linux-kernel - Re: [PATCH v2] printk: Add console owner and waiter logic to load balance console writes

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Message-ID: <20171103071824.1cf629ee@vmware.local.home>
Date:   Fri, 3 Nov 2017 07:18:24 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.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>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "yuwang.yuwang" <yuwang.yuwang@...baba-inc.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH v2] printk: Add console owner and waiter logic to load
 balance console writes

On Fri, 3 Nov 2017 11:19:53 +0100
Jan Kara <jack@...e.cz> wrote:

> Hi,
> 
> On Thu 02-11-17 13:06:05, Steven Rostedt wrote:
> > +			if (spin) {
> > +				/* We spin waiting for the owner to release us */
> > +				spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
> > +				/* Owner will clear console_waiter on hand off */
> > +				while (!READ_ONCE(console_waiter))
> > +					cpu_relax();  
> 
> Hum, what prevents us from rescheduling here? And what if the process
> stored in console_owner is scheduled out? Both seem to be possible with
> CONFIG_PREEMPT kernel? Unless I'm missing something you will need to
> disable preemption in some places...

Yes you are missing something ;-)

> 
> Other than that I like the simplicity of your approach.
> 
> 								Honza
> 
> > +
> > +				spin_release(&console_owner_dep_map, 1, _THIS_IP_);
> > +				printk_safe_exit_irqrestore(flags);

The above line re-enables interrupts. And is done for both the
console_owner and the console_waiter. These are only held with
interrupts disabled. Nothing will preempt it. In fact, if it could,
lockdep would complain (it did in when I screwed it up at first ;-)


-- Steve