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

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Date:   Thu, 14 Dec 2017 22:18:31 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Tejun Heo <tj@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>, 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
Subject: Re: [RFC][PATCHv6 00/12] printk: introduce printing kernel thread

On Fri, 15 Dec 2017 11:10:24 +0900
Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:

> Steven, your approach works ONLY when we have the following preconditions:
> 
>  a) there is a CPU that is calling printk() from the 'safe' (non-atomic,
>     etc) context
> 
>         what does guarantee that? what happens if there is NO non-atomic
>         CPU or that non-atomic simplky missses the console_owner != false
>         point? we are going to conclude
> 
>         "if printk() doesn't work for you, it's because you are holding it wrong"?
> 
> 
>         what if that non-atomic CPU does not call printk(), but instead
>         it does console_lock()/console_unlock()? why there is no handoff?
> 
>         CPU0				CPU1 ~ CPU10
> 					in atomic contexts [!]. ping-ponging console_sem
> 					ownership to each other. while what they really
> 					need to do is to simply up() and let CPU0 to
> 					handle it.
> 					printk
> 	console_lock()
> 	 schedule()
> 					...
> 					printk
> 					printk
> 					...
> 					printk
> 					printk
> 
> 					up()
> 
> 	// woken up
> 	console_unlock()
> 
>         why do we make an emphasis on fixing vprintk_printk()?

Where do we do the above? And has this been proven to be an issue? If
it has, I think it's a separate issue than what I proposed. As what I
proposed is to fix the case where lots of CPUs are doing printks, and
only one actually does the write.

> 
> 
>  b) non-atomic CPU sees console_owner set (which is set for a very short
>     period of time)
> 
>         again. what if that non-atomic CPU does not see console_owner?
>         "don't use printk()"?

May I ask, why are we doing the printk in the first place?

> 
>  c) the task that is looping in console_unlock() sees non-atomic CPU when
>     console_owner is set.

I haven't looked at the latest code, but my last patch didn't care
about "atomic" and "non-atomic" issues, because I don't know if that is
indeed an issue in the real world.

> 
> 
> IOW, we need to have
> 
> 
>    the right CPU (a) at the very right moment (b && c) doing the very right thing.
> 
> 
>    * and the "very right moment" is tiny and additionally depends
>      on a foreign CPU [the one that is looping in console_unlock()].
> 
> 
> 
> a simple question - how is that going to work for everyone? are we
> "fixing" a small fraction of possible use-cases?

Still sounds like you are ;-)

> 
> 
> 
> Steven, I thought we reached the agreement [**] that the solution we should
> be working on is a combination of prinkt_kthread and console_sem hand
> off. Simply because it adds the missing "there is a non-atomic CPU wishing
> to console_unlock()" thing.
> 
> 	lkml.kernel.org/r/20171108162813.GA983427@...big577.frc2.facebook.com
> 
> 	https://marc.info/?l=linux-kernel&m=151011840830776&w=2
> 	https://marc.info/?l=linux-kernel&m=151015141407368&w=2
> 	https://marc.info/?l=linux-kernel&m=151018900919386&w=2
> 	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'm still fine with the hybrid approach, but I want to see a problem
first before we fix it.

> 
> 
> what am I missing?

The reproducer. Let Tejun do the test with just my patch, and if it
still has problems, then we can add more logic to the code. I like to
take things one step at a time. What I'm seeing is that there was a
problem that could be solved with my solution, but during this process,
people have found hundreds of theoretical problems and started down the
path to solve each of them. I want to see a real bug, before we go down
the path of having to have external threads and such, to solve a bug
that we don't really know exists yet.

-- Steve