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test read and write speed in file
by 于运超 Oct. 23, 2013

Oct. 23, 2013

hi all, how can i test read and wirte speed in a file in linux OS. the command dd provided how to test the speed of harddisk, i think dd can not accurate test the speed of file.what should i do ? which tool or software can do this .

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Dinning-philosphers problem, solution using counting semaphore and fair scheduling
by Kumar Amit Mehta Oct. 22, 2013

Oct. 22, 2013

Hi All, This query is not about linux kernel in any way, but rather is a very primitive question on scheduling and resource management principles, using the dinning-philosphers analogy. I assume, most of the people here would have faced such situation and hence thought of discussing this in this forum. Also, i think, learning such basic principle will help me understand the scheduling algorithms. I also assume that every computer science student would have come across this classical problem of dinning philosphers problem[1], proposed by Dijkstra. I came across it long back but i never took it seriously, Now I have revisited this same topic and was wondering if using a counting semaphore with completely fair scheduling will solve both of the deadlock and starvation situations. Going back to the classical case, lets assume that we still have just 5 philosphers gathered over weekend at some food joint, sitting in a round-table. The table has again just 5 forks laid out around their plates(please see the diagram in the link[1] below) and to eat that sphagetti, both forks are needed. These guys either talk or eat, nothing else(nobody's going anywhere). To avoid potential deadlock situation(considering each one took one left fork at the same time) and to avoid starvation(a slow philospher never gets a chance to eat, because he is so lazy, picking up the fork) and to improve efficiency(let 2 philosphers eat simultaneously), why not use a counting semaphore instead of binary semaphore. Also, to avoid starvation, lets use a round-robin scheduling. As, there is no point accessing one fork at a time, so, each fork will be paired and acquistion of a semaphore, entitles one to get a pair of forks. The psuedo code is mentioned below: int nforks; //total number of forks, 5 here Semaphore sem; //controls, who gets to eat int num_philosphers; //total number of philosphers, 5 here cfq_t queue; // some kind of waitqueue, At any moment, there will be // 3 elements(philosphers) in the queue add_tail(queue); //add at the end remove_head(queue); //remove from the front if ((nfoks & 0x01) == 0x01) sem = (nforks -1) / 2; else sem = (nforks) / 2; // P(sem) returns 0, if a pair of forks were acquired, else some number // P(sem) and V(sem) has same classical concept philospher(i) { philosphies(); if (hungry) { if (P(sem) != 0) { add_tail(queue); } else { takeup_fork(); eat(); put_down_fork(); V(sem); // release the pair of forks remove_head(queue); // remove the philospher from // the front add_tail(queue); // add myself on the queue, add // at the tail of the queue } } else { remove_head(queue); add_tail(queue); } } scheduler() { int i; for (i = 0; i < num_philosphers; i++) philospher(i); } Please share you thoughts, help me learn. [1] http://en.wikipedia.org/wiki/Dining_philosophers_problem Thanks much, Kumar

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Module that enforces etc/limits.conf
by Paul Davies C Oct. 22, 2013

Oct. 22, 2013

Hi, May I know which kernel module / code part that enforce the limits specified in /etc/limits.conf file?

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Branch annotations
by Nav Kamal Oct. 21, 2013

Oct. 21, 2013

I cam across a line which goes like this : #define likely(condition) (__builtin__ expect (!!(condition),1)) Please somebody explain the double negation used in this macro. Regards Navkamal Rakra

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Why is kernel memory not pagable ?
by Nav Kamal Oct. 21, 2013

Oct. 21, 2013

Am very new to kernel programming and one of the concept says that kernel memory isn't pagable ? Can please someone explain its reason ?

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Can Linux kernel do context switch when the interrupt is disabled?
by Grissiom Oct. 21, 2013

Oct. 21, 2013

Hi all, Can Linux kernel do context switching when the interrupt is disabled? I am learning the code in ARM platform but I think this question might be arch-independent. -- Cheers, Grissiom

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down_interruptible() return non-zero value once signal is received and trying down_interruptible again in loop return value is always non-zero
by Mushtaq Khan Oct. 20, 2013

Oct. 20, 2013

Hi, In kernel driver, using Semaphore to block on resources (Descriptors and memory). Semaphore is initialized in locking state using call "init_MTEX_LOCKED()", when resources are not available process calls down_interruptible() and blocks. Once resources are freed up() is called, and blocked process is unblocked. up() is called from interrupt context which frees resources on receiving ACK-Interrupt. Following is the code snippet for blocking: do { ret = down_interruptible(sem) if (ret == 0) { break; } else { printk("Semaphore is not acquired try again\n"); continue; } } while (1); While loop is used to make sure down_interrptible() is called until process acquires the semaphore. If process receives signal, process unblocks from down_interrptible() without acquiring the semaphore. Issue am seeing is once signal is received under the blocking state, again trying to acquire semaphore using down_interruptible() return value is always non-zero and process is looping in while loop. Can someone explain this behavior, I wrote the code with expectation that after signal is received again trying to acquire lock will either block the process or process will get semaphore. Please help me on this. Thanks, Mushtaq Khan

