Re: [PATCH v2] arm64: insn: Simulate nop instruction for better uprobe performance

["Liao, Chang" <liaochang1@huawei.com>]

在 2024/10/16 2:58, Catalin Marinas 写道:
> On Mon, 09 Sep 2024 07:11:14 +0000, Liao Chang wrote:
>> v2->v1:
>> 1. Remove the simuation of STP and the related bits.
>> 2. Use arm64_skip_faulting_instruction for single-stepping or FEAT_BTI
>>    scenario.
>>
>> 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.
>>
>> [...]
> 
> Applied to arm64 (for-next/probes), thanks! I fixed it up according to
> Mark's comments.
> 
> [1/1] arm64: insn: Simulate nop instruction for better uprobe performance
>       https://git.kernel.org/arm64/c/ac4ad5c09b34
> 

Mark, Catalin and Andrii,

I am just back from a long vacation, thanks for reviewing and involvement for
this patch.

I've sent a patch [1] that simulates STP at function entry, It maps user
stack pages to kernel address space, allowing kernel to use STP directly
to push fp/lr onto stack. Unfortunately, the profiling results below show
reveals this approach increases the uprobe-push throughput by 29.3% (from
0.868M/s/cpu to 1.1238M/s/cpu) and uretprobe-push by 15.9% (from 0.616M/s/cpu
to 0.714M/s/cpu). As Andrii pointed out, this approach is a bit complex and
overkill for STP simluation. So I look forward to more input about this patch,
is it possible to reach a better result? Or should I pause this work for now
and wait for Arm64 to add some instruction for storing pairs of registers to
unprivileged memory in privileged exception level? Thanks.

xol-stp
-------
uprobe-push     ( 1 cpus):    0.868 ± 0.001M/s  (  0.868M/s/cpu)
uretprobe-push  ( 1 cpus):    0.616 ± 0.001M/s  (  0.616M/s/cpu)

simulated-stp
-------------
uprobe-push     ( 1 cpus):    1.128 ± 0.002M/s  (  1.128M/s/cpu)
uretprobe-push  ( 1 cpus):    0.714 ± 0.001M/s  (  0.714M/s/cpu)

[1] https://lore.kernel.org/all/20240910060407.1427716-1-liaochang1@huawei.com/

-- 
BR
Liao, Chang