Re: [RFC PATCH] VFIO: Add a parameter to force nonthread IRQ

[Paolo Bonzini <pbonzini@redhat.com>]

On 27/10/2015 07:35, Yunhong Jiang wrote:
> On Mon, Oct 26, 2015 at 09:37:14PM -0600, Alex Williamson wrote:
>> On Mon, 2015-10-26 at 18:20 -0700, Yunhong Jiang wrote:
>>> An option to force VFIO PCI MSI/MSI-X handler as non-threaded IRQ,
>>> even when CONFIG_IRQ_FORCED_THREADING=y. This is uselful when
>>> assigning a device to a guest with low latency requirement since it
>>> reduce the context switch to/from the IRQ thread.
>>
>> Is there any way we can do this automatically?  Perhaps detecting that
>> we're on a RT kernel or maybe that the user is running with RT priority?
>> I find that module options are mostly misunderstood and misused.
> 
> Alex, thanks for review.
> 
> It's not easy to detect if the user is running with RT priority, since 
> sometimes the user start the thread and then set the scheduler priority 
> late.
> 
> Also should we do this only for in kernel irqchip scenario and not for user 
> space handler, since in kernel irqchip has lower overhead?

The overhead of the non-threaded IRQ handler is the same for kernel or
userspace irqchip, since the handler just writes 1 to the eventfd.

On RT kernels however can you call eventfd_signal from interrupt
context?  You cannot call spin_lock_irqsave (which can sleep) from a
non-threaded interrupt handler, can you?  You would need a raw spin lock.

Paolo