Kernel Timing Management covers Timing architecture with Hertz, jiffies, Kernel Timers, Delays in linux kernel

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Kernel Timing Management

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What to Expect
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Timing Architecture
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Ticking in jiffies
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Delaying the process
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Kernel Timers

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What is Time keeping?
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User spaces deals with the absolute time
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2 ‘O clock for back up
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12 ‘O clock for shutdown
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Kernel space deals with relative timing
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Data transfer should finish in 5 msecs
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Operations mostly in mses, worst case seconds

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HZ – ticks per second
– Varies from 50 to 1000
– Typically 1000 for desktops & 100 for embedded
systems
– Can be modified using the kernel configuration
– 1 tick = 1ms (Desktop), 10ms (embedded systems)
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Jiffy
– Internal kernel counter
– Increments with each timer tick
– jiffies & jiffies_64
Unit of time in kernel

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API’s
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#include <linux/jiffies.h>
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get_jiffies_64()
– For accessing the 64 bit jiffies
– Direct access not recommended
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Comparison
– int time_before(a, b)
– int time_after(a, b)
– int time_before_eq(a, b)
– int time_after_eq(a, b)

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Long Delays
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Busy wait: cpu_relax
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while (time_before(jiffies, j1))
– cpu_relax
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Yielding: schedule/schedule_timeout
– while(time_before(jiffies,j1))
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schedule()

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Header: <linux/delay.h>
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Arch specific header: <asm/delay.h>
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APIs
– void ndelay(unsigned long delays)
– void udelay(unsigned long udelays)
– void mdelay(unsigned long mdelays)
Short delays but busy waiting

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Long Delays, Back to Yielding
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Header: <linux/delay.h>
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APIs
– void msleep(unsigned int millisecs)
– unsigned long msleep_interruptible(unsigned int
millisecs)
– void ssleep(unsigned int secs)

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Kernel Timers
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Allows to schedule a task sometime in future
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Executes a user defined function at user
defined time
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Example usage
– Monitoring the button in absence of interrupts
– Finishing some lengthy shutdowns
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Executes asynchronously & in interrupt context.
So, schedule out is not allowed

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Data Structures for Kernel Timers
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Back end of the various delays
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Header: <linux/timer.h>
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Type: struct timer_list
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APIs
– DEFINE_TIMER(my_timer, fn)
– Void add_timer(struct timer_list *)
– timer_setup(struct timer_list *t, fn, flags);
– void del_timer(struct timer_list *)
– mod_timer(struct timer_list *, unsigned long);
– int del_timer_sync(struct timer_list *)

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What all did we learn?
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Timing Architecture
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Ticking in jiffies
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Delaying the process
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Kernel Timers