Re: [PATCH v4] printk: Add console owner and waiter logic to load balance console writes

From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Byungchul Park <byungchul.park@lge.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	LKML <linux-kernel@vger.kernel.org>,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	Cong Wang <xiyou.wangcong@gmail.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
	Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	yuwang.yuwang@alibabab-inc.com,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jan Kara <jack@suse.cz>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	rostedt@rostedt.homelinux.com, kernel-team@lge.com
Subject: Re: [PATCH v4] printk: Add console owner and waiter logic to load balance console writes
Date: Tue, 28 Nov 2017 15:23:26 +0900	[thread overview]
Message-ID: <20171128062326.GA1907@jagdpanzerIV> (raw)
In-Reply-To: <20171127084822.GA15859@X58A-UD3R>

Hi,

On (11/27/17 17:48), Byungchul Park wrote:
[..]
> > +				/* Owner will clear console_waiter on hand off */
> > +				while (READ_ONCE(console_waiter))
> > +					cpu_relax();
> > +
> > +				spin_release(&console_owner_dep_map, 1, _THIS_IP_);
> > +				printk_safe_exit_irqrestore(flags);
> > +
> > +				/*
> > +				 * The owner passed the console lock to us.
> > +				 * Since we did not spin on console lock, annotate
> > +				 * this as a trylock. Otherwise lockdep will
> > +				 * complain.
> > +				 */
> > +				mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
> 
> I'm afraid if it's ok even not to lock(or trylock) actually here. Is there
> any problem if you call console_trylock() instead of mutex_acquire() here?

console_trylock() will not work. console_trylock() implies that the
current printing process does up(), which a) opens a race with possible
console_lock()/console_trylock() from foreign CPUs, and b) additionally
wakes up a task from console_sem wait list [if there was one]. so chances
are some other CPU potentially can acquire the lock ahead of waiter,
forcing the waiter to continue spinning on console_sem. and the bad news
are a) it's spinning with local IRQs disabled and b) that another CPU,
which has acquired the console_sem, can schedule under console_sem.

anyway, we are not going to merge this patch. we already have discussed
that in V3 thread:

https://marc.info/?l=linux-kernel&m=151019815721161&w=2
https://marc.info/?l=linux-kernel&m=151020275921953&w=2
https://marc.info/?l=linux-kernel&m=151020404622181&w=2
https://marc.info/?l=linux-kernel&m=151020565222469&w=2

I took some parts of the Steven's patch set, tho, and backported them
to the current printk_kthread series.

	-ss