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?
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@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+7uer4chs0r99zln02ezz5yruv...@mail.gmail.com/ >>> >>> Signed-off-by: Liao Chang <liaocha...@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
