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

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Date:   Wed, 17 Jan 2018 15:29:20 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Tejun Heo <tj@...nel.org>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        rostedt@...e.goodmis.org, Byungchul Park <byungchul.park@....com>,
        Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

On (01/16/18 11:13), Petr Mladek wrote:
[..]
> IMHO, it would make sense if flushing the printk buffer behaves
> the same when called either from printk() or from any other path.
> I mean that it should be aggressive and allow an effective
> hand off.
> 
> It should be safe as long as foo_specific_work() does not take
> too much time.
> 
> From other side. The cond_resched() in console_unlock() should
> be obsoleted by the hand-shake code.

hm, let's not have too optimistic expectations. hand off works in very
specific conditions. console is not exclusively owned by printk, and
console_sem is not printk's own lock. even things like

	systemd -> n_tty_write -> do_output_char -> con_write

involves console_lock() and console_unlock(). IOW user space
logging/debugging can cause printk stalls, and vice versa.

by the way, do_con_write() explicitly calls console_conditional_schedule()
under console_sem, before it goes to console_unlock(). so the scope of
"situation normal, console_sem locked, the owner scheduled out" is much
bigger than just vprintk_emit() -> console_unlock(). IMHO.

and there are even more things there. personally, I don't think
that hand off is enough to obsolete anything in that area.

[...]
> They both were obvious solutions that helped to reduce the risk
> of soft-lockups. The first one handled evidently safe scenarios.
> The second one was even more aggressive. I would say that
> they both were more or less add-hoc solutions that did not
> take into account the other side effects (delaying output,
> even loosing messages).

agreed.

> I would not say that one is a diametric difference between them.
> Therefore if we remove one for a reason, we should think about
> reverting the other as well. But again. I am fine if we remove
> only one now.
> 
> Does this make any sense?

I see cond_resched() as a mirroring of console_lock()->console_unlock()
behaviour on PREEMPT systems, and as such it looks valid to me, so we
probably better keep it there. IMHO.

	-ss