October 2018 - kernelnewbies@kernelnewbies.org

Linux Kernel Tester's Guide is still valid?
by Manuel Quintero Fonseca Oct. 22, 2018

Oct. 22, 2018

Hi, Linux Kernel Tester's Guide is still valid? http://www.unixy.pl/maciek/doc/lktg/handbook-en.pdf

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Re: [RFC: sound/i2c/cs8427] clang warns about cast to signed char
by Takashi Iwai Oct. 18, 2018

Oct. 18, 2018

On Thu, 18 Oct 2018 12:10:28 +0200, Philipp K wrote: > > Hi, > > I tried compiling the cs8247 sound module with clang and hit the > following warning: > > sound/i2c/cs8427.c:141:31: warning: implicit conversion from 'int' to > 'char' changes value from 160 to -96 [-Wconstant-conversion] > data[0] = CS8427_REG_AUTOINC | CS8427_REG_CORU_DATABUF; > ~ ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~ > > data is a char array, and CS8427_REG_AUTOINC is defined as 0x80 (dec > 128) which is too big for a (signed) char. > > Looking at other code in the same file, we usually use unsigned chars. > > So is there a reason for data being signed in this case? Otherwise we > could just change data to unsigned char and be on the safe side. No any reason I know of. It must be just an old convention. > If you want I can send in a patch. Yes, please. thanks, Takashi

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[RFC: sound/i2c/cs8427] clang warns about cast to signed char
by Philipp K Oct. 18, 2018

Oct. 18, 2018

Hi, I tried compiling the cs8247 sound module with clang and hit the following warning: sound/i2c/cs8427.c:141:31: warning: implicit conversion from 'int' to 'char' changes value from 160 to -96 [-Wconstant-conversion] data[0] = CS8427_REG_AUTOINC | CS8427_REG_CORU_DATABUF; ~ ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~ data is a char array, and CS8427_REG_AUTOINC is defined as 0x80 (dec 128) which is too big for a (signed) char. Looking at other code in the same file, we usually use unsigned chars. So is there a reason for data being signed in this case? Otherwise we could just change data to unsigned char and be on the safe side. If you want I can send in a patch. Regards, Philipp

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Understand numa_faults_memory
by Henry C Oct. 18, 2018

Oct. 18, 2018

Hi, I am on CentOS 7 with kernel package 3.10.0-862.2.3.el7.x86_64. I was looking at the scheduling info of my application by cat /proc/[pid]/sched: ------------------------------------------------------------------- se.exec_start : 78998944.120048 se.vruntime : 78337609.962134 se.sum_exec_runtime : 78337613.040860 se.nr_migrations : 6 nr_switches : 41 nr_voluntary_switches : 31 nr_involuntary_switches : 10 se.load.weight : 1024 policy : 0 prio : 120 clock-delta : 13 mm->numa_scan_seq : 925 numa_migrations, 0 numa_faults_memory, 0, 0, 0, 0, 1 numa_faults_memory, 1, 0, 0, 0, 1 numa_faults_memory, 0, 1, 1, 1, 0 numa_faults_memory, 1, 1, 0, 1, 9 And I am trying to understand the numbers after numa_faults_memory. I dived into the source code: https://elixir.bootlin.com/linux/v3.18.75/source/kernel/sched/debug.c I believe line 539 prints the last 4 lines. Given my machine has 2 NUMA nodes, and the outer loop will loop through the NUMA nodes and the inner loop is hardcoded to loop twice, the # of lines in the output match. So my questions are: 1. What the inner loop is trying to do? Why looping twice? 2. What is the last number in numa_faults_memory (e.g. 9 for last line)? 3. When will this counter be reset? According to the comment at https://elixir.bootlin.com/linux/v3.18.75/source/include/linux/sched.h#L1577, "Exponential decaying average of faults on a per-node basis. Scheduling placement decisions are made based on the these counts. The values remain static for the duration of a PTE scan", it sounds like numa_faults_memory is reset or recompute for each PTE scan. What's PTE scan? What does the "scheduling" refer to? Like scheduling the migration of a chunk of memory from one NUMA node to another due to NUMA balancing? If yes, does it mean if I turn off NUMA balancing("echo 0 > /proc/sys/kernel/numa_balancing"), PTE scan will stop and numa_faults_memory will remain 0 all the time (assuming there is sufficient memory all the time)? 4. And for my machine task_struct::numa_faults_memory[] would have 4 elements. Then, I am lost what it is tracking. As I thought it is tracking the number of numa faults per NUMA node. And the number of elements is always # of NUMA node * 2. Thanks!

