linux-kernel - Re: [RFC PATCH v1 00/25] printk: new implementation

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Message-ID: <87zhefu6fr.fsf@linutronix.de>
Date:   Wed, 22 Jan 2020 11:33:12 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Eugeniu Rosca <erosca@...adit-jv.com>
Cc:     <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>,
        Andrew Gabbasov <andrew_gabbasov@...tor.com>,
        Sanjeev Chugh <sanjeev_chugh@...tor.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Dean Jenkins <dean_jenkins@...tor.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dirk Behme <dirk.behme@...bosch.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        <linux-serial@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Eugeniu Rosca <roscaeugeniu@...il.com>
Subject: Re: [RFC PATCH v1 00/25] printk: new implementation

On 2020-01-22, Eugeniu Rosca <erosca@...adit-jv.com> wrote:
> So, what's specific to R-Car3, based on my testing, is that the issue
> can only be reproduced if the printk storm originates on CPU0 (it does
> not matter if from interrupt or task context, both have been
> tested). If the printk storm is initiated on any other CPU (there are
> 7 secondary ones on R-Car H3), there is no regression in the audio
> quality/latency.
>
> I cannot fully explain this empirical observation, but it directs my
> mind to the following workaround, for which I have a PoC:
>  - employ vprintk_safe() any time CPU0 is the owner/caller of printk
>  - tie CPU0-private printk internal IRQ workers to another CPU
>
> The above makes sure nothing is printed to the serial console on
> behalf of CPU0. I don't even hope this to be accepted by community,
> but can you please share your opinion the idea itself is sane?

It is a problem-specific hack. You will need to be certain that CPU1-7
will never have problems with console printing storms.

Be aware that vprintk_safe() is not particularly reliable in many crash
scenarios. If seeing oops output on the console is important, this can
be a risky hack.

Also, be aware that it has its own config option for the safe buffer
size: PRINTK_SAFE_LOG_BUF_SHIFT

>> The printk rework focusses on making printk non-interfering by
>> decoupling console printing from printk() callers. However, the
>> console printing itself will still do just as much interrupt
>> disabling as before. That is driver-related, not printk-related.
>
> I didn't dive into the internals of this series, but decoupling the
> execution context of the serial driver from the execution context of
> the printk callers sounds very good to me (this is what i try to
> achieve via vanilla vprintk_safe). I wonder if it's easier to remove
> CPU0 from equation with this series applied.

Yes, it would be quite easy. The console printers run as dedicated
kthreads. It is only a matter of setting the CPU affinity for the
related kthread.

>> The linux-rt patches (which include this printk rework) *are* being
>> ported to mainline now. My recommendation is to continue using the
>> linux-rt patches (with PREEMPT_RT=y) until PREEMPT_RT is available
>> mainline.
>
> If there is any roadmap publicly available, I would appreciate a
> reference.

I am only aware of the quilt "series" file [0] that is roughly
documenting the status of the effort.

John Ogness

[0] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tree/patches/series?h=linux-5.4.y-rt-patches