From: Matthew Whitehead <tedheadster@gmail.com>
To: john.stultz@linaro.org, tglx@linutronix.de, linux-kernel@vger.kernel.org
Cc: rostedt@goodmis.org, mwhitehe@redhat.com
Subject: nohz problem with idle time on old hardware
Date: Wed, 13 Nov 2013 06:39:27 -0500 [thread overview]
Message-ID: <20131113113927.GA13875@mwhitehe.csb> (raw)
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@gmail.com>
next reply other threads:[~2013-11-13 11:39 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-11-13 11:39 Matthew Whitehead [this message]
2013-11-13 14:02 ` nohz problem with idle time on old hardware Thomas Gleixner
2013-11-13 15:21 ` Steven Rostedt
2013-11-13 15:31 ` Steven Rostedt
2013-11-13 15:50 ` Thomas Gleixner
2013-11-13 16:17 ` Paul E. McKenney
2013-11-13 15:57 ` Steven Rostedt
2013-11-13 16:07 ` Thomas Gleixner
2013-11-13 16:12 ` Steven Rostedt
2013-11-13 16:18 ` Paul E. McKenney
2013-11-13 16:23 ` Steven Rostedt
2013-11-13 16:35 ` Paul E. McKenney
2013-11-13 20:01 ` Thomas Gleixner
2013-11-13 20:07 ` Steven Rostedt
2013-11-13 21:49 ` Matthew Whitehead
2013-11-18 22:44 ` Paul E. McKenney
2013-11-19 18:07 ` [tip:timers/urgent] NOHZ: Check for nohz active instead of nohz enabled tip-bot for Thomas Gleixner
2014-04-09 13:51 ` nohz problem with idle time on old hardware Viresh Kumar
2014-04-09 14:31 ` Steven Rostedt
2014-04-09 15:20 ` Viresh Kumar
2014-04-09 15:29 ` Steven Rostedt
2014-04-09 15:31 ` Steven Rostedt
2014-04-09 15:34 ` Viresh Kumar
2014-04-09 15:39 ` Steven Rostedt
2014-04-09 15:56 ` Viresh Kumar
2014-04-09 16:15 ` Steven Rostedt
2014-04-09 15:11 ` Frederic Weisbecker
2013-11-13 16:21 ` Thomas Gleixner
2013-11-13 16:31 ` Steven Rostedt
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20131113113927.GA13875@mwhitehe.csb \
--to=tedheadster@gmail.com \
--cc=john.stultz@linaro.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mwhitehe@redhat.com \
--cc=rostedt@goodmis.org \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).