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problem mapping physical address from /dev/mem
by Vaibhav Jain Oct. 7, 2011

Oct. 7, 2011

Hi, I am trying to run a program that scans memory from a given physical address using /dev/mem. It uses mmap to map physical address from /dev/mem. So to start with I used /proc/iomem to look up the physical memory mapping and found the address 00010000 to be the starting address for System ram. But whenever I provide this address to the program it throws an error of "Operation not permitted". Browsing through some articles I found the option of compiling the kernel with CONFIG_STRICT_DEVMEM option disabled. I have tried this but this isn't working either. Please help me figure out the problem. Thanks Vaibhav Jain

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Re: [lm-sensors] [PATCH] hwmon class driver registration with a device number
by Himanshu Chauhan Oct. 7, 2011

Oct. 7, 2011

On Thu, Oct 06, 2011 at 12:07:52PM -0700, Guenter Roeck wrote: > And why, and what for. The initial idea of posting to kernelnewbies was to get a hint on how the patch would be taken as. I wanted to know if developers will like the idea behind it or not. I guess, Guenter is not convinced with any of my reasoning. I am willing to clean it up further only if I get a positive hint. But it doesn't seem to be going anywhere. Thanks Guenter, Alan, and Greg for taking your time and reviewing it. Greg: To answer your last question, if this was taken positively, I was thinking of having functionality similar to misc device registration. -Himanshu

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Why do processes with higher priority to be allocated more timeslice?
by Parmenides Oct. 7, 2011

Oct. 7, 2011

Hi, It seems that Linux's scheduler tends to allocate longer timeslice for processes with higher priority. Actually, the CFS scheduler which is a new scheduler in Linux kernel also does the same thing. But, I think this way does not fit with scheduler's principle. The goal chased by a scheduler is low latency and high thoughput. Normally, a I/O-bound process has higher priority, while a CPU-bound process has lower priority. So, a I/O-bound process (which has enough timeslice) can preempt a CPU-bound process easily. This way ensures lower latency. It is also necessary that CPU-bound processes are to be allocated longer timeslice to improve throughput owing to less process switch costs. That means lower priority processes (CPU-bound) should be allocated longer timeslice, whichs obviously conflicts with the actual practice taken by the Linux's scheduler. Any explanation? Thanks.

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How to open and write to a block driver within a character driver?
by Yang Xiang Oct. 7, 2011

Oct. 7, 2011

I am taksed to develop a Block device driver which will be used within a character device driver so the write to the character driver will be also formatted, converted and duplicated to the block drvice. I am new at this. And I am doing all this within kernel space so I really don't know I can open a block device without using "open" and "write" syscalls. I know there is a generic_make_request call to block driver but it requires the bio vec to be filled out with data and I don't know where to start on that. Any quick solution to this plase? Thanks so much in advance, yx

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[PATCH] hwmon class driver registration with a device number
by Himanshu Chauhan Oct. 6, 2011

Oct. 6, 2011

This patch adds a way to register a hwmon class driver with a device number rather than the default MK_DEV(0,0). This would help in creating required "dev" file under sysfs which in turn can be used in populating /dev tree during bootup (helpful to driver which want to show up a char interface along with sysfs). Signed-off-by: Himanshu Chauhan <hschauhan(a)nulltrace.org> --- drivers/hwmon/hwmon.c | 28 ++++++++++++++++++++++------ include/linux/hwmon.h | 1 + 2 files changed, 23 insertions(+), 6 deletions(-) diff --git a/drivers/hwmon/hwmon.c b/drivers/hwmon/hwmon.c index a61e781..36961c5 100644 --- a/drivers/hwmon/hwmon.c +++ b/drivers/hwmon/hwmon.c @@ -31,15 +31,17 @@ static DEFINE_IDR(hwmon_idr); static DEFINE_SPINLOCK(idr_lock); /** - * hwmon_device_register - register w/ hwmon - * @dev: the device to register + * hwmon_device_register_numbered - register hwmon class dev + * with specific device number. + * @dev: Device to register. + * @dev_id: device number to register. * - * hwmon_device_unregister() must be called when the device is no - * longer needed. + * hwmon_device_unregister() must be called when the device + * is no longer needed. * * Returns the pointer to the new device. */ -struct device *hwmon_device_register(struct device *dev) +struct device* hwmon_device_register_numbered(struct device *dev, dev_t dev_id) { struct device *hwdev; int id, err; @@ -58,7 +60,7 @@ again: return ERR_PTR(err); id = id & MAX_ID_MASK; - hwdev = device_create(hwmon_class, dev, MKDEV(0, 0), NULL, + hwdev = device_create(hwmon_class, dev, dev_id, NULL, HWMON_ID_FORMAT, id); if (IS_ERR(hwdev)) { @@ -71,6 +73,20 @@ again: } /** + * hwmon_device_register - register w/ hwmon + * @dev: the device to register + * + * hwmon_device_unregister() must be called when the device is no + * longer needed. + * + * Returns the pointer to the new device. + */ +struct device *hwmon_device_register(struct device *dev) +{ + return hwmon_device_register_numbered(dev, MK_DEV(0,0)); +} + +/** * hwmon_device_unregister - removes the previously registered class device * * @dev: the class device to destroy diff --git a/include/linux/hwmon.h b/include/linux/hwmon.h index 6b6ee70..d816cbe 100644 --- a/include/linux/hwmon.h +++ b/include/linux/hwmon.h @@ -16,6 +16,7 @@ #include <linux/device.h> +struct device *hwmon_device_register_numbered(struct device *dev, dev_t dev_id); struct device *hwmon_device_register(struct device *dev); void hwmon_device_unregister(struct device *dev); -- 1.7.4.1

