV1wM Jg6OxFYd01z+a+oL
  • Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Dave P Martin <dave.martin@xxxxxxx>
  • Delivery-date: Wed, 20 Feb 2019 21:47:57 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 2/20/19 3:46 PM, Julien Grall wrote:
> (+ Andrew and Jan for feedback on the event channel interrupt)
>
> Hi Boris,
>
> Thank you for the your feedback.
>
> On 2/20/19 8:04 PM, Boris Ostrovsky wrote:
>> On 2/20/19 1:05 PM, Julien Grall wrote:
>>> Hi,
>>>
>>> On 20/02/2019 17:07, Boris Ostrovsky wrote:
>>>> On 2/20/19 9:15 AM, Julien Grall wrote:
>>>>> Hi Boris,
>>>>>
>>>>> Thank you for your answer.
>>>>>
>>>>> On 20/02/2019 00:02, Boris Ostrovsky wrote:
>>>>>> On Tue, Feb 19, 2019 at 05:31:10PM +0000, Julien Grall wrote:
>>>>>>> Hi all,
>>>>>>>
>>>>>>> I have been looking at using Linux RT in Dom0. Once the guest is
>>>>>>> started,
>>>>>>> the console is ending to have a lot of warning (see trace below).
>>>>>>>
>>>>>>> After some investigation, this is because the irq handler will now
>>>>>>> be threaded.
>>>>>>> I can reproduce the same error with the vanilla Linux when passing
>>>>>>> the option
>>>>>>> 'threadirqs' on the command line (the trace below is from 5.0.0-rc7
>>>>>>> that has
>>>>>>> not RT support).
>>>>>>>
>>>>>>> FWIW, the interrupt for port 6 is used to for the guest to
>>>>>>> communicate with
>>>>>>> xenstore.
>>>>>>>
>>>>>>>    From my understanding, this is happening because the interrupt
>>>>>>> handler is now
>>>>>>> run in a thread. So we can have the following happening.
>>>>>>>
>>>>>>>       Interrupt context            |     Interrupt thread
>>>>>>>                                    |
>>>>>>>       receive interrupt port 6     |
>>>>>>>       clear the evtchn port        |
>>>>>>>       set IRQF_RUNTHREAD            |
>>>>>>>       kick interrupt thread        |
>>>>>>>                                    |    clear IRQF_RUNTHREAD
>>>>>>>                                    |    call evtchn_interrupt
>>>>>>>       receive interrupt port 6     |
>>>>>>>       clear the evtchn port        |
>>>>>>>       set IRQF_RUNTHREAD           |
>>>>>>>       kick interrupt thread        |
>>>>>>>                                    |    disable interrupt port 6
>>>>>>>                                    |    evtchn->enabled = false
>>>>>>>                                    |    [....]
>>>>>>>                                    |
>>>>>>>                                    |    *** Handling the second
>>>>>>> interrupt ***
>>>>>>>                                    |    clear IRQF_RUNTHREAD
>>>>>>>                                    |    call evtchn_interrupt
>>>>>>>                                    |    WARN(...)
>>>>>>>
>>>>>>> I am not entirely sure how to fix this. I have two solutions in
>>>>>>> mind:
>>>>>>>
>>>>>>> 1) Prevent the interrupt handler to be threaded. We would also
>>>>>>> need to
>>>>>>> switch from spin_lock to raw_spin_lock as the former may sleep on
>>>>>>> RT-Linux.
>>>>>>>
>>>>>>> 2) Remove the warning
>>>>>>
>>>>>> I think access to evtchn->enabled is racy so (with or without the
>>>>>> warning) we can't use it reliably.
>>>>>
>>>>> Thinking about it, it would not be the only issue. The ring is sized
>>>>> to contain only one instance of the same event. So if you receive
>>>>> twice the event, you may overflow the ring.
>>>>
>>>> Hm... That's another argument in favor of "unthreading" the handler.
>>>
>>> I first thought it would be possible to unthread it. However,
>>> wake_up_interruptible is using a spin_lock. On RT spin_lock can sleep,
>>> so this cannot be used in an interrupt context.
>>>
>>> So I think "unthreading" the handler is not an option here.
>>
>> That sounds like a different problem. I.e. there are two issues:
>> * threaded interrupts don't work properly (races, ring overflow)
>> * evtchn_interrupt() (threaded or not) has spin_lock(), which is not
>> going to work for RT
>
> I am afraid that's not correct, you can use spin_lock() in threaded
> interrupt handler.

In non-RT handler -- yes, but not in an RT one (in fact, isn't this what
you yourself said above?)

>
>> The first can be fixed by using non-threaded handlers.
>
> The two are somewhat related, if you use a non-threaded handler then
> you are not going to help the RT case.
>
> In general, the unthreaded solution should be used in the last resort.
>
>>>
>>>>
>>>>>
>>>>>>
>>>>>> Another alternative could be to queue the irq if !evtchn->enabled
>>>>>> and
>>>>>> handle it in evtchn_write() (which is where irq is supposed to be
>>>>>> re-enabled).
>>>>> What do you mean by queue? Is it queueing in the ring?
>>>>
>>>>
>>>> No, I was thinking about having a new structure for deferred
>>>> interrupts.
>>>
>>> Hmmm, I am not entirely sure what would be the structure here. Could
>>> you expand your thinking?
>>
>> Some sort of a FIFO that stores {irq, data} tuple. It could obviously be
>> implemented as a ring but not necessarily as Xen shared ring (if that's
>> what you were referring to).
>
> The underlying question is what happen if you miss an interrupt. Is it
> going to be ok?

This I am not sure about. I thought yes since we are signaling the
process only once.

-boris

> If no, then we have to record everything and can't kill/send an error
> to the user app because it is not its fault.
>
> This means a FIFO would not be a viable. How do you size it? Static
> (i.e pre-defined) size is not going to be possible because you don't
> know how many interrupt you are going to receive before the thread
> handler runs. You can't make it grow dynamically as it make become
> quite big for the same reason.
>
> Discussing with my team, a solution that came up would be to introduce
> one atomic field per event to record the number of event received. I
> will explore that solution tomorrow.
>




_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel

 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.