【Linux命令】诊断系统故障命令dmesg详解
本帖最后由 sunsili 于 2023-7-1 22:34 编辑【Linux命令】诊断系统故障命令dmesg详解
前言
本文讲解诊断系统故障命令dmesg
dmesg命令简介
dmesg 命令用于显示系统开机信息，可用于诊断系统故障。内核会将系统开机信息存储在ring buffer中，可以使用dmesg命令来查看，开机信息保存在/var/log/dmesg文件中。
dmesg用法
查看帮助
dmesg -h
Usage:
dmesg
Display or control the kernel ring buffer.
Options:
-C, --clear clear the kernel ring buffer
-c, --read-clear read and clear all messages
-D, --console-off disable printing messages to console
-E, --console-on enable printing messages to console
-F, --file <file> use the file instead of the kernel log buffer
-f, --facility <list> restrict output to defined facilities
-H, --human human readable output
-k, --kernel display kernel messages
-L, --color[=<when>] colorize messages (auto, always or never)
colors are enabled by default
-l, --level <list> restrict output to defined levels
-n, --console-level <level> set level of messages printed to console
-P, --nopager do not pipe output into a pager
-p, --force-prefix force timestamp output on each line of multi-line messages
-r, --raw print the raw message buffer
-S, --syslog force to use syslog(2) rather than /dev/kmsg
-s, --buffer-size <size> buffer size to query the kernel ring buffer
-u, --userspace display userspace messages
-w, --follow wait for new messages
-x, --decode decode facility and level to readable string
-d, --show-delta show time delta between printed messages
-e, --reltime show local time and time delta in readable format
-T, --ctime show human-readable timestamp (may be inaccurate!)
-t, --notime don't show any timestamp with messages
--time-format <format>show timestamp using the given format:
Suspending/resume will make ctime and iso timestamps inaccurate.
-h, --help display this help
-V, --version display version
Supported log facilities:
kern - kernel messages
user - random user-level messages
mail - mail system
daemon - system daemons
auth - security/authorization messages
syslog - messages generated internally by syslogd
lpr - line printer subsystem
news - network news subsystem
Supported log levels (priorities):
emerg - system is unusable
alert - action must be taken immediately
crit - critical conditions
err - error conditions
warn - warning conditions
notice - normal but significant condition
info - informational
debug - debug-level messages
For more details see dmesg(1).
dmesg帮助信息解释
命令语法dmesg
选项说明-c#显示信息后，清除ring buffer中的内容-s<缓冲区大小>#默认值为8196，刚好等于ring buffer的大小-n#设置记录信息的层级-D#禁用打印消息到控制台-E#启用打印消息到控制台-h#打印帮助文本并退出-k#打印内核消息-n#设置将消息记录到控制台的级别-r#打印原始消息缓冲区-s#使用多少大小的缓冲区来查询内核环缓冲区。默认情况下为16392-T#打印人类可读时间戳-t#不打印内核的时间戳-u#打印用户空间消息-V#输出版本信息并退出-x#将设施和级别（优先级）编号解码为可读的前缀
应用举例
在openwrt上查看开机信息dmesg
[ 0.000000] Linux version 5.4.143 (*******) (gcc version 8.4.0 (OpenWrt GCC 8.4.0 r16279-5cc 0535800)) #0 Tue Aug 31 22:20:08 2021
[ 0.000000] Board has DDR2
[ 0.000000] Analog PMU set to hw control
[ 0.000000] Digital PMU set to hw control
[ 0.000000] SoC Type: MediaTek MT7628AN ver:1 eco:2
[ 0.000000] printk: bootconsole enabled
[ 0.000000] CPU0 revision is: 00019655 (MIPS 24KEc)
[ 0.000000] MIPS: machine is Jotale JS76x8 (32M)
[ 0.000000] Initrd not found or empty - disabling initrd
[ 0.000000] Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes.
