Re: Interrupt to CPU routing in HVM domains - again

[Steve Ofsthun <sofsthun@virtualiron.com>]

James Harper wrote:
> (Bart - I hope you don't mind me sending your email to the list)
> 
> Keir,
> 
> As per a recent discussion I modified the IRQ code in the Windows GPLPV
> drivers so that only the vcpu_info[0] structure is used, instead of
> vcpu_info[current_cpu] structure. As per Bart's email below though, this
> has caused him to experience performance issues.
> 
> Have I understood correctly that only cpu 0 of the vcpu_info[] array is
> ever used even if the interrupt actually occurs on another vcpu? Is this
> true for all versions of Xen? It seems that Bart's experience is exactly
> the opposite of mine - the change that fixed up the performance issues
> for me caused performance issues for him...

While the event channel delivery code "binds" HVM event channel interrupts to VCPU0, the interrupt is delivered via the emulated IOAPIC.  The guest OS may program this "hardware" to deliver the interrupt to other VCPUs.  For linux, this gets done by the irqbalance code among others.  Xen overrides this routing for the timer 0 interrupt path in vioapic.c under the #define IRQ0_SPECIAL_ROUTING.  We hacked our version of Xen to piggyback on this code to force all event channel interrupts for HVM guests to also avoid any guest rerouting:

#ifdef IRQ0_SPECIAL_ROUTING
    /* Force round-robin to pick VCPU 0 */
    if ( ((irq == hvm_isa_irq_to_gsi(0)) && pit_channel0_enabled()) ||
         is_hvm_callback_irq(vioapic, irq) )
        deliver_bitmask = (uint32_t)1;
#endif

This routing override provides a significant performance boost [or rather avoids the performance penalty] for SMP PV drivers up until the time that VCPU0 is saturated with interrupts.  You can probably achieve the same thing but forcing the guest OS to set it's interrupt affinity to VCPU0.  Under linux for example, you can disable the irqbalance code.

Steve

> 
> Bart: Can you have a look through the xen-devel list archives and have a
> read of a thread with a subject of "HVM windows - PCI IRQ firing on both
> CPU's", around the middle of last month? Let me know if you interpret
> that any differently to me...
> 
> Thanks
> 
> James
> 
> 
> 
>> -----Original Message-----
>> From: bart brooks [mailto:bart_brooks@hotmail.com]
>> Sent: Friday, 5 September 2008 01:19
>> To: James Harper
>> Subject: Performance - Update GPLPV drivers -0.9.11-pre12
>> Importance: High
>>
>> Hi James,
>>
>>
>>
>> We have tracked down the issue where performance has dropped off after
>> version 0.9.11-pre9 and still exists in version 0.9.11-pre12.
>>
>> Event channel interrupts for transmit are generated only on VCPU-0,
>> whereas for receive they are generated on all VCPUs in a round robin
>> fashion. Post 0.9.11-pre9 it is assumed that all the interrupts are
>> generated on VCPU-0, so the network interrupts generated on other
> VPCUs
>> are only processed if there is some activity going on VCPU-0 or an
>> outstanding DPC. This caused the packets to be processed out-of-order
> and
>> retransmissions. Retransmissions happened after a timeout (200ms) with
> no
>> activity during that time. Overall it bought down the bandwidth a lot
> with
>> huge gaps of no activity.
>>
>>
>>
>> Instead of assuming that everything is on CPU-0, the following change
> was
>> made in the xenpci driver in the file evtchn.c in the function
>> EvtChn_Interrupt()
>>
>> int cpu = KeGetCurrentProcessorNumber() & (MAX_VIRT_CPUS - 1);
>>
>> This is the same code found in version  0.9.11-pre9
>>
>>
>>
>> After this change, we are getting numbers comparable to 0.9.11-pre9 .
>>
>> Bart
>>
>>
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>>
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