Messages in this thread Patch in this message | | | Subject | Re: [PATCH] arm64: entry: Improve the performance of system calls | From | "Leizhen (ThunderTown)" <> | Date | Tue, 14 Sep 2021 19:48:54 +0800 |
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On 2021/9/14 19:23, Leizhen (ThunderTown) wrote: > > > On 2021/9/14 17:55, Mark Rutland wrote: >> Hi, >> >> On Fri, Sep 03, 2021 at 08:19:50PM +0800, Zhen Lei wrote: >>> Commit 582f95835a8f ("arm64: entry: convert el0_sync to C") converted lots >>> of functions from assembly to C, this greatly improves readability. But >>> el0_svc()/el0_svc_compat() is in response to system call requests from >>> user mode and may be in the hot path. >>> >>> Although the SVC is in the first case of the switch statement in C, the >>> compiler optimizes the switch statement as a whole, and does not give SVC >>> a small boost. >>> >>> Use "likely()" to help SVC directly invoke its handler after a simple >>> judgment to avoid entering the switch table lookup process. >>> >>> After: >>> 0000000000000ff0 <el0t_64_sync_handler>: >>> ff0: d503245f bti c >>> ff4: d503233f paciasp >>> ff8: a9bf7bfd stp x29, x30, [sp, #-16]! >>> ffc: 910003fd mov x29, sp >>> 1000: d5385201 mrs x1, esr_el1 >>> 1004: 531a7c22 lsr w2, w1, #26 >>> 1008: f100545f cmp x2, #0x15 >>> 100c: 540000a1 b.ne 1020 <el0t_64_sync_handler+0x30> >>> 1010: 97fffe14 bl 860 <el0_svc> >>> 1014: a8c17bfd ldp x29, x30, [sp], #16 >>> 1018: d50323bf autiasp >>> 101c: d65f03c0 ret >>> 1020: f100705f cmp x2, #0x1c >> >> It would be helpful if you could state which toolchain and config was >> used to generate the above. > > gcc version 7.3.0 (GCC), make defconfig > >> >> For comparison, what was the code generation like before? I assume >> el0_svc wasn't the target of the first test and branch? Assuming so, how >> many tests and branches were there before the call to el0_svc()? >
Sorry, the old assembly code was not compiled with the latest mainline. But the key point is no different.
0000000000000fe0 <el0t_64_sync_handler>: fe0: d503233f paciasp fe4: a9bf7bfd stp x29, x30, [sp, #-16]! fe8: 910003fd mov x29, sp fec: d5385201 mrs x1, esr_el1 ff0: 531a7c22 lsr w2, w1, #26 ff4: f100f05f cmp x2, #0x3c ff8: 54000068 b.hi 1004 <el0t_64_sync_handler+0x24> // b.pmore ffc: 7100f05f cmp w2, #0x3c 1000: 540000c9 b.ls 1018 <el0t_64_sync_handler+0x38> // b.plast 1004: 97fffce9 bl 3a8 <el0_inv> 1008: a8c17bfd ldp x29, x30, [sp], #16 100c: d50323bf autiasp 1010: d65f03c0 ret 1014: d503201f nop 1018: 90000003 adrp x3, 0 <el0_da> 101c: 91000063 add x3, x3, #0x0 1020: 38624862 ldrb w2, [x3, w2, uxtw] 1024: 10000063 adr x3, 1030 <el0t_64_sync_handler+0x50> 1028: 8b228862 add x2, x3, w2, sxtb #2 102c: d61f0040 br x2 1030: 97fffc3a bl 118 <el0_dbg> 1034: 17fffff5 b 1008 <el0t_64_sync_handler+0x28> 1038: 97fffc96 bl 290 <el0_fpsimd_exc> 103c: 17fffff3 b 1008 <el0t_64_sync_handler+0x28> 1040: 97fffc08 bl 60 <el0_sp> 1044: 17fffff1 b 1008 <el0t_64_sync_handler+0x28> 1048: 97fffbee bl 0 <el0_da> 104c: 17ffffef b 1008 <el0t_64_sync_handler+0x28> 1050: 97fffeea bl bf8 <el0_pc> 1054: 17ffffed b 1008 <el0t_64_sync_handler+0x28> 1058: 97fffeb2 bl b20 <el0_ia> 105c: 17ffffeb b 1008 <el0t_64_sync_handler+0x28> 1060: 97fffc46 bl 178 <el0_fpac> 1064: 17ffffe9 b 1008 <el0t_64_sync_handler+0x28> 1068: 97fffc72 bl 230 <el0_sve_acc> 106c: 17ffffe7 b 1008 <el0t_64_sync_handler+0x28> 1070: 97ffff18 bl cd0 <el0_svc> 1074: 17ffffe5 b 1008 <el0t_64_sync_handler+0x28> 1078: 97fffcb6 bl 350 <el0_bti> 107c: 17ffffe3 b 1008 <el0t_64_sync_handler+0x28> 1080: 97fffc54 bl 1d0 <el0_fpsimd_acc> 1084: 17ffffe1 b 1008 <el0t_64_sync_handler+0x28> 1088: 97fffc9a bl 2f0 <el0_sys> 108c: 17ffffdf b 1008 <el0t_64_sync_handler+0x28> 1090: 97fffc0c bl c0 <el0_undef> 1094: 17ffffdd b 1008 <el0t_64_sync_handler+0x28>
