Messages in this thread | | | Date | Mon, 3 Apr 2017 20:53:11 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [RFC][PATCH 0/4] printk: introduce printing kernel thread |
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On (03/06/17 21:45), Sergey Senozhatsky wrote: [..] > printk kthread changes the behavior of printk in one _corner case_. > The corner case is quite interesting and actually consists of two corner > cases. Suppose on SMP system there is only one CPU that printk()-s a lot, > the rest of CPUs don't lock console_sem and don't printk(). Previously > that printing CPU had been throttling itself (*) because of console drivers > call for every printk(): > > CPU0 > > printk("a") > console_unlock() > call_console_drivers("a") > > ... > > printk("z") > console_unlock() > call_console_drivers("z") > > * Given that no other CPU locks the console_sem. > > With printk kthread the case turns into this one: > > CPU0 CPU1 > > printk("a") > wake_up printk_kthread > ... printk_kthread > printk("k") console_unlock() > ... call_console_drivers("a") > printk("z") call_console_drivers("b") > call_console_drivers("c") > ... > > > The second 'corner case' part here is that CPU0 may be much faster > than printing CPU, which may result in dropped printk messages. > > This all is absolutely possible even with out the printk-kthread. > A single console_lock() call from CPUx will result in exactly the > same condition. So it's not necessarily a regression. But there may > be some scenarios in the kernel that may suffer from this change. > From the top of my head -- sysrq backtrace dump, and, probably, OOM > print out and backtrace dump.
there is another possibility here.
being always reschedulable potentially can put us at risk of having unpleasant situations when printk_kthread is getting preempted too often (well, who knows what can happen on the system), which can slow down logbuf emit process (printk_kthread) up to the point when printk() CPUs will force log_store() to begin dropping the messages. this can happen.
-ss
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