linux-kernel - Re: [PATCH v5 1/2] printk: Add console owner and waiter logic to load balance console writes

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Message-ID: <20180118102139.43c04de5@gandalf.local.home>
Date:   Thu, 18 Jan 2018 10:21:39 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        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,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tejun Heo <tj@...nel.org>, Pavel Machek <pavel@....cz>,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH v5 1/2] printk: Add console owner and waiter logic to
 load balance console writes

On Thu, 18 Jan 2018 13:01:46 +0900
Byungchul Park <byungchul.park@....com> wrote:

> > I disagree. It is like a spinlock. You can say a spinlock() that is
> > blocked is also waiting for an event. That event being the owner does a
> > spin_unlock().  
> 
> That's exactly what I was saying. Excuse me but, I don't understand
> what you want to say. Could you explain more? What do you disagree?

I guess I'm confused at what you are asking for then.


> > I find your way confusing. I'm simulating a spinlock not a wait for
> > completion. A wait for completion usually initiates something then  
> 
> I used the word, *event* instead of *completion*. wait_for_completion()
> and complete() are just an example of a pair of waiter and event.
> Lock and unlock can also be another example, too.
> 
> Important thing is that who waits and who triggers the event. Using the
> pair, we can achieve various things, for examples:
> 
>     1. Synchronization like wait_for_completion() does.
>     2. Control exclusively entering into a critical area.
>     3. Whatever.
> 
> > waits for it to complete. This is trying to get into a critical area
> > but another task is currently in it. It's simulating a spinlock as far
> > as I can see.  
> 
> Anyway it's an example of "waiter for an event, and the event".
> 
> JFYI, spinning or sleeping does not matter. Those are just methods to
> achieve a wait. I know you're not talking about this though. It's JFYI.

OK, if it is just FYI.

-- Steve