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Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop
From: |
Peter Maydell |
Subject: |
Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency? |
Date: |
Fri, 12 Jun 2015 19:03:37 +0100 |
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 :-)
--- 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 (current_cpu && current_cpu != cs) {
+ cpu_exit(current_cpu);
+ }
}
static void arm_cpu_kvm_set_irq(void *opaque, int irq, int level)
-- PMM
- [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Alex Züpke, 2015/06/12
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?,
Peter Maydell <=
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Alex Züpke, 2015/06/15
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Peter Maydell, 2015/06/15
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Alex Züpke, 2015/06/15
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Peter Maydell, 2015/06/15
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Peter Maydell, 2015/06/15
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Alex Zuepke, 2015/06/15
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Peter Maydell, 2015/06/16
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Peter Maydell, 2015/06/16
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Alex Züpke, 2015/06/16
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Peter Maydell, 2015/06/16