On Sat 2017-04-08 00:13:06, Sergey Senozhatsky wrote: > On (04/07/17 14:44), Pavel Machek wrote: > [..] > > > [..] > > > > I believe "spend at most 2 seconds in printk(), then print a warning > > > > and offload" is a solution closer to what we had before. > > > > > > a warning here can be very noisy. > > > > Well, on normally-configured it should be ok. We don't commonly see > > printk problems... If it is too noisy, perhaps we should increase from > > 2 seconds, but I don't think it will be problem. > > we are looking at different typical setups :) serial console being 45 > seconds behind logbuf does not surprise me anymore. > > [..] > > > what we have been thinking about is something like printk-stall detection. > > > we probably (there are some if-s) can detect in printk() that offloading > > > does not work and we must automatically switch to printk_emergency mode. > > > that, in theory, can relax our dependency on printk_emergency_begin/end > > > being in the right place at the right time. need to think more about it. > > > > So... I don't really like the begin/end interface. I would rather have > > printk_emergency(KERN_ ...). > > you mean a single printk_emergency() switches printk to emergency mode > or printk_emergency(KERN_ ... ) is a single message that must be printed > in emergency mode?
The latter. Having state is ugly.
> printk() depends on console_trylock(). we can't expect printk_emergency(KERN_
> ...)
> to always do more than just log_store().
>
> the idea behind begin/end interface is that you can do
>
> emergency_begin
> printk
> pr_cont
> pr_cont
> pr_cont
> printk
> dump_stack
> emergency_end
>
> with out the need of rewriting dump_stack() or anything else to use
> printk_emergency(). we, for example, do this in sysrq patch from this
> series.
Well.. I guess it is less work to include emergency_begin/end() but I
also believe result will state-less solution will be cleaner.
> > Second... I don't think "stuck detector" is that helpful. What I
> > usually seen was some rather innocent kernel message followed by
> > hard-lock. That's where "message delayed" is useful..
>
> a side note,
> that's rather unclear to me how would "message delayed" really help.
> if your system hard-lockup so badly and there are no printk messages
> even from NMI watchdog, then we won't be able to print that message.
We are talking about
printk("unusual condition");
do_something_clever(); /* Which unfortunately hard-crashes the machine */
that works with my proposal, but not with yours. Seen it happen many
times before.
Pavel
--
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