linux-kernel - Excessive latency for dev

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Message-ID: <Pine.LNX.4.44L0.1205231627140.1248-100000@iolanthe.rowland.org>
Date:	Wed, 23 May 2012 16:37:29 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Kernel development list <linux-kernel@...r.kernel.org>
cc:	Jason Baron <jbaron@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, Greg KH <greg@...ah.com>
Subject: Excessive latency for dev_dbg?

I just tried doing some testing of 3.4.  Sequences of literally
adjacent dev_dbg() calls -- with local interrupts disabled -- show up
in the log buffer spaced at time intervals between 1 and 4
milliseconds!

This is with CONFIG_DYNAMIC_DEBUG not enabled, and CONFIG_PRINTK_TIME,
CONFIG_PREEMPT, CONFIG_HIGH_RES_TIMERS, and CONFIG_HPET_TIMER all set
to y.

What's going on here?  Did printk suddenly slow down a lot?  Or has the
timer code gone crazy?

Alan Stern

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