April 2014 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

Kworker and jbd2 constantly writing to hdd
by Jay Aurabind April 25, 2014

April 25, 2014

Hi, My kernel is 3.13.0-24, ubuntu 14.04. I have an ext4 root partition, and I can see 2 threads (kworker,jbd2) periodically writing to hdd at about roughly (1,5) second interval. I disabled journalling so that jbd2 is gone. Still, the kworker thread is bugging me. Is it really necessary for the system to work properly? cat /proc/<PID>/stack of the kworker is: [<ffffffff810846f1>] worker_thread+0x1d1/0x410 [<ffffffff8108b312>] kthread+0xd2/0xf0 [<ffffffff8172637c>] ret_from_fork+0x7c/0xb0 [<ffffffffffffffff>] 0xffffffffffffffff It doesnt make any sense to me. On tracing this kworker, I found this line: kworker/u8:1-68 [000] d... 1135.724955: workqueue_queue_work: \ work struct=ffff8800701e00b8 function= \ ata_sff_pio_task workqueue=ffff880073e6f600 req_cpu=256 cpu=0 So it seems the function ata_sff_pio_task in drivers/ata/libata-sff.c#L1343 is being run. Why is it getting called periodically? Is it possible to suppress it ? Someone else has also posted about it in arch linux forum: https://bbs.archlinux.org/viewtopic.php?id=178632, smelling a kernel bug. But I dont feel it could be a bug, because some other kernel thread must be requesting some operation, which could be the reason for a "generic" ata code to pop up. I'm concerned because my harddisk heats up unnecessarily (though not violently), and I never had such a situation while using windows (couple of years ago). I am a noob when it comes to kernel programming, (just started ldd3), so please correct me if I'm wrong. Please also suggest something to fix this issue, or to investigate more into this. Thanks and Regards, Jay

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Re: cgroups: cannot write on file memory.oom_control
by Hedi Boufaied April 25, 2014

