Does perf-annotate work correctly?

From: Du, Changbin
Date: Tue Sep 12 2017 - 06:16:53 EST


When a annotate a symbol, I find the annotated C source code doesn't match assembly code.
So I cannot determine which line of C code has much overhead withou gdb's help.

Here is a example result of function apic_has_interrupt_for_ppr() in kvm module.

â580 __clear_bit(KVM_APIC_PV_EOI_PENDING, &vcpu->arch.apic_attention); â
â581 } â
â â
â583 static int apic_has_interrupt_for_ppr(struct kvm_lapic *apic, u32 ppr) â
â584 { â
0.88 â30: cmpb $0x0,0x91(%rdi) â
2.54 â â je 63 â
0.20 â mov 0xa0(%rdi),%rcx â
â581 int highest_irr; â
â582 if (kvm_x86_ops->sync_pir_to_irr && apic->vcpu->arch.apicv_active) â
4.91 â mov $0xe0,%eax x â
1.46 â45: mov %eax,%edx x â
0.02 â sar $0x5,%edx x â
3.57 â shl $0x4,%edx x â
3.34 â movslq %edx,%rdx x â
1.25 â mov 0x200(%rcx,%rdx,1),%edx x â
42.44 â test %edx,%edx x â
0.01 â âââjne 88 x â
3.48 â â sub $0x20,%eax x â
2.24 â â cmp $0xffffffe0,%eax x â
â586âapic_find_highest_irr(): â
â â â
â407â /* â
â408â * Note that irr_pending is just a hint. It will be always â
â409â * true with virtual interrupt delivery enabled. â
â410â */ â
â411â if (!apic->irr_pending) â
â ââ jne 45 â
0.62 â63:â mov $0xffffffff,%eax â
0.83 â â leaveq â
13.52 â ââ retq â
â6a:â mov %esi,-0x4(%rbp) â
â â mov %rdx,%rdi â
â418âfind_highest_vector(): â
â340âstatic int find_highest_vector(void *bitmap) â
â341â{ â
â342â int vec; â
â343â u32 *reg; â
â â â
â345â for (vec = MAX_APIC_VECTOR - APIC_VECTORS_PER_REG; â
â ââ callq *%rax â
â â mov -0x4(%rbp),%esi â
â343â vec >= 0; vec -= APIC_VECTORS_PER_REG) { â
â344â reg = bitmap + REG_POS(vec); â
â345â if (*reg) â
0.05 â75:â cmp $0xffffffff,%eax â
â ââ je 63 â
1.95 â â mov %eax,%edx â
1.45 â â and $0xf0,%edx


Look at the assembly code block where I have put a 'x' on the right. Apparently the
assembly code doesn't match the C source code arrounded. Let's look the correct disassemble
result from gdb:

340 for (vec = MAX_APIC_VECTOR - APIC_VECTORS_PER_REG;
0x000000000003b4e0 <+64>: mov $0xe0,%eax

342 reg = bitmap + REG_POS(vec);
343 if (*reg)
0x000000000003b4e5 <+69>: mov %eax,%edx
0x000000000003b4e7 <+71>: sar $0x5,%edx
0x000000000003b4ea <+74>: shl $0x4,%edx
0x000000000003b4ed <+77>: movslq %edx,%rdx
0x000000000003b4f0 <+80>: mov 0x200(%rcx,%rdx,1),%edx
0x000000000003b4f7 <+87>: test %edx,%edx
0x000000000003b4f9 <+89>: jne 0x3b528 <apic_has_interrupt_for_ppr+136>

341 vec >= 0; vec -= APIC_VECTORS_PER_REG) {
0x000000000003b4fb <+91>: sub $0x20,%eax

340 for (vec = MAX_APIC_VECTOR - APIC_VECTORS_PER_REG;
0x000000000003b4fe <+94>: cmp $0xffffffe0,%eax
0x000000000003b501 <+97>: jne 0x3b4e5 <apic_has_interrupt_for_ppr+69>


Compared to gdb, perf-annoate has messed up. is it a bug or just perf is not as perfect as gdb?

--
Thanks,
Changbin Du

Attachment: signature.asc
Description: PGP signature