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adding a system call in Linux kernel v3.11
by David Oct. 20, 2013

Oct. 20, 2013

I’ve been trying to do an assignment and I have never worked in a kernel or linux. I’m currently using vmware player to emulate fedora live desktop x86_64-19-1. I followed the code posted on http://lists.kernelnewbies.org/pipermail/kernelnewbies/2013-July/008598.html but I get a warning when I go to make modules. It is: CALL scripts/checksyscalls.sh <stdin>:1226:2: warning: #warning syscall helloworld not implemented [-Wcpp] I was wondering how do I fix this so it doesn’t appear.

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Smart card reader detection on Linux
by पारस Oct. 19, 2013

Oct. 19, 2013

I have a smart card reader called SK-284. It support two communication methods, RS232 and USB. In my case only USB is enable. When I connect this device to my Linux laptop, via USB, It wont get detected. I run '$dmesg' command to check on which ttyUSB port it get detected, I got following output, [ 2321.208330] usb 2-1.2: new full-speed USB device number 7 using ehci_hcd [ 2321.384628] generic-usb 0003:8203:8288.0003: hiddev0,hidraw1: USB HID v1.10 Device [\xffffffe6\xffffffb7\xffffffb1\xffffffb7\xffffffb1\xffffffe5\xffffff9c\xffffffb3\xffffff9c\xffffffb3\xffffffe5\xffffff88\xffffff9b\xffffff88\xffffff9b\xffffffe8\xffffff87\xffffffaa\xffffff87\xffffffaa\xffffffe6\xffffff8a\xffffff80\xffffff8a\xffffff80\xffffffe6\xffffff9c\xffffffaf\xffffff9c\xffffffaf\xffffffe6\xffffff9c\xffffff89\xffffff9c\xffffff89\xffffffe9\xffffff99\xffffff90\xffffff99\xffffff90\xffffffe5\xffffff85\xffffffac\xffffff85\xffffffac\xffffffe5\xffffff8f\xffffffb8\xffffff8f\xffffffb8 CRT450 USB 1.1] on usb-0000:00:1d.0-1.2/input0 [ 2326.634925] usb 2-1.2: USB disconnect, device number 7 [ 2326.855479] usb 2-1.2: new full-speed USB device number 8 using ehci_hcd [ 2326.954303] usb 2-1.2: config index 0 descriptor too short (expected 9, got 0) [ 2326.954310] usb 2-1.2: can't read configurations, error -22 [ 2327.027211] usb 2-1.2: new full-speed USB device number 9 using ehci_hcd [ 2327.171371] generic-usb 0003:CEA0:2840.0004: hiddev0,hidraw1: USB HID v1.10 Device [\xffffffe6\xffffffb7\xffffffb1\xffffffb7\xffffffb1\xffffffe5\xffffff9c\xffffffb3\xffffff9c\xffffffb3\xffffffe5\xffffff88\xffffff9b\xffffff88\xffffff9b\xffffffe8\xffffff87\xffffffaa\xffffff87\xffffffaa\xffffffe6\xffffff8a\xffffff80\xffffff8a\xffffff80\xffffffe6\xffffff9c\xffffffaf\xffffff9c\xffffffaf\xffffffe6\xffffff9c\xffffff89\xffffff9c\xffffff89\xffffffe9\xffffff99\xffffff90\xffffff99\xffffff90\xffffffe5\xffffff85\xffffffac\xffffff85\xffffffac\xffffffe5\xffffff8f\xffffffb8\xffffff8f\xffffffb8 CRT284 USB 1.1] on usb-0000:00:1d.0-1.2/input0 My laptop specification: OS: Ubuntu 12.04 Kernel: 3.2.0-54-generic-pae When I connect same device to Windows 7 and run device manager, It got detected under Human Interface Device (HID). and some test applications are running fine. My task is to write a small code on Linux to detect the smart card by device. So where do I get to know on which ttyUSB port is used by this device, or any other method to get connected to this device. Thanks in advance.

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Greg KH showed a trick to compiled one file - how to?
by Matthias Beyer Oct. 14, 2013

Oct. 14, 2013

Hi there! Greg KH showed a method to compile only one file on fosdem (11 I guess, just saw it on youtube) by writing: make M=./path/to/file.c But somehow, this doesn't work anymore. Any tipps or suggestions? -- Mit freundlichen Grüßen, Kind regards, Matthias Beyer Proudly sent with mutt. Happily signed with gnupg.

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