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Kernel Timing

Zephyr provides a robust and scalable timing framework to enable reporting and tracking of timed events from hardware timing sources of arbitrary precision.

Time Units

Kernel time is tracked in several units which are used for different purposes.

Real time values, typically specified in milliseconds or microseconds, are the default presentation of time to application code. They have the advantages of being universally portable and pervasively understood, though they may not match the precision of the underlying hardware perfectly.

The kernel presents a “cycle” count via the k_cycle_get_32() API. The intent is that this counter represents the fastest cycle counter that the operating system is able to present to the user (for example, a CPU cycle counter) and that the read operation is very fast. The expectation is that very sensitive application code might use this in a polling manner to achieve maximal precision. The frequency of this counter is required to be steady over time, and is available from sys_clock_hw_cycles_per_sec() (which on almost all platforms is a runtime constant that evaluates to CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC).

For asynchronous timekeeping, the kernel defines a “ticks” concept. A “tick” is the internal count in which the kernel does all its internal uptime and timeout bookeeping. Interrupts are expected to be delivered on tick boundaries to the extent practical, and no fractional ticks are tracked. The choice of tick rate is configurable via CONFIG_SYS_CLOCK_TICKS_PER_SEC. Defaults on most hardware platforms (ones that support setting arbitrary interrupt timeouts) are expected to be in the range of 10 kHz, with software emulation platforms and legacy drivers using a more traditional 100 Hz value.

Conversion

Zephyr provides an extensively enumerated conversion library with rounding control for all time units. Any unit of “ms” (milliseconds), “us” (microseconds), “tick”, or “cyc” can be converted to any other. Control of rounding is provided, and each conversion is available in “floor” (round down to nearest output unit), “ceil” (round up) and “near” (round to nearest). Finally the output precision can be specified as either 32 or 64 bits.

For example: k_ms_to_ticks_ceil32() will convert a millisecond input value to the next higher number of ticks, returning a result truncated to 32 bits of precision; and k_cyc_to_us_floor64() will convert a measured cycle count to an elapsed number of microseconds in a full 64 bits of precision. See the reference documentation for the full enumeration of conversion routines.

On most platforms, where the various counter rates are integral multiples of each other and where the output fits within a single word, these conversions expand to a 2-4 operation sequence, requiring full precision only where actually required and requested.

Uptime

The kernel tracks a system uptime count on behalf of the application. This is available at all times via k_uptime_get(), which provides an uptime value in milliseconds since system boot. This is expected to be the utility used by most portable application code.

The internal tracking, however, is as a 64 bit integer count of ticks. Apps with precise timing requirements (that are willing to do their own conversions to portable real time units) may access this with k_uptime_ticks().

Timeouts

The Zephyr kernel provides many APIs with a “timeout” parameter. Conceptually, this indicates the time at which an event will occur. For example:

  • Kernel blocking operations like k_sem_take() or k_queue_get() may provide a timeout after which the routine will return with an error code if no data is available.

  • Kernel struct k_timer objects must specify delays for their duration and period.

  • The kernel k_delayed_work API provides a timeout parameter indicating when a work queue item will be added to the system queue.

All these values are specified using a k_timeout_t value. This is an opaque struct type that must be initialized using one of a family of kernel timeout macros. The most common, K_MSEC(), defines a time in milliseconds after the current time (strictly: the time at which the kernel receives the timeout value).

Other options for timeout initialization follow the unit conventions described above: K_NSEC(), K_USEC(), K_TICKS() and K_CYC() specify timeout values that will expire after specified numbers of nanoseconds, microseconds, ticks and cycles, respectively.

Precision of k_timeout_t values is configurable, with the default being 32 bits. Large uptime counts in non-tick units will experience complicated rollover semantics, so it is expected that timing-sensitive applications with long uptimes will be configured to use a 64 bit timeout type.

Finally, it is possible to specify timeouts as absolute times since system boot. A timeout initialized with K_TIMEOUT_ABS_MS() indicates a timeout that will expire after the system uptime reaches the specified value. There are likewise nanosecond, microsecond, cycles and ticks variants of this API.

