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

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Date:   Wed, 17 Jan 2018 21:19:53 -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 10:53:37 +0900
Byungchul Park <byungchul.park@....com> wrote:

> Hello,
> 
> This is a thing simulating a wait for an event e.g.
> wait_for_completion() doing spinning instead of sleep, rather
> than a spinlock. I mean:
> 
>     This context
>     ------------
>     while (READ_ONCE(console_waiter)) /* Wait for the event */
>        cpu_relax();
> 
>     Another context
>     ---------------
>     WRITE_ONCE(console_waiter, false); /* Event */

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 why I said this's the exact case of cross-release. Anyway
> without cross-release, we usually use typical acquire/release
> pairs to cover a wait for an event in the following way:
> 
>     A context
>     ---------
>     lock_map_acquire(wait); /* Or lock_map_acquire_read(wait) */
>                             /* Read one is better though..    */
> 
>     /* A section, we suspect, a wait for an event might happen. */
>     ...
>     lock_map_release(wait);
> 
> 
>     The place actually doing the wait
>     ---------------------------------
>     lock_map_acquire(wait);
>     lock_map_acquire(wait);
> 
>     wait_for_event(wait); /* Actually do the wait */
> 
> You can see a simple example of how to use them by searching
> kernel/cpu.c with "lock_acquire" and "wait_for_completion".
> 
> However, as I said, if you suspect that cpu_relax() includes
> the wait, then it's ok to leave it. Otherwise, I think it
> would be better to change it in the way I showed you above.

I find your way confusing. I'm simulating a spinlock not a wait for
completion. A wait for completion usually initiates something then
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.

-- Steve