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

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Message-ID: <alpine.DEB.2.21.1908091306520.21433@nanos.tec.linutronix.de>
Date:   Fri, 9 Aug 2019 13:15:05 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Brendan Higgins <brendanhiggins@...gle.com>
Subject: Re: [RFC PATCH v4 9/9] printk: use a new ringbuffer implementation

On Thu, 8 Aug 2019, Linus Torvalds wrote:

> On Thu, Aug 8, 2019 at 5:48 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > I've never tried, but are you saying that even with the "10 second
> > hold" the laptop's DRAM may still have old data that is accessible?
> 
> The power doesn't go off when you *start* the 10s hold. It goes off
> after ten seconds.
> 
> So power is off only for the time it then takes you to press the power
> button again to turn it on again. So just a second or two if you react
> quickly to the "ok, power light finally went off". Longer if you
> don't.
> 
> But yes, DRAM has retention time in the seconds. See for example
> 
>     https://www.pdl.cmu.edu/PDL-FTP/NVM/dram-retention_isca13.pdf
> 
> and look at the kinds of times they are looking at - their graphs
> aren't in  milliseconds, they are in 1-5 seconds (and the retention is
> pretty high for that time).
> 
> But I don't know what a power-off-in-laptop scenario really looks like..

That's random behaviour. It's hardware & BIOS & value add. What do you
expect?

I tried on a few machines. My laptop does not retain any useful information
and on some server box (which takes ages to boot) the memory is squeaky
clean, i.e. the BIOS wiped it already. Some others worked with a two second
delay between turning the remote power switch on and off.

Thanks,

	tglx