Messages in this thread | | | Date | Tue, 14 Sep 2021 16:17:24 +0100 | From | Joey Gouly <> | Subject | Re: [PATCH] arm64: entry: Improve the performance of system calls |
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Hi,
On Tue, Sep 14, 2021 at 10:55:16AM +0100, Mark Rutland wrote: > Hi, > > 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...
I tested Mark's branch which implements this (found at https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/entry/switch-table)
I also took lmbench from https://github.com/intel/lmbench.git and built `lat_syscall` with:
gcc lat_syscall.c lib_*.c -l m -o lat_syscall -static
These are the results I got from benchmarking on my MacBook Air M1, with the following command:
./lat_syscall null &> /dev/null ; uname -a ; for i in 0 1 2 3 4 ; do ./lat_syscall null ; done
The kernel was based on arm64_defconfig that was then stripped of as much as possible. GCC 11.1.0 from kernel.org crosstool page. Clang build fom git b041b613e6fff713fc9ad6dbc73024286fb2fc93.
gcc: master: 0.14300 switch-table: 0.14350 likely: 0.13962
clang: master: 0.14354 switch-table: 0.14642 likely: 0.14256
The generated code looks similar to what Leizhen has posted, so I didn't post it again.
So it seems the table approach actually performs worse in my testing, and Leizhen's approach is slightly better than master (d0ee23f9d78be5531c4b055ea424ed0b489dfe9b).
Thanks, Joey
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