August 2011 - kernelnewbies@kernelnewbies.org

Exact scenario to use __wait_on_bit() ?
by Indraneel Mukherjee Aug. 8, 2011

Aug. 8, 2011

I have a queue where processes queue up I/O. Driver must pick any one I/O request from the queue(based on some criteria), do the I/O and then wake up corresponding process. Problem: How to selectively wake up a particular process from the middle of the Wait Queue? Instead, can I use __wait_on_bit() for this scenario? TIA, Indro

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Question on Alignment requiremnet
by subin gangadharan Aug. 7, 2011

Aug. 7, 2011

Hi All, I have some doubts on the alignment requirement.It would be really helpful,if someone can shed some light on this. Why there are so many different types of alignment like 4 byte, 8 byte,16 byte ?.My exact question is, in a 32 bit machine(I assume processor reads data in 4 bytes),how 16 byte alignment makes different from 4 byte alignment ?.How this will influence the processor performance ?. - With Regards Subin Gangadharan Everything should be made as simple as possible,but not simpler.

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Re: Compile kernel for various times and problem of too long time consuming
by Amirali Shambayati Aug. 7, 2011

Aug. 7, 2011

Would you say how you rebuild kernel? On Sun, Aug 7, 2011 at 10:34 PM, Randy Dunlap <rdunlap(a)xenotime.net> wrote: > On Sun, 7 Aug 2011 21:39:12 +0430 Amirali Shambayati wrote: > >> Hi all, >> Since I want to evaluate performance improvements by changing just one >> file in kernel(noop-iosched) and compiling kernel is too much >> time-consuming, I want to know that if I compile kernel for more than >> one time, does it take similar time to first time? >> >> I use following commands. If I don't use "make-kpkg clean" for next >> times, does it just compile the files which have been modified or it >> compile whole kernel package again? >> >> make-kpkg clean >> fakeroot make-kpkg --initrd --append-to-version=-some-string-here >> kernel-image kernel-headers > > Since I never use make-kpkg, I can only tell you that when I modified > block/noop-iosched.c and rebuilt the kernel, here is what was rebuilt: > > GEN /lnx/src/NEXT/linux-next-20110805/X32/Makefile > CHK include/linux/version.h > Using /lnx/src/NEXT/linux-next-20110805 as source for kernel > CHK include/generated/utsrelease.h > CALL /lnx/src/NEXT/linux-next-20110805/scripts/checksyscalls.sh > CHK include/generated/compile.h > CHK kernel/config_data.h > CC block/noop-iosched.o > LD block/built-in.o > LD vmlinux.o > MODPOST vmlinux.o > GEN .version > CHK include/generated/compile.h > UPD include/generated/compile.h > CC init/version.o > LD init/built-in.o > LD .tmp_vmlinux1 > KSYM .tmp_kallsyms1.S > AS .tmp_kallsyms1.o > LD .tmp_vmlinux2 > KSYM .tmp_kallsyms2.S > AS .tmp_kallsyms2.o > LD vmlinux > SYSMAP System.map > SYSMAP .tmp_System.map > Building modules, stage 2. > VOFFSET arch/x86/boot/voffset.h > CC arch/x86/boot/version.o > OBJCOPY arch/x86/boot/compressed/vmlinux.bin > LZMA arch/x86/boot/compressed/vmlinux.bin.lzma > MODPOST 575 modules > MKPIGGY arch/x86/boot/compressed/piggy.S > AS arch/x86/boot/compressed/piggy.o > LD arch/x86/boot/compressed/vmlinux > ZOFFSET arch/x86/boot/zoffset.h > OBJCOPY arch/x86/boot/vmlinux.bin > AS arch/x86/boot/header.o > LD arch/x86/boot/setup.elf > OBJCOPY arch/x86/boot/setup.bin > BUILD arch/x86/boot/bzImage > Setup is 14972 bytes (padded to 15360 bytes). > System is 1287 kB > CRC 64ecf4d > Kernel: arch/x86/boot/bzImage is ready (#18) > > > so it's a minimal rebuild, not a full kernel rebuild. > > --- > ~Randy > *** Remember to use Documentation/SubmitChecklist when testing your code *** > -- Amirali Shambayati Bachelor Student Computer Engineering Department Sharif University of Technology Tehran, Iran

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Compile kernel for various times and problem of too long time consuming
by Amirali Shambayati Aug. 7, 2011

Aug. 7, 2011

Hi all, Since I want to evaluate performance improvements by changing just one file in kernel(noop-iosched) and compiling kernel is too much time-consuming, I want to know that if I compile kernel for more than one time, does it take similar time to first time? I use following commands. If I don't use "make-kpkg clean" for next times, does it just compile the files which have been modified or it compile whole kernel package again? make-kpkg clean fakeroot make-kpkg --initrd --append-to-version=-some-string-here kernel-image kernel-headers Thanks for your help in advance, -- Amirali Shambayati Bachelor Student Computer Engineering Department Sharif University of Technology Tehran, Iran

