Re: [RFC PATCH v4 9/9] printk: use a new ringbuffer implementation

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	John Ogness <john.ogness@linutronix.de>,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andrea Parri <andrea.parri@amarulasolutions.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Brendan Higgins <brendanhiggins@google.com>
Subject: Re: [RFC PATCH v4 9/9] printk: use a new ringbuffer implementation
Date: Fri, 9 Aug 2019 13:20:50 -0700	[thread overview]
Message-ID: <CAHk-=wgTEs7AFTxQPsqpHS1JN9OU1eiY6eCdFjxnKSU_1POx=g@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1908092141490.21433@nanos.tec.linutronix.de>

On Fri, Aug 9, 2019 at 1:07 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> I'm all for it. I just tried it out and the ratio was 3 out of 5 retained
> the data +/- a few bitflips with ~2 seconds power off. The other two were
> the laptop and that server machine which wipes everything.

Perfect. That actually says "the theory works". My desktop worked only
on warm reboot - which isn't really the interesting case (it does
cover things like triple boots etc and "press reset button when it
hangs, so it *can* be helpful, but even on desktops reset buttons seem
to be getting less common).

But yes, the whole thing where BIOSes wipe everything is problematic,
but that's where I just need to ping the right people inside Intel
again.

I did send the patch to inside Intel earlier, but I think the timing
for that might have been bad (people were on vacation), so I should
just reach out to more Intel people.

It would be better to have a more polished patch (the whole "fixed
address at around 12GB physical" really is such a horrible hack), but
I dreaded actually parsing the e280 memory map to do some "static for
one particular configuration" thing.

I should just do that and have something that Intel HW and FW people
can test on any hardware.

> If that can be avoided with some ACPI tweak especially on the laptop, that
> would be great. I'm not so worried about the server case.

Yeah, the server case I think we have covered other ways. Plus people
running them tend to have serious developer resources anyway.

They might still use something like this for some convenient
first-order debugging if we end up having generally available, of
course, but the target really is "random laptop or home user that uses
a distro and can't be expected to even try to sanely report - much
less debug - a hung machine condition".

                     Linus