Kernel timing issues

Kernel timing issues An introduction to the use of kernel timers and work queues

Kernel semaphores • Our ‘stash.c’ device-driver exhibited some ‘race conditions’ when we used it with two or more ‘reader’ processes (or with two or more ‘writer’ processes) • We can eliminate such ‘races’ if our driver enforces a ‘one writer/one reader’ policy • This is easy to do by using ‘semaphores’

Mutual-exclusion syntax • Declare a semaphore: struct semaphore sem; • To initialize this semaphore: init_MUTEX( &sem ); • To acquire this semaphore: down_interruptible( &sem ); • To release this semaphore: up( &sem);

‘open()’ uses file->f_fmode • You can implement an ‘open()’ method in your device-driver that lets only one task at a time open your device for reading: { if ( file->f_fmode & FMODE_READ ) down_interruptible( &sem ); return 0; // success }

‘release()’ uses file->f_fmode • You can implement a ‘release()’ method in your device-driver that lets a ‘reader’ task release its earlier acquired semaphore: { if ( file->f_fmode & FMODE_READ ) up( &sem ); return 0; // success }

struct file_operations struct file_operations my_fops = { owner: THIS_MODULE, read: my_read, write: my_write, open: my_open, release: my_release, };

‘newstash.c’ • We wrote a new version of ‘stash.c’ that illustrates the use of two semaphores to restrict device-file access to one ‘writer’ and one ‘reader’ at a time • Other tasks that want to ‘read’ or ‘write’ are put to sleep if they try to ‘open’ the device-file, but are woken up when the appropriate semaphore gets ‘released’

Kernel timers • Another facility Linux offers lets drivers put a process to sleep until a fixed amount of time has elapsed (as measured in jiffies) • When the timer expires, a driver-defined action will be performed, which can ‘wake up’ the process that was put to sleep, or could perform some alternative action (for example, the kernel timer could start over)

Programming syntax • Declare a timer: struct timer_list mytimer; • Initialize this timer: init_timer( &mytimer ); mytimer.func = mytimeraction; mytimer.data = (unsigned long)mydata; mytimer.expires = <number-of-jiffies> • Install this timer: add_timer( &mytimer ); • Modify this timer: mod_timer( &mytimer, <jifs> ); • Delete this timer: del_timer( &mytimer ); • Delete it safely: del_timer_sync( &mytimer);

A kernel-timer caution • A kernel timer’s timeout-action cannot do anything that could cause the current task to ‘sleep’ (such as copying data between user-space and kernel-space, or trying to allocate more kernel memory) • However, to aid debugging, a timer CAN use ‘printk()’ within its timeout-routine

‘trytimer.c’ • We have posted an example that shows how a Linux kernel timer can be used to perform a periodic action (such as using ‘printk()’ to issue a message every time the time expires, then restart the timer • Notice that our demo is not able to issue messages directly to the console – its timer-function executes without a ‘tty’

Delaying work • If a device-driver needs to perform actions that require using process resources (like a tty), or that may possibly ‘sleep’, then it can defer that work – using a ‘workqueue’

Programming syntax • Declare: struct workqueue_struct *myqueue; struct work_struct thework; • Define: void dowork( void *data ) { /* actions */ }; • Initialize: myqueue = create_singlethread_workqueue( “mywork” ); INIT_WORK( &thework, dowork, <data-pointer> ); • Schedule: queue_dalayed_work( myqueue, &thework, <delay> ); • Cleanup: if ( !cancel_delayed_work( &thework ) ) flush_workqueue( myqueue ); destroy_workqueue( myqueue );

‘tryworkq.c’ and ‘defermsg.c’ • We have posted demo-modules that show the use of workqueues to perform actions later, either as soon as a ‘process context’ is available, or after a prescribed time • Further details on the options for using an existing kernel workqueue or a workqueue of your own creation may be found in our textbook (Chapter 7 of LDD3)

Applying these ideas • To demonstrate a programming situation in which using kernel timers is valuable, we created the ‘foo.c’ device-driver, plus an application that uses it (‘watchfoo.cpp’) • You can compile and install the module, then execute the application: $ watchfoo • But you will notice there are two ‘problems’ (excess cpu usage and loop-termination)

Reducing CPU’s usage • The ‘watchfoo’ program rereads ‘/dev/foo’ constantly (numerous times per second), much faster than the human eye can see • If you run the ‘top’ utility, you will see that up to 99-percent of the available CPU time is being consumed by ‘watchfoo’ • You can add a kernel timer to the ‘foo.c’ driver to curtail this excessive reading

In-class exercise • Modify ‘foo.c’ (call it ‘timedfoo.c’) as follows • Create an integer flag-variable (‘ready’) as a global object in your module • When your ‘read()’ function gets called, it should sleep until ‘ready’ equals TRUE; it should set ‘ready’ equal to FALSE when it awakens, but should set a timer to expire after 1/10 seconds • Your timer’s action-function should set ‘ready’ back to TRUE, then wake up any sleeping tasks

Implementation hints • You need a wait-queue (so your driver’s ‘reader’ tasks can sleep on it) • You need a timer action-function • You need to organize your timer-function’s data-items into a single structure (because the timer-function has only one argument) • Your timer-function must do two things: • Change ‘ready’ to TRUE • Wake up any ‘sleepers’

Kernel timing issues