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changing parameters in build
by Carter Cheng Oct. 17, 2018

Oct. 17, 2018

Hi, I am having exploring how much work it would be to do whole kernel optimization using clang on the current kernel. My understanding is the kernel uses a recursive make file system. I am curious if there are any easy ways to get the kernel to build llvm bit code for all files without going through and hand editing hundreds of makefiles. Thanks in advance, Carter.

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stap shows kfree() is 5 times more than kallocs, how could be?
by Lev Olshvang Oct. 15, 2018

Oct. 15, 2018

Hello all, I am debugging kernel module and use SystemTap to monitor requested and freed memory. I see that SystemTap statistics shows that kfree() is called 5 times more than kalloc. It happens not only on my module, it happens on VirtualBox vboxsf driver as well. See hits count below kmalloc hits: 21561 kfree hits: 102175 ----- probe hit report: kernel.function("__kmalloc@mm/slub.c:3706"), (./kmalloc_argus.stp:10:1), hits: 21561, cycles: 21228min/52226avg/334688max, from: kernel.function("__kmalloc@mm/slub.c:3706") from: kernel.function("__kmalloc"), index: 0 kernel.function("__kmalloc@mm/slub.c:3706").return, (./kmalloc_argus.stp:25:1), hits: 21561, cycles: 18843min/51228avg/406816max, from: kernel.function("__kmalloc@mm/slub.c:3706").return from: kernel.function("__kmalloc").return, index: 1 kernel.function("kfree@mm/slub.c:3843"), (./kmalloc_argus.stp:38:1), hits: 102175, cycles: 400min/1557avg/499754max, from: kernel.function("kfree@mm/slub.c:3843") from: kernel.function("kfree"), index: 2 Please comment. Lev

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How to see the accepted in baseline patch content
by Lev Olshvang Oct. 14, 2018

Oct. 14, 2018

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Generic UDP Encapsulation
by Kanthi P Oct. 13, 2018

Oct. 13, 2018

Hi, I am looking at the ways in which we can encapsulate/decapsulate GUE packets. I get the packets using BPF/XDP as raw ethernet buffers. What are the ways in which I can encapsulate this in GUE and send it to a remote server And how can I decapsulate the GUE packet and put it back to the kernel stack. Really appreciate your help! Thanks!

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link time analysis for the kernel.
by Carter Cheng Oct. 12, 2018

Oct. 12, 2018

Hi, There are some detaills about the current procedures for linking the kernel that I am unfamiliar with. My understanding is that GCC and Clang both have the ability to do link time analysis and transforms on code but is it possible to write link time passes that will run on the kernel since the linking phase is a bit different (i.e. doesnt produce an ELF file)? Regards, Carter.

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Fwd: Learning Linux Kernel Development
by Carter Cheng Oct. 12, 2018

Oct. 12, 2018

---------- Forwarded message --------- From: Carter Cheng <cartercheng(a)gmail.com> Date: Wed, Oct 10, 2018 at 10:52 PM Subject: Learning Linux Kernel Development To: <kernelnewbies(a)kernelnewbies.org> Hello, I am actually kind of interested in learning linux kernel development and have been over the last few weeks going through Love's Linux Kernel Development book which details the situation in the kernel around linux 2.6. Obviously for a book of such limited length he can probably only go into some of the details and give a taste of what is actually is going on. I have a couple questions however and I was wondering if this was the correct place to ask- 1. After finishing the book and perhaps Understanding the Linux Kernel and Linux Device Drivers. What is the best way to dig deeper. 2. Is there some good way to figure out how to update knowledge gained from this book to what is in the 4.x series of kernels? Thanks, Carter.

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