[PATCH] arm64: insn: Simulate nop and push instruction for better uprobe performance
Andrii Nakryiko
andrii.nakryiko at gmail.com
Thu Sep 5 13:17:15 PDT 2024
On Fri, Aug 30, 2024 at 2:25 AM Liao, Chang <liaochang1 at huawei.com> wrote:
>
>
>
> 在 2024/8/30 3:26, Andrii Nakryiko 写道:
> > On Tue, Aug 27, 2024 at 4:34 AM Liao, Chang <liaochang1 at huawei.com> wrote:
> >>
> >> Hi, Mark
> >>
> >> Would you like to discuss this patch further, or do you still believe emulating
> >> STP to push FP/LR into the stack in kernel is not a good idea?
> >>
> >
> > Please send an updated version of your patches taking into account
> > various smaller issues Mark pointed out. But please keep STP
> > emulation, I think it's very important, even if it's not covering all
> > possible uprobe tracing scenarios.
>
> OK, I will send it out over the weekend, including an enhancement of STP
> emuation that addresses the MTE and POE issues Mark mentioned. I hope this
> will lead to more feedback from him.
>
Hey Liao,
Did you get a chance to prepare and send a new revision of this patch?
> >
> >> Thanks.
> >>
> >>
> >> 在 2024/8/21 15:55, Liao, Chang 写道:
> >>> Hi, Mark
> >>>
> >>> My bad for taking so long to rely, I generally agree with your suggestions to
> >>> STP emulation.
> >>>
> >>> 在 2024/8/15 17:58, Mark Rutland 写道:
> >>>> On Wed, Aug 14, 2024 at 08:03:56AM +0000, Liao Chang wrote:
> >>>>> 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.
> >>>>
> >>>> I appreciate this might be surprising, but does it actually matter
> >>>> outside of a microbenchmark?
> >>>
> >>> I just do a simple comparsion the performance impact of single-stepped and
> >>> emulated STP on my local machine. Three user cases were measured: Redis GET and
> >>> SET throughput (Request Per Second, RPS), and the time taken to execute a grep
> >>> command on the "arch_uprobe_copy_xol" string within the kernel source.
> >>>
> >>> Redis GET (higher is better)
> >>> ----------------------------
> >>> No uprobe: 49149.71 RPS
> >>> Single-stepped STP: 46750.82 RPS
> >>> Emulated STP: 48981.19 RPS
> >>>
> >>> Redis SET (larger is better)
> >>> ----------------------------
> >>> No uprobe: 49761.14 RPS
> >>> Single-stepped STP: 45255.01 RPS
> >>> Emulated stp: 48619.21 RPS
> >>>
> >>> Grep (lower is better)
> >>> ----------------------
> >>> No uprobe: 2.165s
> >>> Single-stepped STP: 15.314s
> >>> Emualted STP: 2.216s
> >>>
> >>> The result reveals single-stepped STP instruction that used to push fp/lr into
> >>> stack significantly impacts the Redis and grep performance, leading to a notable
> >>> notable decrease RPS and increase time individually. So emulating STP on the
> >>> function entry might be a more viable option for uprobe.
> >>>
> >>>>
> >>>>> 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.
> >>>>
> >>>> Function entry doesn't always start with a STP; these days it's often a
> >>>> BTI or PACIASP, and for non-leaf functions (or with shrink-wrapping in
> >>>> the compiler), it could be any arbitrary instruction. This might happen
> >>>> to be the common case today, but there are certain;y codebases where it
> >>>> is not.
> >>>
> >>> Sure, if kernel, CPU and compiler support BTI and PAC, the entry instruction
> >>> is definitly not STP. But for CPU and kernel lack of these supports, STP as
> >>> the entry instruction is still the common case. And I profiled the entry
> >>> instruction for all leaf and non-leaf function, the ratio of STP is 64.5%
> >>> for redis, 76.1% for the BPF selftest bench. So I am thinking it is still
> >>> useful to emulate the STP on the function entry. Perhaps, for CPU and kernel
> >>> with BTI and PAC enabled, uprobe chooses the slower single-stepping to execute
> >>> STP for pushing stack.
