linux-kernel - Re: [RFC][PATCHv6 00/12] printk: introduce printing kernel thread

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Message-ID: <20171214152551.GY3919388@devbig577.frc2.facebook.com>
Date:   Thu, 14 Dec 2017 07:25:51 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv6 00/12] printk: introduce printing kernel thread

Hello, Petr.

On Thu, Dec 14, 2017 at 03:27:09PM +0100, Petr Mladek wrote:
> Ah, I know that it was me who was pessimistic about Steven's approach[1]
> and persuaded you that offloading idea was still alive. But I am less
> sure now.

So, I don't really care which one gets in as long as the livelock
problem is fixed although to my obviously partial eyes the two
alternatives seem overly complex.  That said,

> My three main concerns about Steven's approach were:
> 
> 1. I was afraid that it might introduce new type of deadlocks.
> 
>    But it seems that it is quite safe after all.
> 
> 
> 2. Steven's code, implementing the hand shake, is far from trivial.
>    Few people were confused and reported false bugs.
> 
>    But the basic idea is pretty simple and straightforward. If
>    we manage to encapsulate it into few helpers, it might become
>    rather self-contained and maintainable. In each case, the needed
>    changes are much smaller than I expected.
> 
> 
> 3. Soft-lockups are still theoretically possible with Steven's
>    approach.
> 
>    But it seems to be quite efficient in many real life scenarios,
>    including Tetsuo's stress testing. Or am I wrong?

AFAICS, Steven's approach doesn't fix the livelock that we see quite
often in the fleet where we don't have a safe context to keep flushing
messages.  This isn't theoretical at all.  You simply don't have a
safe context on the cpu to go to.  I said I'd come back with a repro
case but haven't had a chance to yet.  I'll try to do it before the
end of the year, but idk this is pretty obvious.

Thanks.

-- 
tejun