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understand the result of lockstat
by Zheng Da Oct. 6, 2011

Oct. 6, 2011

Hello, I try to analyze the lock contention in the linux FS, so I enable lockstats in the kernel. But I have some troubles to understand the result in lockstats. When I read 4GB data randomly from a file on a ramdisk with 8 threads (I have 8 cores), and it takes 21 seconds. The most contended lock is class name con-bounces contentions waittime-min waittime-max waittime-total acq-bounces acquisitions holdtime-min holdtime-max holdtime-total &(&mapping->tree_lock)->rlock 104380 104615 0 0 61705.23 169228 2862470 0 0 1010557.22 &(&zone->lru_lock)->rlock 1233 1245 0 0 2632.71 21834 255752 0 3.86 402904.1 xtime_lock 722 731 0 0 1916.15 1342 2901 0 0 4812.51 What is unit of the waittime? in nanoseconds? I look into the code of x86, and it seems the clock time the code reads is nanoseconds. But if it's in nanoseconds, the wait time of mapping->tree_lock is very short, only 62us in total? Also, the result shows that there are 104615 contentions out of 2862470 acquisitions. I guess it's not very highly contended. Thanks, Da

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Current and correct CPU clock and asm("cpuid")
by Peter Senna Tschudin Oct. 6, 2011

Oct. 6, 2011

Dear list members, I'm following: http://people.virginia.edu/~chg5w/page3/assets/MeasuringUnix.pdf And I'm trying to measure executing time of simple operations with RDTSC. See the code below: #include <stdio.h> #define CPU_THOUSAND_HZ 800000 typedef unsigned long long ticks; static __inline__ ticks getticks(void) { unsigned a, d; asm("cpuid"); asm volatile("rdtsc" : "=a" (a), "=d" (d)); return (((ticks)a) | (((ticks)d) << 32)); } void main() { ticks tickBegin, tickEnd; tickBegin = getticks(); // code to time tickEnd = getticks(); double time = (tickEnd-tickBegin)/CPU_THOUSAND_HZ; printf ("%Le\n", time); } How can the C code detects the correct value for CPU_THOUSAND_HZ? The problems I see are: - It is needed to collect the information for the CPU that will run the process. On Core i7 processors, different cores can run at different clock speed at same time. - If the clock changes during the execution of process, what should it do? When is the best time for collecting the clock speed? The authors of the paper are not sure about the effects of "asm("cpuid");" Does it ensure that the entire process will run on the same CPU, and will serialize it avoiding out of order execution by the CPU? Thank you very much! :-) Peter -- Peter Senna Tschudin peter.senna(a)gmail.com gpg id: 48274C36

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Re: [PATCH] hwmon class driver registration with a device number
by Himanshu Chauhan Oct. 6, 2011

Oct. 6, 2011

Hi, > I can not comment on the merits of your patch. Unless I am missing > something, which may well be since I only spent a couple of minutes on > it, other device classes don't seem to provide a similar API, so I don't > know if or why it would make sense for hwmon. Maybe a driver which wants > to register a character device interface should do so independently of > hwmon. > The idea here is to sit in the same class directory as of hwmon. Devices registered with this interface will have "dev" under, for example, /sys/class/hwmon/hwmon0/dev. To do the same inside the driver will be a bit more involved than a call. In my opinion other classes should also have similar interfaces. > On the technical side, EXPORT_SYMBOL is missing. > I will take care of that. Regards Himanshu

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schedule history between 2 interrupts
by Amit Nagal Oct. 5, 2011

Oct. 5, 2011

Hi , i have a gap of 2-3 ms between 2 usb interrupts callback . i want to know which kernel/user threads execute in the gap of 2-3 ms . basically i want to log schedule history between 2 usb interrupts callback . which data structures / functions can i use to furnish schedule logging history for this case ? Regards Amit Nagal

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how does linux restores a thread's stack pointer, program counter and return address.
by Smital Desai Oct. 4, 2011

Oct. 4, 2011

Hi. I am using linux 2.6.39 and MIPS core. I am keen to find out how does linux restores a thread's stack pointer, program counter and return address. I have created a binary which spawns four threads. with a ps -eaL , then I kill one of the thread with kill -11 <thread_id>. In the kernel 'do_coredump' handler i can easily find out the stack pointer of the current crashed thread by reading the struct pt_regs->reg29. My concern is to find out the stack pointers of rest of the 3 threads that did not crash. I did it this way: 1. read the reg29 variable of struct thread_struct for each task (By traversing the list of task with list_for_each) 2. Dump the values at address stored in reg29. 3. Look out manually to locate the Stack pointer offset. Tried this by crashing all the threads and found that the stackpointer comes at a fixed offset and assumed that stack pointer to be correct and I went ahead. so you can say that my Stack pointer is stored at => reg29 + offset; My question now is how to find this value of 'offset' dynamically. How does the kernel keeps a track of any thread's stack pointer while restoring the context. Since my observation was that the reg29 field of struct thread_struct at a glance seems to be a stack pointer. but it is not. it is just an address where at some offset we can find our stack pointer stored. Thanks Smital Desai ________________________________ The contents of this e-mail and any attachment(s) may contain confidential or privileged information for the intended recipient(s). Unintended recipients are prohibited from taking action on the basis of information in this e-mail and using or disseminating the information, and must notify the sender and delete it from their system. L&T Infotech will not accept responsibility or liability for the accuracy or completeness of, or the presence of any virus or disabling code in this e-mail" ______________________________________________________________________

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