linux-kernel - Re: [PATCH 01/10] random: make 'add_interrupt

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Message-ID: <20120706130127.GB10798@thunk.org>
Date:	Fri, 6 Jul 2012 09:01:27 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Matt Mackall <mpm@...enic.com>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	w@....eu, ewust@...ch.edu, zakir@...ch.edu, greg@...ah.com,
	nadiah@...ucsd.edu, jhalderm@...ch.edu, tglx@...utronix.de,
	davem@...emloft.net, stable@...nel.org
Subject: Re: [PATCH 01/10] random: make 'add_interrupt_randomness()' do
 something sane

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@....edu> 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?

Thanks,

						- ted
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