Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?

[Alex Züpke]

Hi Peter,

Am 12.06.2015 um 20:03 schrieb Peter Maydell:
> On 12 June 2015 at 17:38, Alex Züpke <address@hidden> wrote:
>> Hi,
>>
>> I'm benchmarking some IPI (== inter-processor-interrupt) synchronization 
>> stuff of my custom kernel on QEMU ARM (qemu-system-arm -M vexpress-a15 -smp 
>> 2) and ran into the following problem: pending IPIs are delayed until the 
>> QEMU main loop receives an event (for example the timer interrupt expires or 
>> I press a key on the console).
>>
>> The following timing diagram tries to show this:
>>
>>   CPU #0                       CPU #1
>>   ======                       ======
>>   ... other stuff ...          WFI (wait for interrupt, like x86 "HLT")
>>   send SGI in MPCore
>>   polls for completeness
>>                  <time passes ...>
>>   polls ...
>>                  <... and passes ...>
>>   still polls ...
>>                  <... and passes ...>
>>   still polls ...
>>                  <... and passes ...>
>>
>>
>>                  <timer interrupt expires>
>>                  <now QEMU switches to CPU #1>
>>                                receives IPI
>>                                signals completeness
>>                                WFI
>>                  <QEMU switches to CPU #0>
>>   polling done
>>   process timer interrupt
>>   ...
> 
> Right. The problem is that we don't have any way of telling
> that CPU 0 is just sat busy waiting for CPU 1.
> 
>> It works as expects (I get thousands of IPIs per second now), but
>> it does not "feel right", so is there a better way to improve the
>> responsiveness of IPI handling in QEMU?
> 
> Probably the best approach would be to have something in
> arm_cpu_set_irq() which says "if we are CPU X and we've
> just caused an interrupt to be set for CPU Y, then we
> should ourselves yield back to the main loop".
> 
> Something like this, maybe, though I have done no more testing
> than checking it doesn't actively break kernel booting :-)


Thanks! One more check for "level" is needed to get it work:

--- a/target-arm/cpu.c
+++ b/target-arm/cpu.c
@@ -325,6 +325,18 @@ static void arm_cpu_set_irq(void *opaque, int irq, int 
level)
     default:
         hw_error("arm_cpu_set_irq: Bad interrupt line %d\n", irq);
     }
+
+    /* If we are currently executing code for CPU X, and this
+     * CPU we've just triggered an interrupt on is CPU Y, then
+     * make CPU X yield control back to the main loop at the
+     * end of the TB it's currently executing.
+     * This avoids problems where the interrupt was an IPI
+     * and CPU X would otherwise sit busy looping for the rest
+     * of its timeslice because Y hasn't had a chance to run.
+     */
+    if (level && current_cpu && current_cpu != cs) {
+        cpu_exit(current_cpu);
+    }
 }
 
 static void arm_cpu_kvm_set_irq(void *opaque, int irq, int level)


Do you need a testcase for this?



Best regards
Alex