Re: xhci_hcd intterrupt affinity in Dom0/DomU limited to single interrupt

[Justin Acker <ackerj67@yahoo.com>]

On Thu, 2015-09-03 at 09:04 -0600, Jan Beulich wrote:
> >>> On 03.09.15 at 14:04, <ackerj67@yahoo.com> wrote:
> >>>> On 02.09.15 at 19:17, <ackerj67@yahoo.com> wrote:
> >>      From: Jan Beulich <jbeulich@suse.com>
> >>  Sent: Wednesday, September 2, 2015 4:58 AM
> >>>>> Justin Acker <ackerj67@yahoo.com> 09/02/15 1:14 AM >>>
> >>> 00:14.0 USB controller: Intel Corporation 7 Series/C210 Series Chipset 
> > Family USB xHCI Host Controller (rev 04) (prog-if 30 [XHCI])
> >>>    Subsystem: Dell Device 053e
> >>>    Flags: bus master, medium devsel, latency 0, IRQ 78
> >>>    Memory at f7f20000 (64-bit, non-prefetchable) [size=64K]
> >>>    Capabilities: [70] Power Management version 2
> >>>    Capabilities: [80] MSI: Enable+ Count=1/8 Maskable- 64bit+
> >> 
> >> This shows that the driver could use up to 8 MSI IRQs, but chose to use just 
> > 
> >> one. If
> >> this is the same under Xen and the native kernel, the driver likely doesn't 
> >> know any
> >> better. If under native more interrupts are being used, there might be an 
> >> issue with
> >> Xen specific code in the kernel or hypervisor code. We'd need to see details 
> > 
> >> to be
> >> able to tell.
> >> 
> >> Please let me know what details I should provide. 
> 
> I'd like to emphasize what I said in my previous reply:

Apologies. The Webmail client doesn't support in-line replies or I
couldn't figure out how to turn these on. Hopefully this is better. 

> 
> > Please, first of all, get your reply style fixed. Just look at the above
> > and tell me how a reader should figure which parts of the text were
> > written by whom.
> >
> >[...]
> >
> >      I am still confused as to whether any device, or in this case xhci_hcd, 
> > can use more than one cpu at any given time. My     understanding based on 
> > David's response is that it cannot due to the event channel mapping. The 
> > device interrupt can be pinned     to a specific cpu by specifying the 
> > affinity. I was hoping there was a way to allow the driver's interrupt to be 
> > scheduled to     use more than 1 CPU at any given time. 
> 
> The problem is that you're mixing up two things: devices and
> interrupts. Any individual interrupt can only be serviced by a single
> CPU at a time, due to the way event channels get bound. Any
> individual device can have more than one interrupt (MSI or MSI-X),
> and then each of these interrupts can be serviced on different
> CPUs.
> 
> Jan
> 

Thanks for clarifying. To the original question, with respect to my
limited understanding of the event channels and interrupts, each
interrupt can be serviced on a different CPU using irqbalance or setting
the affinity manually, but the same interrupt cannot be serviced by more
than 1 CPU at a time? If so, is there a way around the 1:1 binding when
loading the Dom0 kernel - a flag or option to use the native interrupt
scheduling for some set of or all 8 CPUs that the device can schedule
interrupts on when not loading the Dom0? The xhci_hcd, as one example,
seems to perform better when it is able to have interrupts serviced by
multiple CPUs.