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LAPIC timer accounting fix

PATCH In review Ben Guthro 3 participants 3 messages Oct 24, 2007 – Oct 25, 2007

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Ben Guthro

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#0 lore raw

PATCH 1/1 LAPIC timer accounting fix

Offset emulated local APIC timer so it doesn't tick during guest's timer related processing. Otherwise, guests using the local APIC for process accounting can see long sequences of process ticks incorrectly charged to interrupt processing.

Signed-off-by Ben Guthro <[email protected]>
Signed-off-by Gary Grebus <[email protected]>
xen/arch/x86/hvm/vlapic.c +5 -0
diff -r 68521d709355 xen/arch/x86/hvm/vlapic.c
@@ -962,6 +962,11 @@ void vlapic_destroy(struct vcpu *v)
     free_domheap_page(vlapic->regs_page);
 }
 
+int is_lvtt_vector(struct vcpu *v, int vector)
+{
+    return (vector == vlapic_lvt_vector(vcpu_vlapic(v), APIC_LVTT));
+}
+
 int is_lvtt(struct vcpu *v, int vector)
 {
     return vcpu_vlapic(v)->pt.enabled &&
xen/arch/x86/hvm/vpt.c +7 -0
diff -r 68521d709355 xen/arch/x86/hvm/vpt.c
@@ -277,6 +277,13 @@ void create_periodic_time(
     pt->period_cycles = (u64)period * cpu_khz / 1000000L;
     pt->one_shot = one_shot;
     pt->scheduled = NOW() + period;
+    if (is_lvtt_vector(v, irq))
+    {
+        /* Try to offset local APIC ticks from timer ticks.  Otherwise, guests
+           using the local APIC for process accounting can see long sequences
+           of process ticks incorrectly charged to interrupt processing. */
+        pt->scheduled += (period >> 1);
+    }
     pt->cb = cb;
     pt->priv = data;
xen/include/asm-x86/hvm/vlapic.h +1 -0
diff -r 68521d709355 xen/include/asm-x86/hvm/vlapic.h
@@ -92,6 +92,7 @@ struct vlapic *apic_round_robin(
 
 int vlapic_match_logical_addr(struct vlapic *vlapic, uint8_t mda);
 
+int is_lvtt_vector(struct vcpu *v, int vector);
 int is_lvtt(struct vcpu *v, int vector);
 int is_lvtt_enabled(struct vcpu *v);

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#1 lore raw
Quoted 4 lines · Ben Guthro

Offset emulated local APIC timer so it doesn't tick during guest's timer related processing. Otherwise, guests using the local APIC for process accounting can see long sequences of process ticks incorrectly charged to interrupt processing.

Do they really get in sync like that? Seems a bit odd. Couldn't the same happen on real hardware? I wonder why it would be more likely in the virtualised case...

Perhaps just offsetting all periodic timers by some random fraction of the period would be a simpler patch?

-- Keir

#2 lore raw
Quoted 10 lines · Keir Fraser
Quoted 4 lines · Ben Guthro

Offset emulated local APIC timer so it doesn't tick during guest's timer related processing. Otherwise, guests using the local APIC for process accounting can see long sequences of process ticks incorrectly charged to interrupt processing.

Do they really get in sync like that? Seems a bit odd. Couldn't the same happen on real hardware? I wonder why it would be more likely in the virtualised case...

Seems like it could happen on real hardware as well. My guess was that real hardware timers would drift relative to each other, and make it less likely.

The failure was seen on a 2.4 kernel (RHEL3) so the default method for process accounting there might be poor anyway. There was a program that failed because it did a large amount of CPU-intensive work but then saw zero ticks charged to the process.

Quoted 2 lines

Perhaps just offsetting all periodic timers by some random fraction of the period would be a simpler patch?

My goal at the time was to move the local APIC timer as far out of sync as possible, but some random fraction might work as well.

/gary

Signature · 2 lines
Gary Grebus
Virtual Iron Software, Inc.