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[Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop eve
From: |
Alex Züpke |
Subject: |
[Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency? |
Date: |
Fri, 12 Jun 2015 18:38:57 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 |
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
...
My timer is setup to generate an interrupt once a second, so I only get 1 IPI
interrupt per second on QEMU. When I run the test on real hardware (i.MX6Q), I
get millions of IPIs instead.
I tried to "fix" this by forcing QEMU back into the main loop and added a call
to qemu_notify_event() in the IPI-sending path of the ARM interrupt controller:
diff --git a/hw/intc/arm_gic.c b/hw/intc/arm_gic.c
index c1d2e70..20dba75 100644
--- a/hw/intc/arm_gic.c
+++ b/hw/intc/arm_gic.c
@@ -21,6 +21,7 @@
#include "hw/sysbus.h"
#include "gic_internal.h"
#include "qom/cpu.h"
+#include "qemu/main-loop.h"
//#define DEBUG_GIC
@@ -898,6 +899,7 @@ static void gic_dist_writel(void *opaque, hwaddr offset,
target_cpu = ctz32(mask);
}
gic_update(s);
+ qemu_notify_event();
return;
}
gic_dist_writew(opaque, offset, value & 0xffff, attrs);
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?
Best regards
Alex
- [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?,
Alex Züpke <=
- Re: [Qemu-devel] QEMU ARM SMP: IPI delivery delayed until next main loop event // how to improve IPI latency?, Peter Maydell, 2015/06/12
- 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