July 2011 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

Re: interview question how does application connects to device
by Mandeep Sandhu July 7, 2011

July 7, 2011

> How would using sysfs be useful if we were to talk about interacting with, let's say, a file system? ioctl's are quite versatile and rather easy to use when one wants to interact with a given FS. > > For instance, one can manage a Btrfs file system by using an user-level tool, which heavily uses ioctl's. Requiring it to interact with sysfs instead seems like complicating something simple with no apparent purpose. Franlky, I've never used the sysfs interface and mostly know it from 'theoretical' view-point. It was created mostly for providing a uniform way for user-space applications (udev/HAL, driver tools, etc) to interact with a device driver. It exposes the underlying device topology as a directory tree. I'm not sure how sysfs applies to 'filesystem' as it depicts a particular 'device hierarchy tree' within a kernel. As Greg mentioned here, lot filesystem drivers still prefer to use the ioctl interface. However, one use for it which I can see, is for controlling the underlying block device (hard-disk, USB mass storage etc). HTH, -mandeep

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performance about use Linux as a router
by jiangtao.jit July 7, 2011

July 7, 2011

dear all: i have a x86 PC and i use it as a router the kernel is 2.6.38 there are two NICs: eth0 and eth1 i configed eth0 as 10.1.0.1 and eth1 as 11.1.0.1 Linux will route between these two cards and it worked ok in the old days but recently, i had added a new NIC as eth2 for LAN so i add a bridge br0 and config it as 10.1.0.1 then i configed eth0 and eth2 under br0 no ip on eth0 now like br0(10.1.0.1) eth1(11.1.0.1) / \ eth0 eth2 | | LAN LAN i expected that the performance will be better but unfortunately things become worse the PC lost packets now at first, i think maybe there is a loop Under br0, so i disconnected the wire on eth2 or eth0 nothing changed, packets still lost then i remove br0 config is as before its' eth0(10.1.0.1) eth1(11.1.0.1) | LAN then everthing becomes fine, no packet lost i can't explain why, any idea? thanks 2011-07-06 jiangtao.jit

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can we enable perf during the kernel initialization?
by Da Zheng July 7, 2011

July 7, 2011

Hello, Does the current kernel allow us to enable perf during the kernel initialization? perf is initialized in a very early stage, but I cannot find any kernel parameters that can enable perf. Has anyone tried to use perf to profile the kernel during its initialization? Thanks, Da

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How to profile the kernel during its initialization?
by Zheng Da July 7, 2011

July 7, 2011

Hello Can we profile the kernel during its initialization? perf is initialized in an early stage in start_kernel(), but it seems there are no command-line parameters to enable it. Does anyone have any ideas how to profile the kernel during its initialization (before user space programs start to run)? I think someone should have the similar problem as me, and there should be some solutions. Thanks, Da

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performance about use Linux as a router
by jiangtao.jit July 6, 2011

July 6, 2011

dear all: i have a x86 PC and i use it as a router the kernel is 2.6.38 there are two NICs: eth0 and eth1 i configed eth0 as 10.1.0.1 and eth1 as 11.1.0.1 Linux will route between these two cards and it worked ok in the old days but recently, i had added a new NIC as eth2 for LAN so i add a bridge br0 and config it as 10.1.0.1 then i configed eth0 and eth2 under br0 no ip on eth0 now like br0(10.1.0.1) eth1(11.1.0.1) / \ eth0 eth2 | | LAN LAN i expected that the performance will be better but unfortunately things become worse the PC lost packets now at first, i think maybe there is a loop Under br0, so i disconnected the wire on eth2 or eth0 nothing changed, packets still lost then i remove br0 config is as before its' eth0(10.1.0.1) eth1(11.1.0.1) | LAN then everthing becomes fine, no packet lost i can't explain why, any idea? thanks 2011-07-06 jiangtao.jit

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read / write / close sync for device files
by Amit Nagal July 5, 2011

July 5, 2011

Hi , if a application is doing read /write I/O with a usb device , in 2 different threads in userspace , and at usb disconnect event , read thread returns with error and application calls close() subsequently without waiting for write thread to finish . Do the VFS upper layers maintain locks that take care that until both read and write methods in usb driver donot finish and return with error in kernel space close will pend and usb driver release method will not be called ? Regards Amit Nagal

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doubts for copy_from_user
by Vishal Thanki July 3, 2011

July 3, 2011

Hi, We use copy_from_user/copy_to_user functions to copy data transfer user data between user space and kernel space. As per my understanding, these functions are required because we cannot access userspace data directly within kernel space. But, the implementation of these functions actually access the user-space data right? Can someone explain me how this actually happens? Vishal

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OOT: sharing about my research about why stat and ls show difference used block count upon a file
by Mulyadi Santosa July 3, 2011

July 3, 2011

Hi all... As the subject says, here is my blog post that discuss that issue: http://the-hydra.blogspot.com/2011/07/stat-or-ls-which-one-showing-correct.… Few kind people already share their opinions on Linkedin. Here is the summary so far: http://the-hydra.blogspot.com/2011/07/feedback-regarding-my-stat-or-ls-post… Have opinions? Please share and let's discuss it. And thanks in advance for your time reading my blog. PS: I thank Manish Katiyar for his help during this research. Thanks man! -- regards, Mulyadi Santosa Freelance Linux trainer and consultant blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com

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regarding __wait_event_interruptible
by Amit Nagal July 2, 2011

July 2, 2011

Hi , __wait_event_interruptible is defined in include/linux/wait.h as : #define __wait_event_interruptible(wq, condition, ret) \ do { \ DEFINE_WAIT(__wait); \ \ for (;;) { \ prepare_to_wait(&wq, &__wait, TASK_INTERRUPTIBLE); \ if (condition) \ break; \ if (!signal_pending(current)) { \ schedule(); \ continue; \ } \ ret = -ERESTARTSYS; \ break; \ } \ finish_wait(&wq, &__wait); \ } while (0) here , can i be guided why in every for loop iteration , we need to call prepare_to_wait ? instead why not prepare_to_wait() be used once only like finish_wait() above and after schedule() call returns , set the current state = TASK_INTERRUPTIBLE (before continue statement ) something like below : #define __wait_event_interruptible(wq, condition, ret) \ do { \ DEFINE_WAIT(__wait); \ prepare_to_wait(&wq, &__wait, TASK_INTERRUPTIBLE); \ \ for (;;) { \ if (condition) \ break; \ if (!signal_pending(current)) { \ schedule(); \ set_current_state(TASK_INTERRUPTIBLE) \ continue; \ } \ ret = -ERESTARTSYS; \ break; \ } \ finish_wait(&wq, &__wait); \ } while (0) kindly share facts related to above . Regards Amit Nagal

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Invitation to connect on LinkedIn
by Sugnan prabhu July 1, 2011

July 1, 2011

LinkedIn ------------ I'd like to add you to my professional network on LinkedIn. - Sugnan Sugnan prabhu Software engineer at Nokia Bengaluru Area, India Confirm that you know Sugnan prabhu https://www.linkedin.com/e/8z2206-gpkyh1h8-2i/isd/3401948201/_VQgSO-e/ -- (c) 2011, LinkedIn Corporation

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