linux-kernel - Re: [RFC][PATCH -next 2/2] printk: set may_schedule for some of console

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Date:	Fri, 22 Jan 2016 10:48:44 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Jan Kara <jack@...e.com>,
	Kyle McMartin <kyle@...nel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Calvin Owens <calvinowens@...com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH -next 2/2] printk: set may_schedule for some of
 console_trylock callers

On Thu 2016-01-21 14:51:46, Sergey Senozhatsky wrote:
> On (01/21/16 10:25), Sergey Senozhatsky wrote:
> [..]
> > > First, the message "This stops the holder of console_sem just where we
> > > want him" is suspitious.
> > 
> > this comment is irrelevant, as of today. it was, a long time ago, because
> > the entire thing was a bit different (linux-2.4.21 kernel/printk.c)
> > 
> >         /* This stops the holder of console_sem just where we want him */
> >         spin_lock_irqsave(&logbuf_lock, flags);
> > 
> > logbuf_lock does stop the holder, local_irq_save() does not, you are right.
> 
> I meant 'irrelevant on its current place'.

Thanks a lot for confirmation.

> [..]
> > > As a result, I think that we do not need the extra checks
> > > for the save context in printk(). IMHO, it is safe to remove
> > > all the console_may_schedule stuff and also remove the extra
> > > preempt_disable/preempt_enable() in vprintk_emit().
> > > 
> > > Or did I miss anything?
> > 
> > hm... I suspect the reason we have console_may_schedule is
> > console_conditional_schedule() - console_sem owner may want
> > to have an internal logic to re-schedule [fwiw], while still
> > holding the console_sem. tty/vt/vt.c or video/console/fbcon.c
> > for example. (in 2.4 kernel: video/fbcon.c and char/console.c).
> > 
> > cond_resched() helps in console_unlock(); console_conditional_schedule()
> > is called after console_lock() and _before_ console_unlock()....
> 
> for CONFIG_PREEMPT_COUNT kernel we can do something like
> 
> +void __sched console_conditional_schedule(void)
> +{
> +       if (!oops_in_progress && preemptible() && !rcu_preempt_depth())
> +               cond_resched();
> +}
> 
> and in console_unlock()
> 
> -               if (do_cond_resched)
> -                       cond_resched();
> +               console_conditional_schedule();
>
>
> but for !CONFIG_PREEMPT_COUNT we can't. because of currently held spin_locks/etc
> that we don't know about.

Ah, I was not aware that we did not have information about preemption
without PREEMPT_COUNT.


> `console_may_schedule' carries a bit of important information for
> console_conditional_schedule() caller. if it has acquired console_sem
> via console_lock() - then it can schedule, if via console_trylock() - it cannot.
> 
> the last `if via console_trylock() - it cannot' rule is not always true,
> we clearly can have printk()->console_unlock() from non-atomic contexts
> (if we know that its non-atomic, which is not the case with !PREEMPT_COUNT).

By other words, we could automatically detect save context for
cond_resched() only if PREEMPT_COUNT is enabled. Otherwise, we need to
keep the current logic (heuristic). Do I get it correctly, please?

I would personally wait a bit for Jack's async console printing.
It will call console only if oops_in_progress is set. It means that
this partial optimization won't be needed at all.

The other (first) patch still makes sense in the simplified form.


Best Regards,
Petr