Timing Internals

Timeout Queue

All Zephyr k_timeout_t events specified using the API above are managed in a single, global queue of events. Each event is stored in a double-linked list, with an attendant delta count in ticks from the previous event. The action to take on an event is specified as a callback function pointer provided by the subsystem requesting the event, along with a struct _timeout tracking struct that is expected to be embedded within subsystem-defined data structures (for example: a struct wait_q, or a k_tid_t thread struct).

Note that all variant units passed via a k_timeout_t are converted to ticks once on insertion into the list. There no multiple-conversion steps internal to the kernel, so precision is guaranteed at the tick level no matter how many events exist or how long a timeout might be.

Note that the list structure means that the CPU work involved in managing large numbers of timeouts is quadratic in the number of active timeouts. The API design of the timeout queue was intended to permit a more scalable backend data structure, but no such implementation exists currently.

Timer Drivers

Kernel timing at the tick level is driven by a timer driver with a comparatively simple API.

  • The driver is expected to be able to “announce” new ticks to the kernel via the z_clock_announce() call, which passes an integer number of ticks that have elapsed since the last announce call (or system boot). These calls can occur at any time, but the driver is expected to attempt to ensure (to the extent practical given interrupt latency interactions) that they occur near tick boundaries (i.e. not “halfway through” a tick), and most importantly that they be correct over time and subject to minimal skew vs. other counters and real world time.

  • The driver is expected to provide a z_clock_set_timeout() call to the kernel which indicates how many ticks may elapse before the kernel must receive an announce call to trigger registered timeouts. It is legal to announce new ticks before that moment (though they must be correct) but delay after that will cause events to be missed. Note that the timeout value passed here is in a delta from current time, but that does not absolve the driver of the requirement to provide ticks at a steady rate over time. Naive implementations of this function are subject to bugs where the fractional tick gets “reset” incorrectly and causes clock skew.

  • The driver is expected to provide a z_clock_elapsed() call which provides a current indication of how many ticks have elapsed (as compared to a real world clock) since the last call to z_clock_announce(), which the kernel needs to test newly arriving timeouts for expiration.

Note that a natural implementation of this API results in a “tickless” kernel, which receives and processes timer interrupts only for registered events, relying on programmable hardware counters to provide irregular interrupts. But a traditional, “ticked” or “dumb” counter driver can be trivially implemented also:

  • The driver can receive interrupts at a regular rate corresponding to the OS tick rate, calling z_clock_anounce() with an argument of one each time.

  • The driver can ignore calls to z_clock_set_timeout(), as every tick will be announced regardless of timeout status.

  • The driver can return zero for every call to z_clock_elapsed() as no more than one tick can be detected as having elapsed (because otherwise an interrupt would have been received).

SMP Details

In general, the timer API described above does not change when run in a multiprocessor context. The kernel will internally synchronize all access appropriately, and ensure that all critical sections are small and minimal. But some notes are important to detail:

  • Zephyr is agnostic about which CPU services timer interrupts. It is not illegal (though probably undesirable in some circumstances) to have every timer interrupt handled on a single processor. Existing SMP architectures implement symmetric timer drivers.

  • The z_clock_announce() call is expected to be globally synchronized at the driver level. The kernel does not do any per-CPU tracking, and expects that if two timer interrupts fire near simultaneously, that only one will provide the current tick count to the timing subsystem. The other may legally provide a tick count of zero if no ticks have elapsed. It should not “skip” the announce call because of timeslicing requirements (see below).

  • Some SMP hardware uses a single, global timer device, others use a per-CPU counter. The complexity here (for example: ensuring counter synchronization between CPUs) is expected to be managed by the driver, not the kernel.

  • The next timeout value passed back to the driver via z_clock_set_timeout() is done identically for every CPU. So by default, every CPU will see simultaneous timer interrupts for every event, even though by definition only one of them should see a non-zero ticks argument to z_clock_announce(). This is probably a correct default for timing sensitive applications (because it minimizes the chance that an errant ISR or interrupt lock will delay a timeout), but may be a performance problem in some cases. The current design expects that any such optimization is the responsibility of the timer driver.

Time Slicing

An auxiliary job of the timing subsystem is to provide tick counters to the scheduler that allow implementation of time slicing of threads. A thread time-slice cannot be a timeout value, as it does not reflect a global expiration but instead a per-CPU value that needs to be tracked independently on each CPU in an SMP context.