[ 0.000000] Primary data cache 32kB, 4-way, PIPT, no aliases, linesize 32 bytes
[ 0.000000] Zone ranges:
[ 0.000000] Normal
[ 0.000000] Movable zone start for each node
[ 0.000000] Early memory node ranges
[ 0.000000] node 0:
[ 0.000000] Initmem setup node 0
[ 0.000000] On node 0 totalpages: 32768
[ 0.000000] Normal zone: 288 pages used for memmap
[ 0.000000] Normal zone: 0 pages reserved
[ 0.000000] Normal zone: 32768 pages, LIFO batch:7
[ 0.000000] pcpu-alloc: s0 r0 d32768 u32768 alloc=1*32768
[ 0.000000] pcpu-alloc: 0
[ 0.000000] Built 1 zonelists, mobility grouping on.Total pages: 32480
[ 0.000000] Kernel command line: console=ttyS0,115200 rootfstype=squashfs,jffs2
[ 0.000000] Dentry cache hash table entries: 16384 (order: 4, 65536 bytes, linear)
[ 0.000000] Inode-cache hash table entries: 8192 (order: 3, 32768 bytes, linear)
[ 0.000000] Writing ErrCtl register=00000000
[ 0.000000] Readback ErrCtl register=00000000
[ 0.000000] mem auto-init: stack:off, heap alloc:off, heap free:off
*******
[ 11.641746] usbcore: registered new interface driver cdc_subset
[ 11.689505] PPP generic driver version 2.4.2
[ 11.703356] usbcore: registered new interface driver qmi_wwan
[ 11.730537] rndis_host 1-1:1.0 eth1: register 'rndis_host' at usb-101c0000.ehci-1, RNDIS device, ac:4 b:b3:b9:eb:e5
[ 11.741397] usbcore: registered new interface driver rndis_host
[ 11.789009] kmodloader: done loading kernel modules from /etc/modules.d/*
[ 11.855712] urngd: v1.0.2 started.
[ 12.515084] random: crng init done
[ 12.518593] random: 1 urandom warning(s) missed due to ratelimiting
[ 19.978363] rt3050-esw 10110000.esw: link changed 0x00
[ 22.202128] br-lan: port 1(eth0.1) entered blocking state
[ 22.207696] br-lan: port 1(eth0.1) entered disabled state
[ 22.213547] device eth0.1 entered promiscuous mode
[ 22.218473] device eth0 entered promiscuous mode
[ 22.255696] br-lan: port 1(eth0.1) entered blocking state
[ 22.261264] br-lan: port 1(eth0.1) entered forwarding state
[ 23.004559] rt3050-esw 10110000.esw: link changed 0x01
这样打印信息不方便查找关键信息
查看前25行开机信息
dmesg | head -n 25
[ 0.000000] Linux version 5.4.143 (fan@FAN-THINK-PC) (gcc version 8.4.0 (OpenWrt GCC 8.4.0 r16279-5cc0535800)) #0 Tue Aug 31 22:20:08 2021
[ 0.000000] Board has DDR2
[ 0.000000] Analog PMU set to hw control
[ 0.000000] Digital PMU set to hw control
[ 0.000000] SoC Type: MediaTek MT7628AN ver:1 eco:2
[ 0.000000] printk: bootconsole enabled
[ 0.000000] CPU0 revision is: 00019655 (MIPS 24KEc)
[ 0.000000] MIPS: machine is Jotale JS76x8 (32M)
[ 0.000000] Initrd not found or empty - disabling initrd
[ 0.000000] Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes.
