Re: [PATCH] arm64: entry: Improve the performance of system calls
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From: Mark Rutland <mark.rutland@arm.com>
Date: 2021-09-14 09:55:28
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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.
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()?
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...
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. Thanks, Mark.
quoted hunk ↗ jump to hunk
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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