> > >> >> At a high-level, I'm not too keen on special-casing things unless >> necessary. >> >> I wonder if we could get similar results without special-casing by using >> a static const array of handlers indexed by the EC, since (with GCC >> 11.1.0 from the kernel.org crosstool page) that can result in code like: >> >> 0000000000001010 <el0t_64_sync_handler>: >> 1010: d503245f bti c >> 1014: d503233f paciasp >> 1018: a9bf7bfd stp x29, x30, [sp, #-16]! >> 101c: 910003fd mov x29, sp >> 1020: d5385201 mrs x1, esr_el1 >> 1024: 90000002 adrp x2, 0 <el0t_64_sync_handlers> >> 1028: 531a7c23 lsr w3, w1, #26 >> 102c: 91000042 add x2, x2, #:lo12:<el0t_64_sync_handlers> >> 1030: f8637842 ldr x2, [x2, x3, lsl #3] >> 1034: d63f0040 blr x2 >> 1038: a8c17bfd ldp x29, x30, [sp], #16 >> 103c: d50323bf autiasp >> 1040: d65f03c0 ret >> >> ... which might do better by virtue of reducing a chain of potential >> mispredicts down to a single potential mispredict, and dynamic branch >> prediction hopefully does a good job of predicting the common case at >> runtime. That said, the resulting tables will be pretty big... > > > a48: 38624862 ldrb w2, [x3, w2, uxtw] > a4c: 10000063 adr x3, a58 <el0_sync_handler+0x48> > a50: 8b228862 add x2, x3, w2, sxtb #2 > a54: d61f0040 br x2 > > The original implementation also generated a query table, but yours is > more concise. I will try to test it. Looks like a better solution. > >> >>> >>> Execute "./lat_syscall null" on my board (BogoMIPS : 200.00), it can save >>> about 10ns. >>> >>> Before: >>> Simple syscall: 0.2365 microseconds >>> Simple syscall: 0.2354 microseconds >>> Simple syscall: 0.2339 microseconds >>> >>> After: >>> Simple syscall: 0.2255 microseconds >>> Simple syscall: 0.2254 microseconds >>> Simple syscall: 0.2256 microseconds >> >> I appreciate this can be seen by a microbenchmark, but does this have an >> impact on a real workload? I'd imagine that real syscall usage will >> dominate this in practice, and this would fall into the noise. > > The product side has a test plan, but the progress will be slow. > >> >> Thanks, >> Mark. >> >>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com> >>> --- >>> arch/arm64/kernel/entry-common.c | 18 ++++++++++++------ >>> 1 file changed, 12 insertions(+), 6 deletions(-) >>> >>> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c >>> index 32f9796c4ffe77b..062eb5a895ec6f3 100644 >>> --- a/arch/arm64/kernel/entry-common.c >>> +++ b/arch/arm64/kernel/entry-common.c >>> @@ -607,11 +607,14 @@ static void noinstr el0_fpac(struct pt_regs *regs, unsigned long esr) >>> asmlinkage void noinstr el0t_64_sync_handler(struct pt_regs *regs) >>> { >>> unsigned long esr = read_sysreg(esr_el1); >>> + unsigned long ec = ESR_ELx_EC(esr); >>> >>> - switch (ESR_ELx_EC(esr)) { >>> - case ESR_ELx_EC_SVC64: >>> + if (likely(ec == ESR_ELx_EC_SVC64)) { >>> el0_svc(regs); >>> - break; >>> + return; >>> + } >>> + >>> + switch (ec) { >>> case ESR_ELx_EC_DABT_LOW: >>> el0_da(regs, esr); >>> break; >>> @@ -730,11 +733,14 @@ static void noinstr el0_svc_compat(struct pt_regs *regs) >>> asmlinkage void noinstr el0t_32_sync_handler(struct pt_regs *regs) >>> { >>> unsigned long esr = read_sysreg(esr_el1); >>> + unsigned long ec = ESR_ELx_EC(esr); >>> >>> - switch (ESR_ELx_EC(esr)) { >>> - case ESR_ELx_EC_SVC32: >>> + if (likely(ec == ESR_ELx_EC_SVC32)) { >>> el0_svc_compat(regs); >>> - break; >>> + return; >>> + } >>> + >>> + switch (ec) { >>> case ESR_ELx_EC_DABT_LOW: >>> el0_da(regs, esr); >>> break; >>> -- >>> 2.25.1 >>> >> . >>
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