Re: xen/evtchn and forced threaded irq

From: Julien Grall
Date: Wed Feb 20 2019 - 13:05:28 EST


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.

ÂFrom my understanding, this is happening because the interrupt
handler is now
run in a thread. So we can have the following happening.

ÂÂÂ Interrupt contextÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂÂ Interrupt thread
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |
ÂÂÂ receive interrupt port 6ÂÂÂÂ |
ÂÂÂ clear the evtchn portÂÂÂÂÂÂÂ |
ÂÂÂ set IRQF_RUNTHREADÂÂÂÂÂÂÂÂÂÂÂ |
ÂÂÂ kick interrupt threadÂÂÂÂÂÂÂ |
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂ clear IRQF_RUNTHREAD
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂ call evtchn_interrupt
ÂÂÂ receive interrupt port 6ÂÂÂÂ |
ÂÂÂ clear the evtchn portÂÂÂÂÂÂÂ |
ÂÂÂ set IRQF_RUNTHREADÂÂÂÂÂÂÂÂÂÂ |
ÂÂÂ kick interrupt threadÂÂÂÂÂÂÂ |
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂ disable interrupt port 6
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂ evtchn->enabled = false
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂ [....]
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂ *** Handling the second
interrupt ***
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂ clear IRQF_RUNTHREAD
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂ call evtchn_interrupt
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂ 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.




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?

Cheers,

--
Julien Grall