Bug 47341

Summary:	Wrong/impossible cputimes for migration processes
Product:	Timers	Reporter:	gunnar.eggen
Component:	Other	Assignee:	john stultz (john.stultz)
Status:	CLOSED CODE_FIX
Severity:	normal	CC:	a.p.zijlstra, alan, cat, jfaulkne, kernel, roberto.passuello
Priority:	P1
Hardware:	All
OS:	Linux
Kernel Version:	>=3.0.7	Subsystem:
Regression:	Yes	Bisected commit-id:
Attachments:	kernel config

Description gunnar.eggen 2012-09-10 19:54:08 UTC

Created attachment 79701 [details]
kernel config

The cputimes/load reported from top/ps etc. report extreme loads on kernel processes migration/x.   The problem isn't permanent as it takes a while to set off, but once it does it escalates and never stops.  

I am unable to locate what's triggering it, but I did a bisect and found the commit at fault: 
commit d670ec13178d0fd8680e6742a2bc6e04f28f87d8 - posix-cpu-timers: Cure SMP wobbles.

The load reported doesn't seem real, as there's no sign that the system is under heavy load or unresponsive.

Compiling with CONFIG_NO_HZ=N seems to help for some, but I've only seen a delay in the triggering of it.

Reproducible: Always

Steps to Reproduce:
1. Boot normally with kernel >= 3.0.7
2. Wait ~1 hour (unable to figure out what sets it off. Idle vs full load doesn't seem to matter)

Actual Results:  
Cpuload/cputime on some migration/x kernel processes show a constant load between 20-100%.  Cpu frequency scaling (ondemand) seems to be affected as it never scales down the cores/threads affected. 

Expected Results:  
Report normal load on migration processes. Allow ondemand to scale down.

The bug was first reported here: https://bugs.gentoo.org/show_bug.cgi?id=394487

Comment 1 john stultz 2012-09-11 17:49:17 UTC

Adding PeterZ, since he might have more context.

Comment 2 gunnar.eggen 2013-01-12 01:24:01 UTC

The issue seems to be resolved with version >=3.6.11. Changing status to resolved.