linux-kernel - Re: Getting interrupt every million cache misses

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Date:   Thu, 27 Oct 2016 10:46:38 +0200
From:   Pavel Machek <pavel@....cz>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     acme@...hat.com, kernel list <linux-kernel@...r.kernel.org>,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com
Subject: Re: Getting interrupt every million cache misses

Hi!

> > I'd like to get an interrupt every million cache misses... to do a
> > printk() or something like that. As far as I can tell, modern hardware
> > should allow me to do that. AFAICT performance events subsystem can do
> > something like that, but I can't figure out where the code is / what I
> > should call.
> > 
> > Can someone help?
> 
> Can you go back one step and explain why you would want this? What use
> is a printk() on every 1e6-th cache miss.

First, thanks for quick reply.

And actually, printk() is not needed, udelay(50msec) is. Reason is,
that DRAM becomes unreliable if about milion cache misses happen in
under 64msec -- so I'd like to slow the system down in such cases to
prevent bug from biting me.

(Details are here
https://googleprojectzero.blogspot.cz/2015/03/exploiting-dram-rowhammer-bug-to-gain.html
). Bug is exploitable to get local root; it is also exploitable to
gain local code execution from javascript... so it is rather severe.

> That is, why doesn't:
> 
>  $ perf record -e cache-misses -c 1000000 -a -- sleep 5
> 
> suffice?

Thanks for the pointer... I'd really like to do this from kernel, so
that I can "almost synchronously" stop the execution when excessive
cache isses happen.

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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