Bug 13583

Summary:	pdflush uses 5% CPU on otherwise idle system
Product:	IO/Storage	Reporter:	Paul Martin (pm)
Component:	Block Layer	Assignee:	Jens Axboe (axboe)
Status:	CLOSED OBSOLETE
Severity:	normal	CC:	akpm, alan, hydrologiccycle, oskarw85.spam, rjw
Priority:	P1
Hardware:	All
OS:	Linux
Kernel Version:	2.6.30	Subsystem:
Regression:	Yes	Bisected commit-id:
Bug Depends on:
Bug Blocks:	13070
Attachments:	opreport -d output /proc/meminfo contents opreport from 2.6.30.2

Description Paul Martin 2009-06-19 13:33:04 UTC

The second pdflush process is the top process in "top" output, varying between 3% and 6% CPU, even when no file system activity is happening, and this remains true after a forced "sync".

This is a regression from 2.6.29.

Comment 1 Jens Axboe 2009-06-19 18:41:52 UTC

Sounds odd. You should try and run eg oprofile to see where it's spending that time. Also attach /proc/meminfo.

Comment 2 Paul Martin 2009-06-19 22:41:45 UTC

Created attachment 22016 [details]
opreport -d output

Comment 3 Paul Martin 2009-06-19 22:42:15 UTC

Created attachment 22017 [details]
/proc/meminfo contents

Comment 4 Andrew Morton 2009-06-29 21:41:15 UTC

That oprofile report is a bit hard to parse.

I use

opreport -l /boot/vmlinux-$(uname -r) | head -50

to generate a nice sorted list of the hottest functions.  Can you do that please?

Comment 5 Paul Martin 2009-07-05 11:23:24 UTC

Sorry no oprofile report, but I have further information.

Setting /proc/sys/vm/laptop_mode to 2 causes disc activity every two seconds, even if there's nothing to do. This is on xfs, so it might be that filesystem has changed behaviour.

I notice that xfs has recently stopped using pdflush (commit 8b5403a6d772d340541cfb30a668fde119c40ac1). Could this be related?

Comment 6 Oskar Wróbel 2009-07-28 09:40:24 UTC

I encountered similar bug with 2.6.28 to 2.6.30 kernels, but pdflush actually uses 100% (all available) cpu on my system. Also I can't sync or umount root partition on ext4 because of this.
I will attach oprofiled report from 2.6.30 (with gentoo-patches, but it's mostly fbcondecor so it shouldn't matter).

Comment 7 Oskar Wróbel 2009-07-28 09:45:21 UTC

Created attachment 22517 [details]
opreport from 2.6.30.2

Comment 8 hydrologiccycle 2009-11-15 23:57:28 UTC

This is also a problem with 2.6.31.6.  The problem with pdflush seems to relate to the version of gcc.  If the kernel is built with gcc-4.3.4, then it will have this problem with pdflush.  If the kernel is built with gcc-4.1.2, there is no problem.

Comment 9 Alan 2012-06-08 15:25:39 UTC

Closing as obsolete, if this is incorrect please re-open and update the kernel version to a recent one