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Jos.v.W.
Status: Interested
Joined: 25 Jan 2011
Posts: 41
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Posted: Dec 26, 12, 4:10    Strange cpu values for task rcu_preempt
After some time of computing I get some strange %cpu and cpu time values for task rcu_preempt, PID 12:

:: Code ::

ps axo pcpu,comm,pid,user | sort -nr | head -n 3
373415872 rcu_preempt   12 root
58.8 einsteinbinary_  1932 boinc
58.6 einsteinbinary_  1934 boinc

ps axo time,comm,pid,user | sort -nr | head -n 3
213503-23:34:55 rcu_preempt 12 root
00:48:09 einsteinbinary_  1932 boinc
00:48:00 einsteinbinary_  1934 boinc


When using liquorix kernel 3.6 it was rcuc/0, PID 9, who showed similar weird values.
Could this issue be related to the BFS task scheduler since I never noticed this when using a Debian kernel?

My system:
:: Code ::

inxi -bAxx
System:    Host: debian Kernel: 3.7.0-1.dmz.1-liquorix-amd64 x86_64 (64 bit, gcc: 4.7.2)
           Desktop: Xfce 4.10.0git-1919aa9 (Gtk 2.24.13) dm: lightdm Distro: Debian GNU/Linux 7.0
Machine:   System: Acer (portable) product: TravelMate 4530 version: V1.11 serial: LXTPN0C01491707B691601
           Mobo: Acer model: Elgon serial: LXTPN0C01491707B691601 Bios: Acer version: V1.11 date: 08/06/2008
           Chassis: type: 10 serial: LXTPN0C01491707B691601
CPU:       Dual core AMD Athlon X2 QL-64 (-MCP-) clocked at 2100.00 MHz
Graphics:  Card: Advanced Micro Devices [AMD] nee ATI RS780M/RS780MN [Mobility Radeon HD 3200 Graphics] bus-ID: 01:05.0 chip-ID: 1002:9612
           X.org: 1.13.99.901 drivers: ati,radeon tty size: 156x47 Advanced Data: N/A for root
Audio:     Card-1: Advanced Micro Devices [AMD] nee ATI RS780 HDMI Audio [Radeon HD 3000-3300 Series]
           driver: snd_hda_intel bus-ID: 01:05.1 chip-ID: 1002:960f
           Card-2: Advanced Micro Devices [AMD] nee ATI SBx00 Azalia (Intel HDA)
           driver: snd_hda_intel bus-ID: 00:14.2 chip-ID: 1002:4383
           Sound: Advanced Linux Sound Architecture ver: k3.7.0-1.dmz.1-liquorix-amd64
Network:   Card-1: Broadcom NetXtreme BCM5764M Gigabit Ethernet PCIe
           driver: tg3 ver: 3.125 bus-ID: 06:00.0 chip-ID: 14e4:1684
           Card-2: Ralink RT2790 Wireless 802.11n 1T/2R PCIe driver: rt2800pci ver: 2.3.0 bus-ID: 05:00.0 chip-ID: 1814:0781
Drives:    HDD Total Size: 250.1GB (25.5% used)
Info:      Processes: 151 Uptime: 28 min Memory: 1074.8/3698.0MB Runlevel: 5 Gcc sys: 4.7.2
           Client: Shell (bash 4.2.37) inxi: 1.8.24
damentz
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Joined: 09 Sep 2008
Posts: 521
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Posted: Dec 26, 12, 17:25    
Could be, yes. BFS accounts for cpu time much differently than CFS. I'll keep looking a this, it could also be that BFS performs worse with rcu_preempt, and your output is a symptom.
damentz
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Joined: 09 Sep 2008
Posts: 521
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Posted: Dec 26, 12, 19:17    
New kernel is out without rcu_boost, a feature that's known to conflict with BFS. Can you see if you get the same anomalies?
Jos.v.W.
Status: Interested
Joined: 25 Jan 2011
Posts: 41
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Posted: Dec 27, 12, 2:09    
I installed the new kernel.
After 3 hours of computing, no abnormal values.
Looks good but to be sure we'll have to wait until tomorrow.
Jos.v.W.
Status: Interested
Joined: 25 Jan 2011
Posts: 41
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Posted: Dec 28, 12, 0:23    
One day later, again the same anomaly:

:: Code ::

ps axo pcpu,comm,pid,user | sort -nr | head -n 2
28343414 rcu_preempt     9 root
67.5 einsteinbinary_  1714 boinc

ps axo time,comm,pid,user | sort -nr | head -n 2
213503-23:37:07 rcu_preempt  9 root
12:11:50 einsteinbinary_  1714 boinc


Should I distrust BFS?
damentz
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Posts: 521
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Posted: Dec 30, 12, 0:52    
Can you try 1.dmz.4? I the time accounting fix in BFS v426 actually created more bugs than it fixed. Thats been reverted in the newest package.
Jos.v.W.
Status: Interested
Joined: 25 Jan 2011
Posts: 41
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Posted: Dec 30, 12, 4:24    
I already tried 1.dmz.4 and almost immediately got a
similar abnormal rcu_preempt %cpu value.
Running Debian's linux-image-3.7-trunk-amd64, the task manager doesn't show any weird value.
Jos.v.W.
Status: Interested
Joined: 25 Jan 2011
Posts: 41
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Posted: Jan 1, 13, 5:42    
I installed linux-image-3.7.0-1.dmz.5 and get:

:: Code ::

ps axo pcpu,comm,pid,user | sort -nr | head -n 4
215365543 jbd2/sda5-8 1353 root
202384220 rcu_preempt    9 root
56.3 einstein_S6LV1_  1711 boinc
55.6 hsgamma_FGRP2_0  1713 boinc

ps axo time,comm,pid,user | sort -nr | head -n 4
213503-23:34:40 rcu_preempt  9 root
213503-23:34:33 jbd2/sda5-8 1353 root
00:16:36 einstein_S6LV1_  1711 boinc
00:16:26 hsgamma_FGRP2_0  1713 boinc


Another BFS kernel which I have tried, linux-image-3.7.1-pf_1_amd64.deb doesn't show these abnormal values.
damentz
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Joined: 09 Sep 2008
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Posted: Jan 1, 13, 14:22    
Interesting, I'll take a look at the pf kernel and find out what's different.
damentz
Status: Assistant
Joined: 09 Sep 2008
Posts: 521
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Posted: Jan 1, 13, 18:34    
Well, I can't get a straight answer out of Con, but he doesn't think that this is completely BFS's fault. In that case, the 3.7.0-1.dmz.6 and newer now run CFS instead. I don't think it's a good idea to run a scheduler with a known timer bug that can cause all sorts of random unexplainable behavior.

Please let me know if you still get that error. The sudden changes in liquorix were caused by a third confirmation on Con's own blog page here: ck-hack.blogspot.com/2012/12/37-ck1-bfs-426-for-linux-37.html?showComment=1356960043310#c4842952502245392347, which are exactly what you've described here.
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