March 2016 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

Re: kmalloc/malloc for dma ?
by Manoj Nayak March 24, 2016

March 24, 2016

Please check the following document. https://forums.xilinx.com/xlnx/attachments/xlnx/ELINUX/10693/1/Linux%20DMA%… Regards Manoj Nayak

1 0

contribution in kernel community & testing kernel
by shubham k March 24, 2016

March 24, 2016

Hi, I a novice & want to contribute to linux community. I have basic knowledge of kernel. Problem statement: I want to work on x86 architecture. I am not sure how to go about unit testing. I can use qemu for testing but i find kernel on x86 architecture too big to compile. How i can get the smaller size kernel on x86 architecture? Please help me setting up testing environment so that i can test my changes properly & quickly. thanks, Shubham

4 3

Re: Kernelnewbies Digest, Vol 64, Issue 37
by Vishwas Srivastava March 23, 2016

March 23, 2016

On Tue, Mar 22, 2016 at 8:06 PM, <kernelnewbies-request(a)kernelnewbies.org> wrote: > Send Kernelnewbies mailing list submissions to > kernelnewbies(a)kernelnewbies.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > or, via email, send a message with subject or body 'help' to > kernelnewbies-request(a)kernelnewbies.org > > You can reach the person managing the list at > kernelnewbies-owner(a)kernelnewbies.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Kernelnewbies digest..." > > > Today's Topics: > > 1. Re: Is it possible to turn off the gcc optimization when > compiling kernel? (Hao Lee) > 2. Re: kernel_thread() causes segfault (Shashank Khasare) > 3. Re: Is it possible to turn off the gcc optimization when > compiling kernel? (Nicholas Mc Guire) > 4. Re: How to get object virtual address from a kernel core dump > (Arun Sudhilal) > 5. Re: Is it possible to turn off the gcc optimization when > compiling kernel? (Hao Lee) > 6. Re: kernel_thread() causes segfault (Manoj Nayak) > 7. Re :Querry Regarding Memory Alignment (Manoj Nayak) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Tue, 22 Mar 2016 19:24:38 +0800 > From: Hao Lee <haolee.swjtu(a)gmail.com> > Subject: Re: Is it possible to turn off the gcc optimization when > compiling kernel? > To: kernelnewbies(a)kernelnewbies.org > Cc: Nicholas Mc Guire <der.herr(a)hofr.at> > Message-ID: > <CA+PpKPm= > eHxK5oj39r8nrT6XGA4qo6XKMNRcgcNCNdGQW_SPvw(a)mail.gmail.com> > Content-Type: text/plain; charset=UTF-8 > > On Mon, Mar 21, 2016 at 5:51 PM, Nicholas Mc Guire <der.herr(a)hofr.at> > wrote: > > You can not turn it off in all functions as some need particluar > > optimization flags to comile at all, but you can pass > > individual CFLAGS per file via the Makefile > > > > CFLAGS_target.o = -O0 or -flags-to-use > > > > aswell as remove specific CFLAGS with > > > > CFLAGS_REMOVE_target.o = -flags-to-remove > > > > but if you want to debug the kernel it is most likely not > > a good idea to try and disable optimization as the code you then > > are debugging might not have that much to do with the final code > > once optimization is on again. So simply generate the .lst file > > of the target you are trying to debug e.g. for kernel/sched/core.c: > > > > make kernel/sched/core.lst > > > > and then use that .lst file to understand the output of gdb you > > are inspecting. > > Thanks for your reply! > Besides,I also find that use "gcc -c -Q -O1 --help=optimizers" can > print the exact set of optimizations. > > regards, > Hao Lee > > > > ------------------------------ > > Message: 2 > Date: Tue, 22 Mar 2016 16:51:44 +0530 > From: Shashank Khasare <sskkernelnewbie(a)gmail.com> > Subject: Re: kernel_thread() causes segfault > To: kernelnewbies(a)kernelnewbies.org > Message-ID: > < > CAGwMyOAxpQJjR9LRDLCp-ZLgHqKPcaLuJwDWyowBMsHHs5sjug(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > I am trying to implement ideas mentioned in the following OSDI paper: > L. Soares and M. Stumm. FlexSC: flexible system call scheduling with > exception-less system calls. In Proc. OSDI, 2010. > > http://www.cs.cmu.edu/~chensm/Big_Data_reading_group/papers/flexsc-osdi10.p… > > The paper propose a new mechanism for applications to make syscall. > The brief idea is to have two types of threads 1) User thread 2) Kernel > Thread. > These two threads share same address space, file descriptor tables, parent > pid etc. > Whenever user thread wants to make syscall, it would post the information > about syscall number & arguments > to syscall in common shared page. User thread would then wait till the > results are posted on shared page. > The kernel thread reads the syscall arguments from shared page and writes > the results to shared page. > User thread consumes the results and continues execution. > Since the kernel thread and user thread can be scheduled on different cpu > cores, and user thread is ideally never executing > kernel code and vice versa, one can expect gain in instruction per cycle > for application, since the cache pollution is reduced to > some extent*.* > > So to implement this mechanism, it is important for the user and kernel > thread to share address space, fd tables etc. > kernel_thread() works fine with older kernels to achieve this task, but is > no longer an option. > > Is there of any mechanism for sharing fd tables as well? > Please let me know. > > Thanks a lot, > Shashank >