April 25, 2014

Hi Rami, I have looked at the example code in your slides and wrote something similar to handle the OoM condition and it seems to work fine. Thank you for that. Since it would be even better for me to avoid playing with OoM, I explored another option for monitoring memory usage: keeping the OoM killer enabled and using the "memory threshold" feature (see below) to get notified when a group's memory usage exceeds the limit. First, my C code for monitoring the memory usage looked like this (running as a separate process): // Assuming that initial memory usage is below the threshold while(1) { if (read(efd, &counter, sizeof(uint64_t)) != sizeof(uint64_t)) die("read eventfd"); printf("Event received: memory usage ABOVE limit\n"); if (read(efd, &counter, sizeof(uint64_t)) != sizeof(uint64_t)) die("read eventfd"); printf("Event received: memory usage BELOW limit\n"); } Then I thought: what happens if mem usage crosses the threshold more than once between 2 consecutive calls to read() ? (a scenario that cannot be formally excluded). I realised that in this situation the "counter" value returned by read() would contain the number of occurrences crossing events. So for example if, between 2 calls to read(), the memory usage goes beyond then below the threshold, the value of "counter" after the 2nd read will be 2. I ended up using this counter behaviour to determine if the current usage is below or beyond the limit: counter = 0; above_mem_limit = 0; while(1) { if (read(efd, &counter, sizeof(uint64_t)) != sizeof(uint64_t)) die("read eventfd"); if(counter%2) above_mem_limit = !above_mem_limit; printf("Event received! (counter=%d), above_mem_limit=%d\n", counter, above_mem_limit); } My question: I have never seen this "event counter" behaviour documented anywhere. The cgroups docs and forums do not say anything about the 64bit value returned by the read() call. Do you think one can safely rely on this to count the number of crossing events ? Thanks and Regards, Hedi ---- From https://www.kernel.org/doc/Documentation/cgroups/memory.txt: 9. Memory thresholds Memory cgroup implements memory thresholds using the cgroups notification API (see cgroups.txt). It allows to register multiple memory and memsw thresholds and gets notifications when it crosses. To register a threshold, an application must: - create an eventfd using eventfd(2); - open memory.usage_in_bytes or memory.memsw.usage_in_bytes; - write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to cgroup.event_control. Application will be notified through eventfd when memory usage crosses threshold in any direction. It's applicable for root and non-root cgroup. ---- Best Regard, Hedi From: Hedi Boufaied/NCE/AMADEUS To: Rami Rosen <roszenrami(a)gmail.com>@AMAEXTMAIL_MUC, Cc: kernelnewbies <kernelnewbies(a)kernelnewbies.org> Date: 22/04/2014 13:12 Subject: Re: cgroups: cannot write on file memory.oom_control Hi Rami, Thank you for the advice. Yes, I will try using the event handler. Best Regards, Hedi From: Rami Rosen <roszenrami(a)gmail.com> To: Hedi Boufaied <hedi.boufaied(a)amadeus.com>, Cc: kernelnewbies <kernelnewbies(a)kernelnewbies.org> Date: 18/04/2014 22:38 Subject: Re: cgroups: cannot write on file memory.oom_control Sent by: kernelnewbies-bounces(a)kernelnewbies.org Hi Hedi, Well, you can use disabling oom in conjunction with event handler. Thus, you can free memory by changing memory setting of other cgroups when an oom occurs in a specific cgroup, and avoid having an oops in that cgroup. You can see an example of using event handler in conjunction with disabling the oom killer in "Namespaces and cgroups in linux", in slides 101/102, see: http://ramirose.wix.com/ramirosen A shameless plug - I wrote this presentation of 121 slides about namespaces and cgroups. Regards, Rami Rosen בתאריך 17 באפר 2014 11:24, "Hedi Boufaied" <hedi.boufaied(a)amadeus.com> כתב: Hi Rami, Thanks a lot for your sequence. I followed it and it does work. I now see my process paused in sleep state instead of getting killed when the cgroup reaches OoO. This being said: * I do not really understand why the use_hierarchy option needs to be disabled in order to allow disabling of the oom killer... * I'm not sure that disabling the OoO killer is safe/reliable enough: in some of the trials I made, I saw other processes (xterms) not part of the OoO cgroup getting completely stuck and I had to reboot my machine... Best Regards, Hedi From: Rami Rosen <roszenrami(a)gmail.com> To: Hedi Boufaied <hedi.boufaied(a)amadeus.com>, Cc: kernelnewbies <kernelnewbies(a)kernelnewbies.org> Date: 15/04/2014 15:39 Subject: Re: cgroups: cannot write on file memory.oom_control Hi, Hedi, This sequence works for me: echo 0 > /sys/fs/cgroup/memory/memory.use_hierarchy mkdir /sys/fs/cgroup/memory/0 Then, the following two commands changes the value of oom_kill_disable: echo 1 > /sys/fs/cgroup/memory/0/memory.oom_control echo 0 > /sys/fs/cgroup/memory/0/memory.oom_control For more info, please look for the text about enabling/disabling use_hierarchy in section 6, http://lxr.free-electrons.com/source/Documentation/cgroups/memory.txt. Regards, Rami Rosen http://ramirose.wix.com/ramirosen On Mon, Apr 14, 2014 at 12:22 PM, Hedi Boufaied <hedi.boufaied(a)amadeus.com> wrote: > Hi everyone, > > I am working with cgroups on OpenSuse (over VirtualBox) and I cannot disable > the OOM killer by writing to the file memory.oom_control: > >> echo 1 > memory.oom_control >> -bash: echo: write error: Invalid argument > > I can change the memory limit and several other settings by writing to the > appropriate files (like memory.limit_in_bytes) but I could never write to > file memory.oom_control although I am root. > > I saw someone posted a similar issue a few days ago but there was no > reply... > > I have copied below the sequence of command I am using. Any idea what could > be the issue ? > > Thanks in advance for your help! > > Hedi > > > ---- > > /sys/fs> su - root > > /sys/fs> cd cgroup > > /sys/fs/cgroup> cgcreate -g memory:/mygroup > > /sys/fs/cgroup> cd memory/mygroup > > /sys/fs/cgroup/memory/mygroup> echo 32M > memory.limit_in_bytes > > /sys/fs/cgroup/memory/mygroup> cat memory.limit_in_bytes > 33554432 > > /sys/fs/cgroup/memory/mygroup> echo 1 > memory.oom_control > -bash: echo: write error: Invalid argument > > /sys/fs/cgroup/memory/mygroup> cat memory.oom_control > oom_kill_disable 0 > under_oom 0 > > --- > _______________________________________________ > Kernelnewbies mailing list > Kernelnewbies(a)kernelnewbies.org > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > _______________________________________________ Kernelnewbies mailing list Kernelnewbies(a)kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

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Difference between seq_release and single_release ?
by Abhijeet Kasurde April 24, 2014

April 24, 2014

Hi All, What is difference between seq_release and single_release API ? Can someone explain particular use case of these APIs ? Thanks, Abhijeet Kasurde

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ioperm and iopl
by Adil Mujeeb April 24, 2014

April 24, 2014

Hi list, Are we going to remove the support of ioperm and iopl system call in future kernel release? I heard it but not sure if its already removed or candidate for removal. Regards, Adil

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"Time police" - best way to submit?
by Henry Hallam April 23, 2014

April 23, 2014

Hi, my name is Henry and I'm a kernel newbie. Recently plagued by a perplexing issue where *something* would step the clock on an important server by exactly 5 minutes at infrequent, unpredictable intervals, I put together my first kernel module - a trivial hook into do_settimeofday via a jprobe, that prints the PID, process name and time adjustment to the kernel message log. Here's what I have so far: https://github.com/henryhallam/timepolice Questions: 1. Is a module using kprobes/jprobes the right way to do it, vs adding printks to linux/time/timekeeping.c? 2. If a module is the way to go, would it be better to create a read/write /proc entry to allow enabling and disabling, rather than inserting the module to enable and removing to disable? 3. Is this sort of thing worth submitting to mainline? What are the next steps for that, after choosing a better name, changing the silly load/unload messages and ensuring it passes checkpatch.pl? Thanks!

