svn commit: r297190 - head/sys/kern

[Edward Tomasz Napierała]

On 0324T1609, Alexander Motin wrote:
> On 24.03.16 15:42, Edward Tomasz Napierała wrote:
> > On 0324T1032, Jean-Sébastien Pédron wrote:
> >> On 23/03/2016 18:45, Edward Tomasz Napierala wrote:
> >>>> So maybe callouts are disabled in this situation. If there is a way to
> >>>> detect that, then vt(4) can go back to a "synchronous mode" where it
> >>>> refreshes the screen after each typed character, like it does when ddb
> >>>> is active.
> >>>
> >>> Looks like that's the case: for some reason the callouts don't work.
> >>> This trivial hack is a (mostly) working workaround:
> >>>
> >>> Index: svn/head/sys/kern/kern_cons.c
> >>> ===================================================================
> >>> --- svn/head/sys/kern/kern_cons.c	(revision 297210)
> >>> +++ svn/head/sys/kern/kern_cons.c	(working copy)
> >>> @@ -430,6 +430,7 @@ cngets(char *cp, size_t size, int visible)
> >>>  	lp = cp;
> >>>  	end = cp + size - 1;
> >>>  	for (;;) {
> >>> +		pause("meh", 1);
> >>
> >> Could you please explain how this works to me? Does calling pause() here
> >> give a chance to interrupt handlers or other threads of running?
> > 
> > It looks like it allows the callout to run.  I've did an experiment
> > and added a simple callout that printed something each second; during
> > the root mount prompt it doesn't get run unless you type '.', which
> > calls pause(9).
> 
> Kernel threads run with absolute priorities, so if somehow this threads
> happen to have higher or equal priority then callout thread, or the
> kernel is built without PREEMPTION, then the last may never be executed
> until this thread get to sleep or at least call sched_yield().

The callout's td_priority seems to be 40; the thread running the prompt
is 84, so it's lower.

I've just noticed another curious thing, though: when you press ScrLk,
the screen gets immediately refreshed; also, pressing arrows works just
the way it should.  In other words, the refresh is broken except for
the ScrlLk mode, where it works as it should.