Hi Peter,
Am 12.06.2015 um 20:03 schrieb Peter Maydell:
> On 12 June 2015 at 17:38, Alex Züpke <[email protected]> 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
