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

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Message-ID: <CAHk-=whNcrysjJYPDFhKAc4tvd80XGAyh95oZeAY6bmzpv3G-A@mail.gmail.com>
Date:   Thu, 8 Aug 2019 18:15:57 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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>,
        Thomas Gleixner <tglx@...utronix.de>,
        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, 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..

            Linus