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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Feb 20, 2019 at 10:03:57PM +0000, Julien Grall wrote:
> 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.

FWIW, you can also mask the interrupt while waiting for the thread to
execute the interrupt handler. Ie:

1. Interrupt injected
2. Execute guest event channel callback
3. Scan for pending interrupts
4. Mask interrupt
5. Clear pending field
6. Queue threaded handler
7. Go to 3 until all interrupts are drained
[...]
8. Execute interrupt handler in thread
9. Unmask interrupt

That should prevent you from stacking interrupts?

Roger.

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