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kern/57986: kernel lock can't be dtraced
>Number: 57986
>Category: kern
>Synopsis: kernel lock can't be dtraced
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: kern-bug-people
>State: open
>Class: sw-bug
>Submitter-Id: net
>Arrival-Date: Sun Mar 03 00:50:00 +0000 2024
>Originator: Taylor R Campbell
>Release: current, 10
>Organization:
The NetBSD Kernellock
>Environment:
>Description:
Tried to use dtrace to put probes on kernel lock usage, in an attempt to measure who's holding the kernel lock for how long:
# dtrace -x cleanrate=50hz -n 'fbt::_kernel_lock:return { self->starttime = timestamp } fbt::_kernel_unlock:entry /self->starttime/ { @[caller] = max(timestamp - self->starttime); self->starttime = 0 } tick-10s, END { printa("\n%a
%@u", @) }'
dtrace: invalid probe specifier fbt::_kernel_lock:return { self->starttime = timestamp } fbt::_kernel_unlock:entry /self->starttime/ { @[caller] = max(timestamp - self->starttime); self->starttime = 0 } tick-10s, END { printa("\n%a %@u", @) }: probe description fbt::_kernel_lock:return does not match any probes
This used to work in netbsd-9:
https://mail-index.netbsd.org/tech-kern/2022/10/30/msg028499.html
>How-To-Repeat:
dtrace fbt::_kernel_lock:return
>Fix:
Yes, please!
Either:
(a) fix whatever broke fdt on _kernel_lock return, or
(b) sprinkle sdt probes.
Performance may not be great for sdt probes, but it's the legacy kernel lock -- we should discourage using it anyway.
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