linux-kernel - Powertop shows events/0 waking at high rate due to ptys

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Message-ID: <4CA0D660.3010804@goop.org>
Date:	Mon, 27 Sep 2010 10:37:36 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Greg KH <gregkh@...e.de>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Ian Jackson <Ian.Jackson@...citrix.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Powertop shows events/0 waking at high rate due to ptys

 Hi,

I noticed on one of my systems that powertop was showing events/0 waking
at a high rate (~2000 times/sec, HZ=1000).  After a lot of tracing I
found it was due to writes pending to ptys which hadn't been read.

Specifically, in flush_to_ldisc(), these lines seem to be causing the
repeated wakeups:

			if (!tty->receive_room) {
				schedule_delayed_work(&tty->buf.work, 1);
				break;
			}

After reading the pending writes the system settles down and wakeups
drops to a normal low level.

The program provoking this was Xen's xenconsoled, which manages running
domains' consoles; the unread output was various domains console
output.  This doesn't seem to be related to Xen at all: Ian Jackson put
together a small standalone program (attached) which reproduces the
problem; he reports that this wakeup behaviour is a regression compared
to a 2.6.16 Debian kernel.

Compile ptypolling with: gcc -o ptypolling ptypolling.c -g -lutil

After running ptypolling, powertop shows events/0 waking at a high
rate.  ptypolling itself is blocked in a select() and is idle.

Thanks,
    J

View attachment "ptypolling.c" of type "text/x-csrc" (1176 bytes)