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Date:	Tue, 10 Sep 2013 17:31:45 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Theodore Ts'o <tytso@....edu>,
	Stephan Mueller <smueller@...onox.de>,
	LKML <linux-kernel@...r.kernel.org>, dave.taht@...ferbloat.net,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] /dev/random: Insufficient of entropy on many architectures

On 09/10/2013 03:33 PM, Theodore Ts'o wrote:
> On Tue, Sep 10, 2013 at 03:08:12PM -0700, John Stultz wrote:
>> One other area you might look at is using the delta between when the
>> next hrtimer was scheduled for and when we actually expired it? That's
>> something we could cheaply calculate on every hrtimer expiration. Though
>> I probably should be hesitant with my suggestions, as I'm not well
>> versed in RNG theory.
> What we need is a time source which whose granularity is substantially
> finer-grained that the rate at which interrupts are delievered to the
> system (preferably by a factor at least 8 or 16).  It also needs to be
> fast enough (since we will be calling it on every single interrupt)
> that the overhead doesn't cause architecture maintainers to break out
> their torches and pitchforks and come looking for me.  :-)

Yea, that point about "every single interrupt" vs "every timer
interrupt". I suspect that if its every timer interrupt, this can be
done easily w/ clocksources as we already do that read, but every single
interrupt would have potential problems with various other devices with
high irq frequency.



> Reading from a cycle counter is therefore ideal; it doesn't need to be
> synchronized across CPU's, and I don't care if it gets stops ticking
> when the system is suspended, and I don't care if the rate at which it
> increment is dependent on CPU clock speed getting jacked up and down
> for by power management systems.  (And in fact, if it's going to cost
> extra overhead to correct for the CPU being suspended or running at a
> 1.6 MHz instead of 2.8 MHz, that's a bug, not a feature.)  We just
> need something fine-grained.
Right so get_cycles() really sounds like the right thing here.

Although I wonder if you run into issues with counters that wrap
quickly? Or does that not matter?


> The problem is what do we do on platforms that don't have a cycle
> counter.  Stephan Mueller has proposed using the "best" clocksource as
> a default fallback.  Which might be OK, but I still remember that
> really, REALLY angry customer who discovered that gettimeofday() was
> breathtakingly slow on an IBM X440 (at least, when they were calling
> it at super high rates).
Yea, I'm familiar with that box. :)


> I just have no idea what various clock sources might do on different
> architectures, and if many of the more older ones (i.e., sparc 32,
> m32, h8, etc.) are just going to fall back to jiffies, I'm not sure
> it's worth it.

Right. In many cases jiffies is all that we have. Thus the check for
timekeeping_valid_for_hres() can be used to at least flag that case. Not
that I know what you want to do if it comes up false.

thanks
-john

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