> >>>
> >>>>
> >>>> STP (or any instruction that accesses memory) is fairly painful to
> >>>> emulate because you need to ensure that the correct atomicity and
> >>>> ordering properties are provided (e.g. an aligned STP should be
> >>>> single-copy-atomic, but copy_to_user() doesn't guarantee that except by
> >>>> chance), and that the correct VMSA behaviour is provided (e.g. when
> >>>> interacting with MTE, POE, etc, while the uaccess primitives don't try
> >>>> to be 100% equivalent to instructions in userspace).
> >>> Agreed, but I don't think it has to emulate strictly the single-copy-atomic
> >>> feature of STP that is used to push fp/lr into stack. In most cases, only one
> >>> CPU will push registers to the same position on stack. And I barely understand
> >>> why other CPUs would depends on the ordering of pushing data into stack. So it
> >>> means the atomicity and ordering is not so important for this scenario. Regarding
> >>> MTE and POE, a similar stragety to BTI and PAC can be applied: for CPUs and kernel
> >>> with MTE and POE enabled, uprobe chooses the slower single-stepping to execute
> >>> STP for pushing stack.
> >>>
> >>>>
> >>>> For those reasons, in general I don't think we should be emulating any
> >>>> instruction which accesses memory, and we should not try to emulate the
> >>>> STP, but I think it's entirely reasonable to emulate NOP.
> >>>>
> >>>>> 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 at 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.
> >>>>>
> >>>>> [0] https://lore.kernel.org/all/CAEf4BzaO4eG6hr2hzXYpn+7Uer4chS0R99zLn02ezZ5YruVuQw@mail.gmail.com/
> >>>>>
> >>>>> Signed-off-by: Liao Chang <liaochang1 at huawei.com>
> >>>>> ---
> >>>>> arch/arm64/include/asm/insn.h | 21 ++++++++++++++++++
> >>>>> arch/arm64/kernel/probes/decode-insn.c | 18 +++++++++++++--
> >>>>> arch/arm64/kernel/probes/decode-insn.h | 3 ++-
> >>>>> arch/arm64/kernel/probes/simulate-insn.c | 28 ++++++++++++++++++++++++
> >>>>> arch/arm64/kernel/probes/simulate-insn.h | 2 ++
> >>>>> arch/arm64/kernel/probes/uprobes.c | 2 +-
> >>>>> 6 files changed, 70 insertions(+), 4 deletions(-)
> >>>>>
> >>>>> diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h
> >>>>> index 8c0a36f72d6f..a246e6e550ba 100644
> >>>>> --- a/arch/arm64/include/asm/insn.h
> >>>>> +++ b/arch/arm64/include/asm/insn.h
> >>>>> @@ -549,6 +549,27 @@ static __always_inline bool aarch64_insn_uses_literal(u32 insn)
> >>>>> aarch64_insn_is_prfm_lit(insn);
> >>>>> }
> >>>>>
> >>>>> +static __always_inline bool aarch64_insn_is_nop(u32 insn)
> >>>>> +{
> >>>>> + /* nop */
> >>>>> + return aarch64_insn_is_hint(insn) &&
> >>>>> + ((insn & 0xFE0) == AARCH64_INSN_HINT_NOP);
> >>>>> +}
> >>>>
> >>>> This looks fine, but the comment can go.
> >>>
> >>> Removed.
> >>>
> >>>>
> >>>>> +static __always_inline bool aarch64_insn_is_stp_fp_lr_sp_64b(u32 insn)
> >>>>> +{
> >>>>> + /*
> >>>>> + * The 1st instruction on function entry often follows the
> >>>>> + * patten 'stp x29, x30, [sp, #imm]!' that pushing fp and lr
> >>>>> + * into stack.
