Commit ac4ad5c0 authored by Liao Chang's avatar Liao Chang Committed by Catalin Marinas
Browse files

arm64: insn: Simulate nop instruction for better uprobe performance 

v2->v1:
1. Remove the simuation of STP and the related bits.
2. Use arm64_skip_faulting_instruction for single-stepping or FEAT_BTI
   scenario.

As Andrii pointed out, the uprobe/uretprobe selftest bench run into a
counterintuitive result that nop and push variants are much slower than
ret variant [0]. The root cause lies in the arch_probe_analyse_insn(),
which excludes 'nop' and 'stp' from the emulatable instructions list.
This force the kernel returns to userspace and execute them out-of-line,
then trapping back to kernel for running uprobe callback functions. This
leads to a significant performance overhead compared to 'ret' variant,
which is already emulated.

Typicall uprobe is installed on 'nop' for USDT and on function entry
which starts with the instrucion 'stp x29, x30, [sp, #imm]!' to push lr
and fp into stack regardless kernel or userspace binary. In order to
improve the performance of handling uprobe for common usecases. This
patch supports the emulation of Arm64 equvialents instructions of 'nop'
and 'push'. The benchmark results below indicates the performance gain
of emulation is obvious.

On Kunpeng916 (Hi1616), 4 NUMA nodes, 64 Arm64 cores@2.4GHz.

xol (1 cpus)
------------
uprobe-nop:  0.916 ± 0.001M/s (0.916M/prod)
uprobe-push: 0.908 ± 0.001M/s (0.908M/prod)
uprobe-ret:  1.855 ± 0.000M/s (1.855M/prod)
uretprobe-nop:  0.640 ± 0.000M/s (0.640M/prod)
uretprobe-push: 0.633 ± 0.001M/s (0.633M/prod)
uretprobe-ret:  0.978 ± 0.003M/s (0.978M/prod)

emulation (1 cpus)
-------------------
uprobe-nop:  1.862 ± 0.002M/s  (1.862M/prod)
uprobe-push: 1.743 ± 0.006M/s  (1.743M/prod)
uprobe-ret:  1.840 ± 0.001M/s  (1.840M/prod)
uretprobe-nop:  0.964 ± 0.004M/s  (0.964M/prod)
uretprobe-push: 0.936 ± 0.004M/s  (0.936M/prod)
uretprobe-ret:  0.940 ± 0.001M/s  (0.940M/prod)

As shown above, the performance gap between 'nop/push' and 'ret'
variants has been significantly reduced. Due to the emulation of 'push'
instruction needs to access userspace memory, it spent more cycles than
the other.

As Mark suggested [1], it is painful to emulate the correct atomicity
and ordering properties of STP, especially when it interacts with MTE,
POE, etc. So this patch just focus on the simuation of 'nop'. The
simluation of STP and related changes will be addressed in a separate
patch.

[0] https://lore.kernel.org/all/CAEf4BzaO4eG6hr2hzXYpn+7Uer4chS0R99zLn02ezZ5YruVuQw@mail.gmail.com/
[1] https://lore.kernel.org/all/Zr3RN4zxF5XPgjEB@J2N7QTR9R3/



CC: Andrii Nakryiko <andrii.nakryiko@gmail.com>
CC: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: default avatarLiao Chang <liaochang1@huawei.com>
Acked-by: default avatarMark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20240909071114.1150053-1-liaochang1@huawei.com


[catalin.marinas@arm.com: small tweaks following MarkR's comments]
Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
parent 14762109

arch/arm64/include/asm/insn.h

+5 −0
Original line number Diff line number Diff line
@@ -575,6 +575,11 @@ static __always_inline u32 aarch64_insn_gen_nop(void) 
	return aarch64_insn_gen_hint(AARCH64_INSN_HINT_NOP);

}



static __always_inline bool aarch64_insn_is_nop(u32 insn)

{

	return insn == aarch64_insn_gen_nop();

}



u32 aarch64_insn_gen_branch_reg(enum aarch64_insn_register reg,

				enum aarch64_insn_branch_type type);

u32 aarch64_insn_gen_load_store_reg(enum aarch64_insn_register reg,

arch/arm64/kernel/probes/decode-insn.c

+9 −0
Original line number Diff line number Diff line
@@ -75,6 +75,15 @@ static bool __kprobes aarch64_insn_is_steppable(u32 insn) 
enum probe_insn __kprobes

arm_probe_decode_insn(u32 insn, struct arch_probe_insn *api)

{

	/*

	 * While 'nop' instruction can execute in the out-of-line slot,

	 * simulating them in breakpoint handling offers better performance.

	 */

	if (aarch64_insn_is_nop(insn)) {

		api->handler = simulate_nop;

		return INSN_GOOD_NO_SLOT;

	}



	/*

	 * Instructions reading or modifying the PC won't work from the XOL

	 * slot.

arch/arm64/kernel/probes/simulate-insn.c

+6 −0
Original line number Diff line number Diff line
@@ -196,3 +196,9 @@ simulate_ldrsw_literal(u32 opcode, long addr, struct pt_regs *regs) 


	instruction_pointer_set(regs, instruction_pointer(regs) + 4);

}



void __kprobes

simulate_nop(u32 opcode, long addr, struct pt_regs *regs)

{

	arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);

}

arch/arm64/kernel/probes/simulate-insn.h

+1 −0
Original line number Diff line number Diff line
@@ -16,5 +16,6 @@ void simulate_cbz_cbnz(u32 opcode, long addr, struct pt_regs *regs); 
void simulate_tbz_tbnz(u32 opcode, long addr, struct pt_regs *regs);

void simulate_ldr_literal(u32 opcode, long addr, struct pt_regs *regs);

void simulate_ldrsw_literal(u32 opcode, long addr, struct pt_regs *regs);

void simulate_nop(u32 opcode, long addr, struct pt_regs *regs);



#endif /* _ARM_KERNEL_KPROBES_SIMULATE_INSN_H */