Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

Sergey Senozhatsky
2018-01-16T02:23:55Z

On (01/15/18 15:45), Petr Mladek wrote:
[..]
> > With the preempt_disable() there really isn't a delay. I agree, we
> > shouldn't let printk preempt (unless we have CONFIG_PREEMPT_RT enabled,
> > but that's another story).
> >
> > >
> > > so very schematically, for hand-off it's something like
> > >
> > > if (... console_trylock_spinning()) // grabbed the ownership
> > >
> > > << ... preempted ... >>
> > >
> > > console_unlock();
> >
> > Which I think we should stop, with the preempt_disable().
>
> Adding the preempt_disable() basically means to revert the already
> mentioned commit 6b97a20d3a7909daa06625 ("printk: set may_schedule
> for some of console_trylock() callers").
>
> I originally wanted to solve this separately to make it easier. But
> the change looks fine to me. Therefore we reached a mutual agreement.
> Sergey, do you want to send a patch or should I just put it at
> the end of this patchset?

you can add the patch.

[..]
> > I think adding the preempt_disable() would fix printk() but let non
> > printk console_unlock() still preempt.
>
> I would personally remove cond_resched() from console_unlock()
> completely.

hmm, not so sure. I think it's there for !PREEMPT systems which have
to print a lot of messages. the case I'm speaking about in particular
is when we register a CON_PRINTBUFFER console and need to console_unlock()
(flush) all of the messages we currently have in the logbuf. we better
have that cond_resched() there, I think.

> Sleeping in console_unlock() increases the chance that more messages
> would need to be handled. And more importantly it reduces the chance
> of a successful handover.
>
> As a result, the caller might spend there very long time, it might
> be getting increasingly far behind. There is higher risk of lost
> messages. Also the eventual taker might have too much to proceed
> in preemption disabled context.

yes.

> Removing cond_resched() is in sync with printk() priorities.

hmm, not sure. we have sleeping console_lock()->console_unlock() path
for PREEMPT kernels, that cond_resched() makes the !PREEMPT kernels to
have the same sleeping console_lock()->console_unlock().

printk()->console_unlock() seems to be a pretty independent thing,
unfortunately (!), yet sleeping console_lock()->console_unlock()
messes up with it a lot.

> The highest one is to get the messages out.
>
> Finally, removing cond_resched() should make the behavior more
> predictable (never preempted)

but we are always preempted in PREEMPT kernels when the current
console_sem owner acquired the lock via console_lock(), not via
console_trylock(). cond_resched() does the same, but for !PREEMPT.

-ss