1 0

How to get object virtual address from a kernel core dump
by Mohammad Y. Zachariah March 23, 2016

March 23, 2016

Hello everyone, I'm taking the way of analysing kernel core dumps as a learning approach using 'crash tool'. One of the interesting crash commands is 'struct' which can print kernel struct definition and/or the actual contents of the structure. According to struct help page, I need the virtual address of the struct in order to view/print its contents, for example: crash> mm_struct.pgd ffff810022e7d080 -px pgd_t *pgd = 0xffff81000e3ac000 -> { pgd = 0x2c0a6067 } My question is how to find the mm_struct address "ffff810022e7d080" in the above example in the first place?? Thank you for your help in advance. Zach

3 2

Re:kernel_thread() causes segfault
by Manoj Nayak March 22, 2016

March 22, 2016

What is the usecase here ? Do we need to share the entire process address space to a kernel thread ? If address space sharing between userspace thread and kernel thread space is the whole idea then we can mmap process address space and do get_user_pages() to allocate physcial page and pin it. kernel thread can do kmap() and kumap() to use the pages from process address space. http://lxr.free-electrons.com/source/fs/aio.c?v=3.8#L99 Regards Manoj Nayak

3 2

Re :Querry Regarding Memory Alignment
by Manoj Nayak March 22, 2016

March 22, 2016

The CPU always reads at its word size (4 bytes on a 32-bit processor), so when you do an unaligned address access — on a processor that supports it — the processor is going to read multiple words. The CPU will read each word of memory that your requested address straddles. This causes multiple processor read to access the requested data. Hardware deals with word size alignment. Other alignment like page size alignment is a software feature. On X86: Any virtual address provided to cpu is split into page aligned virtual address and an offset.CPU uses page aligned virtual address to check TLB to get page aligned physical address. Now physical address = Page aligned physical address + offset If page size is bigger then less no of TLB entries are required for the processor. Each page needs one TLB entry. Regards Manoj Nayak

1 0

Re: kernel_thread() causes segfault
by Manoj Nayak March 22, 2016

March 22, 2016

Process has files_struct and fs_struct in task_struct. Two thread's task_struct can point to same files_struct and fs_struct if we do the changes through a new system call. Please check the following URL. http://lxr.free-electrons.com/source/kernel/fork.c#L993 Regards Manoj Nayak

1 0

Is it possible to turn off the gcc optimization when compiling kernel?
by Hao Lee March 22, 2016

March 22, 2016

Hi, When I am debugging the linux kernel, I find that the execution sequence of some code is very strange. I think I need to turn off gcc optimization by changing "-O2" to "-O0". But I encounter many errors. So, I want to know is it possible to turn off the gcc optimization or how can I compile some functions without optimization. Thanks.

2 4

Re: How to get object virtual address from a kernel core dump
by Manoj Nayak March 22, 2016

March 22, 2016

task_struct contains mm_struct. If we have pid of the process then task_struct can be obtained from pid using following two methods. 1. Please check find_task_by_pid() function in kernel. We can write a similar macro to convert pid to task_struct. 2. We can write a macro that traverses all task starting from init_task and check the required pid. #define for_each_task(p) \ for (p = &init_task ; (p = p->next_task) != &init_task ; ) If process is the current one then current_thread_info()->task provides task_struct for current task. We can write a macro similar to current_thread_info(). pid-> task_struct->mm_struct. Regards Manoj Nayak

1 0

hrtimer_clock_base and hrtimer_cpu_base
by Pratyush Patel March 22, 2016

March 22, 2016

Hello, I am unable to understand what exactly "clock_base" and "cpu_base", or simply "base" means (I am seeking an intuitive idea). I understand that hrtimer subsystem stores its timers in a red-black tree data structure, and the node for each timer can be found in struct hrtimer. And if I understand correctly, these red-black trees are stored per-CPU, each of which is processed differently, leading to a "hrtimer_cpu_base"? However, as mentioned in a comment in the hrtimer.h, the "active" attribute in "hrtimer_clock_base" stores the root node of the red-black tree. Does this mean that each CPU has multiple clock_bases, and hence multiple red-black trees? Also, could someone please let me know, or perhaps point to a relevant resource, which explains what a "timer_base" means? Relevant files: 1) http://lxr.free-electrons.com/source/include/linux/hrtimer.h 2) http://lxr.free-electrons.com/source/kernel/time/hrtimer.c Thank you, Pratyush

1 0