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

[Yunhong Jiang <yunhong.jiang@linux.intel.com>]

On Tue, Oct 27, 2015 at 10:29:28AM +0100, Paolo Bonzini wrote:
> 
> 
> 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.

IIUC, the handler not only write1 1 to the eventfd, it also invoke the wait 
queue function, and the in kernel irqchip has different callback with the 
user space irqchip, am I right? But I should not state that in kernel 
irqchip has lower overhead since I have no data for it.

> 
> 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.

Thanks for pointing this out. Yes, we can't call spin_lock_irqsave on RT 
kernel. Will do this way on next patch. But not sure if it's overkill to use 
raw_spinlock there since the eventfd_signal is used by other caller also.

Thanks
--jyh


> 
> Paolo
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