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

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Message-ID: <20180116061013.GA19801@jagdpanzerIV>
Date:   Tue, 16 Jan 2018 15:10:13 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...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

Hi,

On (01/15/18 12:50), Petr Mladek wrote:
> On Mon 2018-01-15 11:17:43, Petr Mladek wrote:
> > PS: Sergey, you have many good points. The printk-stuff is very
> > complex and we could spend years discussing the perfect solution.
> 
> BTW: One solution that comes to my mind is based on ideas
> already mentioned in this thread:
> 
> void console_unlock(void)
> {
> 	disable_preemtion();
> 
> 	while(pending_message) {
> 
> 	    call_console_drivers();
> 
> 	    if (too_long_here() && current != printk_kthread) {
> 	       wake_up_process(printk_kthread())
> 
> 	}
> 
> 	enable_preemtion();
> }

unfortunately disabling preemtion in console_unlock() is a bit
dangerous :( we have paths that call console_unlock() exactly
to flush everything (not only new pending messages, but everything)
that is in logbuf and we cannot return from console_unlock()
preliminary in that case.

> bool too_long_here(void)
> {
> 	return should_resched();
> or
> 	return spent_here() > 1 / HZ / 2;
> or
> 	what ever we agree on
> }
> 
> 
> int printk_kthread_func(void *data)
> {
> 	while(1) {
> 		 if (!pending_messaged)
> 			schedule();
> 
> 		if (console_trylock_spinning())
> 			console_unlock();
> 
> 		cond_resched();
> 	}
> }

overall that's very close to what I have in one of my private branches.
console_trylock_spinning() for some reason does not perform really
well on my made-up internal printk torture tests. it seems that I
have a much better stability (no lockups and so on) when I also let
printk_kthread to sleep on console_sem(). but I will look further.

	-ss