Because there may be no other hardware available to drive timeslicing, Zephyr multiplexes the existing timer driver. This means that the value passed to z_clock_set_timeout() may be clamped to a smaller value than the current next timeout when a time sliced thread is currently scheduled.

Legacy Usage and Porting Guide

In earlier versions of Zephyr, the k_timeout_t abstraction did not exist and timeouts were presented to the kernel as signed integer values specified in milliseconds. The K_FOREVER value was defined with a value of -1.

In general, application code that uses the pre-existing constructor macros (K_MSEC() et. al.) will continue to work without change. Code that presents raw milliseconds to the calls can simply wrap the argument in K_MSEC().

Some Zephyr subsystem code, however, was written originally to present their own APIs to the user which accept millisecond values (including K_FOREVER) and take actions like storing the value for later, or performing arithmetic on the value. This will no longer work unmodified in the new scheme.

One option in the immediate term is to use the CONFIG_LEGACY_TIMEOUT_API kconfig. This redefines the k_timeout_t type to be a 32 bit integer and preserves source code compatibility with the older APIs. This comes at the cost of disabling newer features like absolute timeouts and 64 bit precision. This kconfig exists for application code, however, and will be going away in a forthcoming release.

A better scheme is to port the subsystem to the new timeout scheme directly. There are two broad architectures for doing this: using k_timeout_t naturally as an application API, or preserving the millisecond subsystem API and converting internally.

Subsystems that keep millisecond APIs

In general, code like this will port just like applications code will. Millisecond values from the user may be treated any way the subsystem likes, and then converted into kernel timeouts using K_MSEC() at the point where they are presented to the kernel.

Obviously this comes at the cost of not being able to use new features, like the higher precision timeout constructors or absolute timeouts. But for many subsystems with simple needs, this may be acceptable.

One complexity is K_FOREVER. Subsystems that might have been able to accept this value to their millisecond API in the past no longer can, becauase it is no longer an intergral type. Such code will need to use a different, integer-valued token to represent “forever”. K_NO_WAIT has the same typesafety concern too, of course, but as it is (and has always been) simply a numerical zero, it has a natural porting path.

Subsystems using k_timeout_t

Ideally, code that takes a “timeout” parameter specifying a time to wait should be using the kernel native abstraction where possible. But k_timeout_t is opaque, and needs to be converted before it can be inspected by an application.

Some conversions are simple. Code that needs to test for K_FOREVER can simply use the K_TIMEOUT_EQ() macro to test the opaque struct for equality and take special action.

The more complicated case is when the subsystem needs to take a timeout and loop, waiting for it to finish while doing some processing that may require multiple blocking operations on underlying kernel code. For example, consider this design:

void my_wait_for_event(struct my_subsys *obj, s32_t timeout_in_ms)
{
    while (true) {
        u32_t start = k_uptime_get_32();

        if (is_event_complete(obj)) {
            return;
        }

        /* Wait for notification of state change */
        k_sem_take(obj->sem, timeout_in_ms);

        /* Subtract elapsed time */
        timeout_in_ms -= (k_uptime_get_32() - start);
    }
}

This code requires that the timeout value be inspected, which is no longer possible. For situations like this, the new API provides an internal z_timeout_end_calc() routine that converts an arbitrary timeout to the uptime value in ticks at which it will expire. So such a loop might look like:

void my_wait_for_event(struct my_subsys *obj, k_timeout_t timeout_in_ms)
{
    /* Compute the end time from the timeout */
    u64_t end = z_timeout_end_calc(timeout_in_ms);

    while (end < k_uptime_ticks()) {
            if (is_event_complete(obj)) {
                return;
            }

            /* Wait for notification of state change */
            k_sem_take(obj->sem, timeout_in_ms);
    }
}

Note that z_timeout_end_calc() returns values in units of ticks, to prevent conversion aliasing, is always presented at 64 bit uptime precision to prevent rollover bugs, handles special K_FOREVER naturally (as UINT64_MAX), and works identically for absolute timeouts as well as conventional ones.

But some care is still required for subsystems that use it. Note that delta timeouts need to be interpreted relative to a “current time”, and obviously that time is the time of the call to z_timeout_end_calc(). But the user expects that the time is the time they passed the timeout to you. Care must be taken to call this function just once, as synchronously as possible to the timeout creation in user code. It should not be used on a “stored” timeout value, and should never be called iteratively in a loop.