From: Julien Grall <julien.grall@arm.com>
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>,
Stefano Stabellini <sstabellini@kernel.org>,
Andrew Cooper <Andrew.Cooper3@citrix.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Jan Beulich <JBeulich@suse.com>,
xen-devel <xen-devel@lists.xenproject.org>,
Dave P Martin <dave.martin@arm.com>
Subject: Re: xen/evtchn and forced threaded irq
Date: Wed, 20 Feb 2019 22:03:57 +0000 [thread overview]
Message-ID: <1100e6b1-6fa8-06f0-8ecc-b0699a2ce5f4__29857.0235072233$1550700330$gmane$org@arm.com> (raw)
In-Reply-To: <1574a7fe-a5ac-4bc2-d3f0-967d8d01e5f1@oracle.com>
Hi Boris,
On 2/20/19 9:46 PM, Boris Ostrovsky wrote:
> 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?)
In RT-linux, interrupt handlers are threaded by default. So the handler
will not run in the interrupt context. Hence, it will be safe to call
spin_lock.
However, if you force the handler to not be threaded (IRQF_NO_THREAD),
it will run in interrupt context.
>>>>
>>>>>
>>>>>>
>>>>>>>
>>>>>>> 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.
I have CCed Andrew and Jan to see if they can help here.
Cheers,
--
Julien Grall
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next prev parent reply other threads:[~2019-02-20 22:04 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <5e256d9a-572c-e01e-7706-407f99245b00@arm.com>
2019-02-20 0:02 ` xen/evtchn and forced threaded irq Boris Ostrovsky
[not found] ` <20190220000209.GA4091@localhost.localdomain>
2019-02-20 14:15 ` Julien Grall
[not found] ` <a872d480-9f1b-6cd7-e507-ac4fcdf705af@arm.com>
2019-02-20 17:07 ` Boris Ostrovsky
[not found] ` <21214d47-9c68-e6bf-007a-4047cc2b02f9@oracle.com>
2019-02-20 18:05 ` Julien Grall
[not found] ` <8f7445d7-fa50-f3e9-44f5-cc2aebd020f4@arm.com>
2019-02-20 20:04 ` Boris Ostrovsky
[not found] ` <15bc52cb-82d8-4d2c-e5a8-c6ae8de90276@oracle.com>
2019-02-20 20:46 ` Julien Grall
[not found] ` <5df8888a-4a29-fccd-bac2-a9c170244029@arm.com>
2019-02-20 21:46 ` Boris Ostrovsky
[not found] ` <1574a7fe-a5ac-4bc2-d3f0-967d8d01e5f1@oracle.com>
2019-02-20 22:03 ` Julien Grall [this message]
[not found] ` <1100e6b1-6fa8-06f0-8ecc-b0699a2ce5f4@arm.com>
2019-02-21 8:07 ` Roger Pau Monné
[not found] ` <20190221080752.zy2qlzb4vi7tbu3p@Air-de-Roger>
2019-02-21 8:38 ` Julien Grall
2019-02-21 8:52 ` Juergen Gross
2019-02-21 9:14 ` Roger Pau Monné
[not found] ` <20190221091431.vqi53op37mvhi25z@Air-de-Roger>
2019-02-21 20:46 ` Julien Grall
2020-04-27 23:20 ` [Xen-devel] " Stefano Stabellini
2019-02-22 11:44 ` Jan Beulich
2019-02-22 12:38 ` Oleksandr Andrushchenko
[not found] ` <f7e78bf6-6e65-6d6c-c1ad-dca7f1e66b17@gmail.com>
2019-02-22 13:33 ` Julien Grall
[not found] ` <fe44917f-bbe9-3e0e-fdda-2eb4db9f25c2@arm.com>
2019-02-25 13:24 ` Oleksandr Andrushchenko
[not found] ` <13a9616c-2d9a-f90b-3358-f2dcadbbb64d@gmail.com>
2019-02-25 13:55 ` Julien Grall
[not found] ` <dbfd87e9-48fc-f641-9e24-ddb6c4f61135@arm.com>
2019-02-25 14:08 ` Oleksandr Andrushchenko
[not found] ` <1aeda04d-3420-fa50-ad33-a0b3e981f5e4@gmail.com>
2019-02-25 15:26 ` Julien Grall
2019-02-26 9:14 ` Roger Pau Monné
[not found] ` <20190226091420.klgldhotiecezw6h@Air-de-Roger>
2019-02-26 9:30 ` Andrew Cooper
[not found] ` <038b837c-63c0-afb7-ca7b-75f61af7518e@citrix.com>
2019-02-26 9:44 ` Roger Pau Monné
2019-02-26 9:45 ` Paul Durrant
[not found] ` <20190226094459.33y2ygrjei3sf3gk@Air-de-Roger>
2019-02-26 10:03 ` Julien Grall
[not found] ` <21c331d5-0cfa-6f7e-3db4-40b7ece45bc8@arm.com>
2019-02-26 10:17 ` Roger Pau Monné
[not found] ` <20190226101721.kh5vbrqdlnrtvhwh@Air-de-Roger>
2019-02-26 10:26 ` Julien Grall
[not found] ` <cdd5781c-3a60-b9f7-205f-fadeee88206e@arm.com>
2019-02-26 11:02 ` Roger Pau Monné
[not found] ` <20190226110231.46luhevhlmefdldo@Air-de-Roger>
2019-02-27 11:09 ` Julien Grall
2019-02-21 8:17 ` Juergen Gross
2019-02-19 17:31 Julien Grall
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