[#24386] rsyslog: restore kernel timestamp in the /dev/kmsg logs

[rchandramouli]

The imklog plugin of rsyslog collects the kernel logs from /dev/kmsg and enqueues it to the syslog. With CONFIG_PRINTK_TIME the kernel messages are by default prefixed with the elapsed time since boot. The imklog plugin parsing these messages have a few options such as to keep the timestamps as such or to interpret and adjust the syslog's reported time accordingly.

The rsylog release 8.2312.0 has fixes in interpreting these timestamps, leading to the change in behavior observed in sonic-net#24386.

https://salsa.debian.org/debian/rsyslog/-/blob/debian/8.2504.0-1/ChangeLog?ref_type=tags#L619

To restore the earlier behavior or retaining the kernel reported elapsed time, disable KlogParseKernelTimestamp as this leads to removal of timestamp from kernel messages and enable KlogKeepKernelTimestamp explicitly. The later is required as the default is now to discard the kernel timestamp.

With this change, the logs retain the kernel timestamp:

root@sonic:~# cat /var/log/syslog | grep "sonic.*kernel:" | head -n 3
2025 Nov  4 05:15:14.918946 sonic NOTICE kernel: [    0.000000] Linux version 6.12.41+deb13-sonic-amd64 ([email protected]) (x86_64-linux-gnu-gcc-14 (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44) #1 SMP PREEMPT_DYNAMIC Debian 6.12.41-1 (2025-08-12)
2025 Nov  4 05:15:14.919533 sonic INFO kernel: [    0.000000] Command line: BOOT_IMAGE=/image-trixie.0-dirty-20251102.122837/boot/vmlinuz-6.12.41+deb13-sonic-amd64 root=UUID=ac0b6826-f8a3-461f-a8ff-701df60d90b6 rw console=tty0 console=ttyS0,115200n8 quiet processor.max_cstate=1 intel_idle.max_cstate=0 net.ifnames=0 biosdevname=0 loop=image-trixie.0-dirty-20251102.122837/fs.squashfs loopfstype=squashfs apparmor=1 security=apparmor varlog_size=4096 usbcore.autosuspend=-1 intel_iommu=off modprobe.blacklist=gpio_ich,i2c-ismt,i2c_ismt,i2c-i801,i2c_i801 crashkernel=0M-2G:256M,2G-4G:320M,4G-8G:384M,8G-:448M acpi_no_watchdog
2025 Nov  4 05:15:14.919536 sonic INFO kernel: [    0.000000] BIOS-provided physical RAM map:
root@sonic:~# cat /var/log/syslog | grep "sonic.*kernel:" | tail -n 3
2025 Nov  4 05:17:26.831607 sonic WARNING kernel: [  143.527486] PDDF_LED       set_status_led: Set [FANTRAY_LED;1] color[green]
2025 Nov  4 05:17:26.912442 sonic WARNING kernel: [  143.607086] PDDF_LED       set_status_led: Set [FANTRAY_LED;2] color[green]
2025 Nov  4 05:20:32.499634 sonic WARNING kernel: [  329.195319] PDDF_LED       set_status_led: Set [SYS_LED;0] color[amber]
root@sonic:~#

Signed-off-by: Ramasamy Chandramouli [email protected]