[ 0.000000] Primary data cache 32kB, 4-way, PIPT, no aliases, linesize 32 bytes
[ 0.000000] Zone ranges:
[ 0.000000] Normal
[ 0.000000] Movable zone start for each node
[ 0.000000] Early memory node ranges
[ 0.000000] node 0:
[ 0.000000] Initmem setup node 0
[ 0.000000] On node 0 totalpages: 32768
[ 0.000000] Normal zone: 288 pages used for memmap
[ 0.000000] Normal zone: 0 pages reserved
[ 0.000000] Normal zone: 32768 pages, LIFO batch:7
[ 0.000000] pcpu-alloc: s0 r0 d32768 u32768 alloc=1*32768
[ 0.000000] pcpu-alloc: 0
[ 0.000000] Built 1 zonelists, mobility grouping on.Total pages: 32480
[ 0.000000] Kernel command line: console=ttyS0,115200 rootfstype=squashfs,jffs2
查看后25行开机信息
dmesg | tail -n 25
[ 11.352236] GACT probability on
[ 11.361732] Mirror/redirect action on
[ 11.395872] u32 classifier
[ 11.398710] input device check on
[ 11.402442] Actions configured
[ 11.463279] usbcore: registered new interface driver cdc_wdm
[ 11.613359] xt_time: kernel timezone is -0000
[ 11.626466] usbcore: registered new interface driver cdc_ether
[ 11.641746] usbcore: registered new interface driver cdc_subset
[ 11.689505] PPP generic driver version 2.4.2
[ 11.703356] usbcore: registered new interface driver qmi_wwan
[ 11.730537] rndis_host 1-1:1.0 eth1: register 'rndis_host' at usb-101c0000.ehci-1, RNDIS device, ac:4b:b3:b9:eb:e5
[ 11.741397] usbcore: registered new interface driver rndis_host
[ 11.789009] kmodloader: done loading kernel modules from /etc/modules.d/*
[ 11.855712] urngd: v1.0.2 started.
[ 12.515084] random: crng init done
[ 12.518593] random: 1 urandom warning(s) missed due to ratelimiting
[ 19.978363] rt3050-esw 10110000.esw: link changed 0x00
[ 22.202128] br-lan: port 1(eth0.1) entered blocking state
[ 22.207696] br-lan: port 1(eth0.1) entered disabled state
[ 22.213547] device eth0.1 entered promiscuous mode
[ 22.218473] device eth0 entered promiscuous mode
[ 22.255696] br-lan: port 1(eth0.1) entered blocking state
[ 22.261264] br-lan: port 1(eth0.1) entered forwarding state
[ 23.004559] rt3050-esw 10110000.esw: link changed 0x01
查看与内存相关的开机信息
dmesg | grep -i memory
[ 0.000000] Early memory node ranges
[ 0.000000] Memory: 122688K/131072K available (4436K kernel code, 192K rwdata, 1016K rodata, 1192K init, 193K bss, 8384K reserved, 0K cma-reserved)
[ 0.526438] Freeing unused kernel memory: 1192K
[ 0.531064] This architecture does not have kernel memory protection.
查看与USB相关的开机信息
dmesg | grep -i usb
[ 1.884358] usbcore: registered new interface driver usbfs
[ 1.890096] usbcore: registered new interface driver hub
[ 1.895617] usbcore: registered new device driver usb
[ 1.907868] ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver
[ 1.939711] phy phy-10120000.usbphy.0: remote usb device wakeup disabled
[ 1.946525] phy phy-10120000.usbphy.0: UTMI 16bit 30MHz
[ 1.957713] ehci-platform 101c0000.ehci: new USB bus registered, assigned bus number 1
[ 1.984466] ehci-platform 101c0000.ehci: USB 2.0 started, EHCI 1.00
[ 1.991901] hub 1-0:1.0: USB hub found
[ 4.628468] usb 1-1: new high-speed USB device number 2 using ehci-platform
[ 11.463279] usbcore: registered new interface driver cdc_wdm
[ 11.626466] usbcore: registered new interface driver cdc_ether
[ 11.641746] usbcore: registered new interface driver cdc_subset
[ 11.703356] usbcore: registered new interface driver qmi_wwan
[ 11.730537] rndis_host 1-1:1.0 eth1: register 'rndis_host' at usb-101c0000.ehci-1, RNDIS device, ac:4b:b3:b9:eb:e5
[ 11.741397] usbcore: registered new interface driver rndis_host
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