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

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Message-ID: <CA+55aFxAe1pRN+jvdLveHQ+nNRNC6HDZ+S4Y9P202rxgNLN8pg@mail.gmail.com>
Date:	Thu, 5 Jul 2012 14:47:52 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Matt Mackall <mpm@...enic.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	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 5, 2012 at 2:39 PM, Matt Mackall <mpm@...enic.com> wrote:
>
> From my read, this code path gets called on timer interrupts too.

That's hopefully never true for any normal cases (timers are *very*
special - they tend to go through their own architecture-specific
stuff). On modern PC's, for example, the timers happen through the
local apic timers directly.

In fact, with SMP, it's a really bad idea to use a normal irq for
timer interrupts, since you really really want per-CPU-core timers.

But yes, we should probably make sure that *if* the architecture uses
regular interrupts for timers we don't count them. The problematic
embedded platforms are often pretty crap hardware: even when they are
SMP, they might use an external timer irq (and then we broadcast the
thing). I think we have the IRQF_TIMER flag for that, so we could add
a check for that.

                 Linus
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