Messages in this thread | | | Date | Mon, 5 Mar 2018 11:14:16 +0900 | From | Sergey Senozhatsky <> | Subject | Re: Would you help to tell why async printk solution was not taken to upstream kernel ? |
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On (03/04/18 10:43), Steven Rostedt wrote: > On Sun, 04 Mar 2018 23:08:23 +0800 > "Qixuan.Wu" <qixuan.wu@linux.alibaba.com> wrote: > > > Suppose there is one scenario that the system has 100 CPU(0~99). While CPU 0 is > > calling slow console, CPU 1~99 are calling printk at the same time. And suppose > > CPU 1 will be waiter, as per the patch, 2~99 will return directly. After CPU 0 finish > > it's log to console, it will return when it finds CPU 1 are waiting. Then CPU 1 need > > flush all logs of CPU(1~99) to the console, which may cause softlockup or rcu > > stall. Above scenario is very unusual and it's very unlikely to happen. > > Yes, people keep bringing up this scenario.
Yeah.
> It would require a single burst of printks to all CPUs.
That's one possibility. The other one is - console_sem locked by a preemptible context which gets scheduled out.
> And then no more printks after that. The last one will end up printing > the entire buffer out the slow console. The thing is, this is a bounded > time, and no printk will print more than one full buffer worth.
It can print more than "one full buffer worth". In theory and on practice.
> If this is a worry, then set the timeouts for the lockup detection to > be longer than the time it takes to print one full buffer with the > slowest console.
I see your point. But I still think that it makes sense to change that "print it all" approach. With more clear/explicit watchdog-dependent limits - we do direct printk for 1/2 (or 2/3) of a current watchdog threshold value and offload if there is more stuff in the logbuf. Implicit "logbuf size * console throughput" is harder to understand. Disabling watchdog because of printk is a bit too much of a compromise, probably.
IOW, is logbuf worth of messages so critically important after all that we are ready to jeopardize the system stability?
-ss
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