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Reserve physical page
by Prateek Sharma Aug. 7, 2011

Aug. 7, 2011

Hello everyone, I am wondering whether it is possible to reserve a particular physical page in the kernel. The GFP pages can start from _any_ location, and i cannot find a way to specify a starting pfn. I need a few dozen pages to store a bitmap starting at a particular physical page frame number. Perhaps i am trying to do this wrong? I am trying to establish a (one-way) communication between KVM and a guest VM, by assuming that if KVM knows the physical address in the VM , it can access that bitmap. Thanks !

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One more question on alignment
by subin gangadharan Aug. 6, 2011

Aug. 6, 2011

Hi, This is a text excerpt from the article [1]. Below paragraph I didn't understand quite well.Mainly in bold letters.Could anybody please explain how this will lead to an infinite loop. Atomicity All modern processors offer atomic instructions. These special instructions are crucial for synchronizing two or more concurrent tasks. As the name implies, atomic instructions must be *indivisible* -- that's why they're so handy for synchronization: they can't be preempted. It turns out that in order for atomic instructions to perform correctly, the addresses you pass them must be at least four-byte aligned. This is because of a subtle interaction between atomic instructions and virtual memory. If an address is unaligned, it requires at least two memory accesses. But what happens if the desired data spans two pages of virtual memory? This could lead to a situation where the first page is resident while the last page is not. Upon access, in the middle of the instruction, a page fault would be generated, executing the virtual memory management swap-in code, destroying the atomicity of the instruction. To keep things simple and correct, both the 68K and PowerPC require that atomically manipulated addresses always be at least four-byte aligned. *Unfortunately, the PowerPC does not throw an exception when atomically storing to an unaligned address. Instead, the store simply always fails. This is bad because most atomic functions are written to retry upon a failed store, under the assumption they were preempted. These two circumstances combine to where your program will go into an infinite loop if you attempt to atomically store to an unaligned address. Oops.* [1] http://www.ibm.com/developerworks/library/pa-dalign/. -- With Regards Subin Gangadharan Everything should be made as simple as possible,but not simpler.

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Compiling kernel module ...
by Abu Rasheda Aug. 6, 2011

Aug. 6, 2011

I am compiling my driver and have run into following scenario. Socket create function got a new parameter in Vanila kenrel 2.6.33. while Scientific Linux has it back ported to 2.6.32. How do I handle this kind of situation ? so that my driver compiles for both kernel versions.

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Debugging the linux kernel core dump
by Akash Aug. 6, 2011

Aug. 6, 2011

Hello, I have a core dump from following machine : <http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies> # uname -a Linux sing-vto2-dst 2.6.34.8-0.2-desktop #1 SMP PREEMPT 2011-04-06 18:11:26 +0200 x86_64 x86_64 x86_64 GNU/Linux I have opened this core in gdb # gdb vmlinux-2.6.34.8-0.2-desktop kernel.core0 GNU gdb 6.6 (build 2009-10-05) Copyright (C) 2006 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. (no debugging symbols found) Program terminated with signal 11, Segmentation fault. #0 0xffffffff8100ba60 in default_idle () How can I add symbols to gdb ? When I try to add symbols from System.map I get following error : (gdb) add-symbol-file System.map-2.6.34.8-0.2-desktop The address where System.map-2.6.34.8-0.2-desktop has been loaded is missing At what address should the System.map be added ? How can I find out that address ? Thanks for any help. Akash

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How to get current core id in kernel source
by J.Hwan Kim Aug. 6, 2011

Aug. 6, 2011

Hi, everyone I wish to get the current core id in which the code is being processed, in device driver code. What function or global variable can I use ? Thanks in advance. Best Regards, J.Hwan Kim

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how to freeze the 2.6 kernel on SMP m/c
by san deep Aug. 6, 2011

Aug. 6, 2011

Hi, Could you please suggest how to freeze the 2.6 kernel(SMP m/c) with my own dlkm with less number of code. I tried the below mentioned code snippet, it hangs only the terminal on which i executed the insmod command. Please correct the below code to freeze whole kernel. #include <linux/module.h> #include <linux/kernel.h> #include <linux/spinlock.h> #include <linux/delay.h> int init_module(void) { spinlock_t lock = SPIN_LOCK_UNLOCKED; while (1) { spin_lock(&lock); msleep(100); } //spin_unlock(lock); printk(KERN_INFO "Hello world :).\n"); return 0; } void cleanup_module(void) { printk(KERN_INFO "Goodbye world :(.\n"); } Thanks and Regards, Sandeep

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