> >>>>> + */
> >>>>> + return aarch64_insn_is_stp_pre(insn) &&
> >>>>> + (((insn >> 30) & 0x03) == 2) && /* opc == 10 */
> >>>>> + (((insn >> 5) & 0x1F) == 31) && /* Rn is sp */
> >>>>> + (((insn >> 10) & 0x1F) == 30) && /* Rt2 is x29 */
> >>>>> + (((insn >> 0) & 0x1F) == 29); /* Rt is x30 */
> >>>>> +}
> >>>>
> >>>> We have accessors for these fields. Please use them.
> >>>
> >>> Do you mean aarch64_insn_decode_register()?
> >>>
> >>>>
> >>>> Regardless, as above I do not think we should have a helper this
> >>>> specific (with Rn, Rt, and Rt2 values hard-coded) inside <asm/insn.h>.
> >>>
> >>> If we left necessary of emulation of STP aside, where would the best file to
> >>> place these hard-coded decoder helper?
> >>>
> >>>>
> >>>>> enum aarch64_insn_encoding_class aarch64_get_insn_class(u32 insn);
> >>>>> u64 aarch64_insn_decode_immediate(enum aarch64_insn_imm_type type, u32 insn);
> >>>>> u32 aarch64_insn_encode_immediate(enum aarch64_insn_imm_type type,
> >>>>> diff --git a/arch/arm64/kernel/probes/decode-insn.c b/arch/arm64/kernel/probes/decode-insn.c
> >>>>> index 968d5fffe233..df7ca16fc763 100644
> >>>>> --- a/arch/arm64/kernel/probes/decode-insn.c
> >>>>> +++ b/arch/arm64/kernel/probes/decode-insn.c
> >>>>> @@ -73,8 +73,22 @@ static bool __kprobes aarch64_insn_is_steppable(u32 insn)
> >>>>> * INSN_GOOD_NO_SLOT If instruction is supported but doesn't use its slot.
> >>>>> */
> >>>>> enum probe_insn __kprobes
> >>>>> -arm_probe_decode_insn(probe_opcode_t insn, struct arch_probe_insn *api)
> >>>>> +arm_probe_decode_insn(probe_opcode_t insn, struct arch_probe_insn *api,
> >>>>> + bool kernel)
> >>>>> {
> >>>>> + /*
> >>>>> + * While 'nop' and 'stp x29, x30, [sp, #imm]! instructions 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;
> >>>>> + } else if (!kernel && aarch64_insn_is_stp_fp_lr_sp_64b(insn)) {
> >>>>> + api->handler = simulate_stp_fp_lr_sp_64b;
> >>>>> + return INSN_GOOD_NO_SLOT;
> >>>>> + }
> >>>>
> >>>> With the STP emulation gone, you won't need the kernel parameter here.>
> >>>>> +
> >>>>> /*
> >>>>> * Instructions reading or modifying the PC won't work from the XOL
> >>>>> * slot.