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Kernel Projects
by Julio Faracco April 23, 2014

April 23, 2014

Hi, Does anybody know if there is a program to offer small (or big) kernel projects to students or someone who are interested in? I'm looking for programs like Google Summer of Code or http://kernelnewbies.org/KernelProjects I'm interested to contribute with kernel patches, but I don't know how and where to start it. I will be glad if someone give me ideas and/or suggestions. Thanks! *--* *Julio Cesar Faracco* *University of São Paulo - Brazil*

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a question about __nocache_pa macro
by Chan Kim April 23, 2014

April 23, 2014

Hi, can someony explain this macro to me? It's in arch/sparc/include/asm/pgtsrmmu.h (linux 3.3) #define __nocache_pa(VADDR) (((unsigned long)VADDR) - SRMMU_NOCACHE_VADDR + __pa((unsigned long)srmmu_nocache_pool)) it looks like converting virtual address VADDR to physical address for which no cache is used. so I can see SRMMU_NOCACHE_VADDR is the virtual address start value from which no cache is used(in our case it's 0xc8000000), and VADDR is relative to this start location (thus the minus). But why do we add the last srmmu_nocache_pool term? we are adding the offset to physical nocache pool address. So it seems like it is converting a virtual address to a physcal address relative to nocache pool.. Why is it done this way? Thanks in advance. Chan

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pft not found for mmtest.
by Raju April 22, 2014

April 22, 2014

Hi all, May I know from where I can download pft. As pft is not found in http://mcp/mmtests-mirror/pft/ <http://mcp/mmtests-mirror/pft//pft-0.12x.tar.gz> and http://free.linux.hp.com/~lts/Tools/ <http://free.linux.hp.com/%7Elts/Tools/pft-0.12x.tar.gz> Log while running mmtest pft-install: Fetching from mirrorhttp://mcp/mmtests-mirror/pft//pft-0.12x.tar.gz <http://mcp/mmtests-mirror/pft//pft-0.12x.tar.gz> pft-install: Fetching from internethttp://free.linux.hp.com/~lts/Tools/pft-0.12x.tar.gz <http://free.linux.hp.com/%7Elts/Tools/pft-0.12x.tar.gz> FATAL pft-install: pft-install: Could not downloadhttp://free.linux.hp.com/~lts/Tools/pft-0.12x.tar.gz <http://free.linux.hp.com/%7Elts/Tools/pft-0.12x.tar.gz> FATAL pft-bench: pft install script returned error FATAL: pft returned failure, unable to continue test exit :: pft 255 Regards

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mmtest is failed as pft is not found
by Hariharan T.S April 22, 2014

April 22, 2014

Hi All, mmtest fails with the following error. The pft (Page fault is not found in any of the locations show below. pft-install: Fetching from mirror http://mcp/mmtests-mirror/pft//pft-0.12x.tar.gz pft-install: Fetching from internet http://free.linux.hp.com/~lts/Tools/pft-0.12x.tar.gz FATAL pft-install: pft-install: Could not download http://free.linux.hp.com/~lts/Tools/pft-0.12x.tar.gz FATAL pft-bench: pft install script returned error FATAL: pft returned failure, unable to continue test exit :: pft 255 May I know from where I can download or find pft. Thanks Hariharan T S

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cgroups: cannot write on file memory.oom_control
by Hedi Boufaied April 22, 2014

April 22, 2014

Hi everyone, I am working with cgroups on OpenSuse (over VirtualBox) and I cannot disable the OOM killer by writing to the file memory.oom_control: > echo 1 > memory.oom_control > -bash: echo: write error: Invalid argument I can change the memory limit and several other settings by writing to the appropriate files (like memory.limit_in_bytes) but I could never write to file memory.oom_control although I am root. I saw someone posted a similar issue a few days ago but there was no reply... I have copied below the sequence of command I am using. Any idea what could be the issue ? Thanks in advance for your help! Hedi ---- /sys/fs> su - root /sys/fs> cd cgroup /sys/fs/cgroup> cgcreate -g memory:/mygroup /sys/fs/cgroup> cd memory/mygroup /sys/fs/cgroup/memory/mygroup> echo 32M > memory.limit_in_bytes /sys/fs/cgroup/memory/mygroup> cat memory.limit_in_bytes 33554432 /sys/fs/cgroup/memory/mygroup> echo 1 > memory.oom_control -bash: echo: write error: Invalid argument /sys/fs/cgroup/memory/mygroup> cat memory.oom_control oom_kill_disable 0 under_oom 0 ---

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