sched_setattr()

The sched_setattr() system call sets the scheduling policy and associated attributes for the thread whose ID is specified in pid. If pid equals zero, the scheduling policy and attributes of the calling thread will be set.

Currently, Linux supports the following "normal" (i.e., non-real-time) scheduling policies as values that may be specified in policy:

SCHED_OTHER

the standard round-robin time-sharing policy;

SCHED_BATCH

for "batch" style execution of processes; and

SCHED_IDLE

for running very low priority background jobs.

Various "real-time" policies are also supported, for special time-critical applications that need precise control over the way in which runnable threads are selected for execution. For the rules governing when a process may use these policies, see sched(7). The real-time policies that may be specified in policy are:

SCHED_FIFO

a first-in, first-out policy; and

SCHED_RR

a round-robin policy.

Linux also provides the following policy:

SCHED_DEADLINE

a deadline scheduling policy; see sched(7) for details.

The attr argument is a pointer to a structure that defines the new scheduling policy and attributes for the specified thread. This structure has the following form:

struct sched_attr {
    u32 size;              /* Size of this structure */
    u32 sched_policy;      /* Policy (SCHED_*) */
    u64 sched_flags;       /* Flags */
    s32 sched_nice;        /* Nice value (SCHED_OTHER,
                              SCHED_BATCH) */
    u32 sched_priority;    /* Static priority (SCHED_FIFO,
                              SCHED_RR) */
    /* For SCHED_DEADLINE */
    u64 sched_runtime;
    u64 sched_deadline;
    u64 sched_period;
    /* Utilization hints */
    u32 sched_util_min;
    u32 sched_util_max;
};

The fields of the sched_attr structure are as follows:

size

This field should be set to the size of the structure in bytes, as in sizeof(struct sched_attr). If the provided structure is smaller than the kernel structure, any additional fields are assumed to be '0'. If the provided structure is larger than the kernel structure, the kernel verifies that all additional fields are 0; if they are not, sched_setattr() fails with the error E2BIG and updates size to contain the size of the kernel structure.

The above behavior when the size of the user-space sched_attr structure does not match the size of the kernel structure allows for future extensibility of the interface. Malformed applications that pass oversize structures won't break in the future if the size of the kernel sched_attr structure is increased. In the future, it could also allow applications that know about a larger user-space sched_attr structure to determine whether they are running on an older kernel that does not support the larger structure.

sched_policy

This field specifies the scheduling policy, as one of the SCHED_* values listed above.

sched_flags

This field contains zero or more of the following flags that are ORed together to control scheduling behavior:

sched_nice

This field specifies the nice value to be set when specifying sched_policy as SCHED_OTHER or SCHED_BATCH. The nice value is a number in the range -20 (high priority) to +19 (low priority); see sched(7).

sched_priority

This field specifies the static priority to be set when specifying sched_policy as SCHED_FIFO or SCHED_RR. The allowed range of priorities for these policies can be determined using sched_get_priority_min(2) and sched_get_priority_max(2). For other policies, this field must be specified as 0.

sched_runtime

This field specifies the "Runtime" parameter for deadline scheduling. The value is expressed in nanoseconds. This field, and the next two fields, are used only for SCHED_DEADLINE scheduling; for further details, see sched(7).

sched_deadline

This field specifies the "Deadline" parameter for deadline scheduling. The value is expressed in nanoseconds.

sched_period

This field specifies the "Period" parameter for deadline scheduling. The value is expressed in nanoseconds.

sched_util_min

sched_util_max (both since Linux 5.3)

These fields specify the expected minimum and maximum utilization, respectively. They are ignored unless their corresponding SCHED_FLAG_UTIL_CLAMP_MIN or SCHED_FLAG_UTIL_CLAMP_MAX is set in sched_flags.

Utilization is a value in the range [0, 1024], representing the percentage of CPU time used by a task when running at the maximum frequency on the highest capacity CPU of the system. This is a fixed point representation, where 1024 corresponds to 100%, and 0 corresponds to 0%. For example, a 20% utilization task is a task running for 2ms every 10ms at maximum frequency and is represented by a utilization value of 0.2 * 1024 = 205.

A task with a minimum utilization value larger than 0 is more likely scheduled on a CPU with a capacity big enough to fit the specified value. A task with a maximum utilization value smaller than 1024 is more likely scheduled on a CPU with no more capacity than the specified value.

A task utilization boundary can be reset by setting its field to UINT32_MAX (since Linux 5.11).

The flags argument is provided to allow for future extensions to the interface; in the current implementation it must be specified as 0.

sched_getattr()

The sched_getattr() system call fetches the scheduling policy and the associated attributes for the thread whose ID is specified in pid. If pid equals zero, the scheduling policy and attributes of the calling thread will be retrieved.

The size argument should be set to the size of the sched_attr structure as known to user space. The value must be at least as large as the size of the initially published sched_attr structure, or the call fails with the error EINVAL.

The retrieved scheduling attributes are placed in the fields of the sched_attr structure pointed to by attr. The kernel sets attr.size to the size of its sched_attr structure.

If the caller-provided attr buffer is larger than the kernel's sched_attr structure, the additional bytes in the user-space structure are not touched. If the caller-provided structure is smaller than the kernel sched_attr structure, the kernel will silently not return any values which would be stored outside the provided space. As with sched_setattr(), these semantics allow for future extensibility of the interface.

The flags argument is provided to allow for future extensions to the interface; in the current implementation it must be specified as 0.