> >>>>> @@ -157,7 +171,7 @@ arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi)
> >>>>> else
> >>>>> scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE;
> >>>>> }
> >>>>> - decoded = arm_probe_decode_insn(insn, &asi->api);
> >>>>> + decoded = arm_probe_decode_insn(insn, &asi->api, true);
> >>>>>
> >>>>> if (decoded != INSN_REJECTED && scan_end)
> >>>>> if (is_probed_address_atomic(addr - 1, scan_end))
> >>>>> diff --git a/arch/arm64/kernel/probes/decode-insn.h b/arch/arm64/kernel/probes/decode-insn.h
> >>>>> index 8b758c5a2062..ec4607189933 100644
> >>>>> --- a/arch/arm64/kernel/probes/decode-insn.h
> >>>>> +++ b/arch/arm64/kernel/probes/decode-insn.h
> >>>>> @@ -28,6 +28,7 @@ enum probe_insn __kprobes
> >>>>> arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi);
> >>>>> #endif
> >>>>> enum probe_insn __kprobes
> >>>>> -arm_probe_decode_insn(probe_opcode_t insn, struct arch_probe_insn *asi);
> >>>>> +arm_probe_decode_insn(probe_opcode_t insn, struct arch_probe_insn *asi,
> >>>>> + bool kernel);
> >>>>>
> >>>>> #endif /* _ARM_KERNEL_KPROBES_ARM64_H */
> >>>>> diff --git a/arch/arm64/kernel/probes/simulate-insn.c b/arch/arm64/kernel/probes/simulate-insn.c
> >>>>> index 22d0b3252476..0b1623fa7003 100644
> >>>>> --- a/arch/arm64/kernel/probes/simulate-insn.c
> >>>>> +++ b/arch/arm64/kernel/probes/simulate-insn.c
> >>>>> @@ -200,3 +200,31 @@ 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)
> >>>>> +{
> >>>>> + instruction_pointer_set(regs, instruction_pointer(regs) + 4);
> >>>>> +}
> >>>>
> >>>> Hmm, this forgets to update the single-step state machine and PSTATE.BT,
> >>>> and that's an extant bug in arch_uprobe_post_xol(). This can be:
> >>>
> >>> For emulated instruction, uprobe won't enable single-step mode of CPU,
> >>> please check the handle_swbp() in kernel/events/uprobes.c:
> >>>
> >>> if (arch_uprobe_skip_sstep(&uprobe->arch, regs))
> >>> goto out;
> >>>
> >>> if (!pre_ssout(uprobe, regs, bp_vaddr))
> >>> return;
> >>>
> >>> For emualted instruction, It will skip entire single-stepping and associated
> >>> exception, which typically begins with pre_ssout() and ends with
> >>> arch_uprobe_post_xol(). Therefore, using instruction_pointer_set() to emulate
> >>> NOP is generally not a bad idea.
> >>>
> >>>>
> >>>> | void __kprobes
> >>>> | simulate_nop(u32 opcode, long addr, struct pt_regs *regs)
> >>>> | {
> >>>> | arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
> >>>> | }
> >>>>
> >>>>> +
> >>>>> +void __kprobes
> >>>>> +simulate_stp_fp_lr_sp_64b(u32 opcode, long addr, struct pt_regs *regs)
> >>>>> +{
> >>>>> + long imm7;
> >>>>> + u64 buf[2];
> >>>>> + long new_sp;
> >>>>> +
> >>>>> + imm7 = sign_extend64((opcode >> 15) & 0x7f, 6);
> >>>>> + new_sp = regs->sp + (imm7 << 3);
> >>>>
> >>>> We have accessors for these fields, please use them.
> >>>
> >>> Do you mean aarch64_insn_decode_immediate()?
> >>>
> >>>>
> >>>>> +
> >>>>> + buf[0] = regs->regs[29];
> >>>>> + buf[1] = regs->regs[30];
> >>>>> +
> >>>>> + if (copy_to_user((void __user *)new_sp, buf, sizeof(buf))) {
> >>>>> + force_sig(SIGSEGV);
> >>>>> + return;
> >>>>> + }
> >>>>
> >>>> As above, this won't interact with VMSA features (e.g. MTE, POE) in the
> >>>> same way as an STP in userspace, and this will not have the same
> >>>> atomicity properties as an STP>
> >>>>> +
> >>>>> + regs->sp = new_sp;
> >>>>> + instruction_pointer_set(regs, instruction_pointer(regs) + 4);
> >>>>
> >>>> Likewise, this sould need ot use arm64_skip_faulting_instruction(),
> >>>> though as above I think we should drop STP emulation entirely.
> >>>
> >>> I explain the reason why using instruction_pointer_set() under your comments
> >>> for simulate_nop().
> >>>
> >>> Thanks.
> >>>
> >>>>
> >>>> Mark.
> >>>>
> >>>
> >>
> >> --
> >> BR
> >> Liao, Chang
> >>
>
> --
> BR
> Liao, Chang
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