Re: [PATCH 01/10] random: make 'add_interrupt_randomness()' dosomething sane

From: Theodore Ts'o
Date: Fri Jul 06 2012 - 09:01:42 EST

On Thu, Jul 05, 2012 at 07:59:00PM -0700, Linus Torvalds wrote:
> On Thu, Jul 5, 2012 at 4:21 PM, Theodore Ts'o <tytso@xxxxxxx> wrote:
> > On Thu, Jul 05, 2012 at 05:31:22PM -0500, Matt Mackall wrote:
> >>
> >> It's better to mix and not credit than to not mix at all. Instead just
> >> check the fast count against HZ before the credit.
> >
> > I'm not sure what you mean by this?
> So what I think Matt meant was to check number of timer interrupts
> against the fast count.
> IOW, always call "add_interrupt_randomness()", but then in that
> function, when you determine the amount of entropy, check if there
> were non-timer interrupts in the last HZ cycle. If there were purely
> timer interrupts, you can still mix in the cycle information, but it's
> likely to be fairly weak.

I'm going to have to plead ignorance to how the timer code works these
days. With the advent of clockevents and NOHZ, it's gotten a lot more
complicated. What in the world is "fast count"? I've grepped for it,
and I can't find it.

I can simply not credit entropy of the timer is on the irq, but I
think you and Matt were suggesting more subtle. I just have no idea
how to tell if there were non-timer interrupts during the last HZ
cycle. Can you give me a hint?


- ted
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