Re: [PATCH RFCv2 00/18] uprobes: Add support to optimize usdt probes on x86_64

* Jiri Olsa [off-list ref] wrote:

hi,
this patchset adds support to optimize usdt probes on top of 5-byte
nop instruction.

The generic approach (optimize all uprobes) is hard due to emulating
possible multiple original instructions and its related issues. The
usdt case, which stores 5-byte nop seems much easier, so starting
with that.

The basic idea is to replace breakpoint exception with syscall which
is faster on x86_64. For more details please see changelog of patch 8.

The run_bench_uprobes.sh benchmark triggers uprobe (on top of different
original instructions) in a loop and counts how many of those happened
per second (the unit below is million loops).

There's big speed up if you consider current usdt implementation
(uprobe-nop) compared to proposed usdt (uprobe-nop5):

  # ./benchs/run_bench_uprobes.sh 

          usermode-count :  818.386 ± 1.886M/s
          syscall-count  :    8.923 ± 0.003M/s
  -->     uprobe-nop     :    3.086 ± 0.005M/s
          uprobe-push    :    2.751 ± 0.001M/s
          uprobe-ret     :    1.481 ± 0.000M/s
  -->     uprobe-nop5    :    4.016 ± 0.002M/s
          uretprobe-nop  :    1.712 ± 0.008M/s
          uretprobe-push :    1.616 ± 0.001M/s
          uretprobe-ret  :    1.052 ± 0.000M/s
          uretprobe-nop5 :    2.015 ± 0.000M/s

So I had to dig into patch #12 to see the magnitude of the speedup:

# current:
#         usermode-count :  818.836 ± 2.842M/s
#         syscall-count  :    8.917 ± 0.003M/s
#         uprobe-nop     :    3.056 ± 0.013M/s
#         uprobe-push    :    2.903 ± 0.002M/s
#         uprobe-ret     :    1.533 ± 0.001M/s
# -->     uprobe-nop5    :    1.492 ± 0.000M/s
#         uretprobe-nop  :    1.783 ± 0.000M/s
#         uretprobe-push :    1.672 ± 0.001M/s
#         uretprobe-ret  :    1.067 ± 0.002M/s
# -->     uretprobe-nop5 :    1.052 ± 0.000M/s
# 
# after the change:
# 
#         usermode-count :  818.386 ± 1.886M/s
#         syscall-count  :    8.923 ± 0.003M/s
#         uprobe-nop     :    3.086 ± 0.005M/s
#         uprobe-push    :    2.751 ± 0.001M/s
#         uprobe-ret     :    1.481 ± 0.000M/s
# -->     uprobe-nop5    :    4.016 ± 0.002M/s
#         uretprobe-nop  :    1.712 ± 0.008M/s
#         uretprobe-push :    1.616 ± 0.001M/s
#         uretprobe-ret  :    1.052 ± 0.000M/s
# -->     uretprobe-nop5 :    2.015 ± 0.000M/s

That's a +169% and a +91% speedup - pretty darn impressive!

Thanks,

	Ingo