linux-kernel - Re: [PATCH v1] watchdog: Use a reference cycle counter to avoid scaling issues

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 23 Apr 2015 17:51:33 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Don Zickus <dzickus@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Anton Blanchard <anton@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v1] watchdog: Use a reference cycle counter to avoid
 scaling issues

> We can just detect the deviation in the callback itself:
> 
>        u64 now = ktime_get_mono_fast_ns();
> 
>        if (now - __this_cpu_read(nmi_timestamp) < period)
>        	       return;
> 
>        __this_cpu_write(nmi_timestamp, now);
> 
> It's that simple.

It's a simple short term hac^wsolution. But if we had a (hypothetical) system with
let's say 10*TSC max you may end up with quite a few false ticks, as in 
unnecessary interrupts. With 100*TSC it would be really bad.

There were systems in the past that ran TSC at a much slower frequency,
such as the early AMD Barcelona systems.

So the problem may eventually come back if not solved properly.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/