linux-kernel - nohz problem with idle time on old hardware

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-next>] [day] [month] [year] [list]

Message-ID: <20131113113927.GA13875@mwhitehe.csb>
Date:	Wed, 13 Nov 2013 06:39:27 -0500
From:	Matthew Whitehead <tedheadster@...il.com>
To:	john.stultz@...aro.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org
Cc:	rostedt@...dmis.org, mwhitehe@...hat.com
Subject: nohz problem with idle time on old hardware

I was testing the 3.12 kernel on some _old_ hardware and I uncovered a bug.
It arises when nohz=on and goes away with nohz=off. On a crusty dual Pentium-1 
system that is completely idle, the sar utility reports 0% idle time on cpu0 
and 100% idle on cpu1. Cpu0 _should_ also be reporting 100% idle, but instead
it reports around 75% system time and 25% user time.

The problem was diagnosed by Steve Rostedt with help from John Stultz. The old
system declares the dual TSCs unstable, and backs down to a timesource of 
refined-jiffies. Apparently refined-jiffies and jiffies are not a usable 
timesourcefor nohz, but we don't check for that case because most modern
systems have several reliable hardware timesources.

John suggested that we turn off nohz unless a usable hardware timesource is
present.

- Matthew Whitehead <tedheadster@...il.com>

--
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/