Messages in this thread | | | Subject | Re: [PATCH] printk: Add loglevel for "do not print to consoles". | From | Tetsuo Handa <> | Date | Tue, 28 Apr 2020 22:11:19 +0900 |
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On 2020/04/28 21:18, Michal Hocko wrote: > On Tue 28-04-20 20:33:21, Tetsuo Handa wrote: >> On 2020/04/27 15:21, Sergey Senozhatsky wrote: >>>> KERN_NO_CONSOLES is for type of messages where "saved for later analysis" is >>>> important but "printed for immediate notification" is not important. >>>> In other words, KERN_NO_CONSOLES is NOT for dying messages where "printed for >>>> immediate notification" is important. >>> >>> per-console loglevel is a user configurable parameter. >>> KERN_NO_CONSOLES is a hard-coded policy. >> >> But given that whether to use KERN_NO_CONSOLES is configurable via e.g. sysctl, >> KERN_NO_CONSOLES will become a user configurable parameter. What's still wrong? > > How do I as a kernel developer know that KERN_NO_CONSOLES should be > used? In other words, how can I assume what a user will consider > important on the console? >
Existing KERN_$LEVEL allows a user to determine whether he/she wants that message to be printed on consoles (even if it spams his/her operation doing on consoles), and at the same time constrains that user whether that message is saved to log files. KERN_NO_CONSOLES allows a user to control whether he/she wants that message to be saved to log files (without spamming his/her operation doing on consoles).
Consoles which are limited by "rows" * "columns" size should receive (ideally) up to a few lines of messages. Printing more lines on such consoles disturbs users using such consoles. Let alone printing one thousand lines of messages (e.g. dump_tasks()) on such consoles. Printing one thousand lines of messages on read-only consoles (e.g. netconsole) might be fine, but non-urgent messages can afford waiting for userspace to do more sophisticated handling (e.g. evaluate and filter).
Even without per-console loglevel feature, we can reduce spam messages for consoles by using KERN_NO_CONSOLES. For example, send only the summary part of OOM-killer and allocation failure messages to both consoles and log files (by not using KERN_NO_CONSOLES), and send the non-summary part to only log files (by using KERN_NO_CONSOLES with KERN_$LEVEL used for summary part in order to make sure that userspace daemon will save both the summary and non-summary parts).
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