Re: [PATCH] arm64: entry: Improve the performance of system calls
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From: Leizhen (ThunderTown) <hidden>
Date: 2021-09-14 11:51:00
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On 2021/9/14 19:23, Leizhen (ThunderTown) wrote:
On 2021/9/14 17:55, Mark Rutland wrote:quoted
Hi, On Fri, Sep 03, 2021 at 08:19:50PM +0800, Zhen Lei wrote:quoted
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, #0x1cIt would be helpful if you could state which toolchain and config was used to generate the above.gcc version 7.3.0 (GCC), make defconfigquoted
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>
quoted
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.quoted
quoted
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 microsecondsI 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.quoted
Thanks, Mark.quoted
Signed-off-by: Zhen Lei <redacted> --- 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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