Ubuntu Manpage: sched_setattr, sched_getattr - set and get scheduling policy and attributes

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NAME

       sched_setattr, sched_getattr - set and get scheduling policy and attributes

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sched.h>            /* Definition of SCHED_* constants */
       #include <sys/syscall.h>      /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_sched_setattr, pid_t pid, struct sched_attr *attr,
                   unsigned int flags);
       int syscall(SYS_sched_getattr, pid_t pid, struct sched_attr *attr,
                   unsigned int size, unsigned int flags);

       Note: glibc provides no wrappers for these system calls, necessitating the use of syscall(2).

DESCRIPTION

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_FLAG_RESET_ON_FORK
Children created by fork(2) do not inherit privileged scheduling policies. See sched(7)
for details.

SCHED_FLAG_RECLAIM (since Linux 4.13)
This flag allows a SCHED_DEADLINE thread to reclaim bandwidth unused by other real-time
threads.

SCHED_FLAG_DL_OVERRUN (since Linux 4.16)
This flag allows an application to get informed about run-time overruns in SCHED_DEADLINE
threads. Such overruns may be caused by (for example) coarse execution time accounting or
incorrect parameter assignment. Notification takes the form of a SIGXCPU signal which is
generated on each overrun.

This SIGXCPU signal is process-directed (see signal(7)) rather than thread-directed. This
is probably a bug. On the one hand, sched_setattr() is being used to set a per-thread
attribute. On the other hand, if the process-directed signal is delivered to a thread
inside the process other than the one that had a run-time overrun, the application has no
way of knowing which thread overran.

SCHED_FLAG_UTIL_CLAMP_MIN
SCHED_FLAG_UTIL_CLAMP_MAX (both since Linux 5.3)
These flags indicate that the sched_util_min or sched_util_max fields, respectively, are
present, representing the expected minimum and maximum utilization of the thread.

The utilization attributes provide the scheduler with boundaries within which it should
schedule the thread, potentially informing its decisions regarding task placement and
frequency selection.

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.

RETURN VALUE

       On success, sched_setattr() and sched_getattr() return 0.  On error, -1 is returned, and errno is set  to
       indicate the error.

ERRORS

sched_getattr() and sched_setattr() can both fail for the following reasons:

EINVAL attr is NULL; or pid is negative; or flags is not zero.

ESRCH The thread whose ID is pid could not be found.

In addition, sched_getattr() can fail for the following reasons:

E2BIG The buffer specified by size and attr is too small.

EINVAL size is invalid; that is, it is smaller than the initial version of the sched_attr structure (48
bytes) or larger than the system page size.

In addition, sched_setattr() can fail for the following reasons:

E2BIG The buffer specified by size and attr is larger than the kernel structure, and one or more of the
excess bytes is nonzero.

EBUSY SCHED_DEADLINE admission control failure, see sched(7).

EINVAL attr.sched_policy is not one of the recognized policies.

EINVAL attr.sched_flags contains a flag other than SCHED_FLAG_RESET_ON_FORK.

EINVAL attr.sched_priority is invalid.

EINVAL attr.sched_policy is SCHED_DEADLINE, and the deadline scheduling parameters in attr are invalid.

EINVAL attr.sched_flags contains SCHED_FLAG_UTIL_CLAMP_MIN or SCHED_FLAG_UTIL_CLAMP_MAX, and
attr.sched_util_min or attr.sched_util_max are out of bounds.

EOPNOTSUPP
SCHED_FLAG_UTIL_CLAMP was provided, but the kernel was not built with CONFIG_UCLAMP_TASK support.

EPERM The caller does not have appropriate privileges.

EPERM The CPU affinity mask of the thread specified by pid does not include all CPUs in the system (see
sched_setaffinity(2)).

STANDARDS

       Linux.

HISTORY

       Linux 3.14.

NOTES

       glibc does not provide wrappers for these system calls; call them using syscall(2).

       sched_setattr()  provides  a  superset  of the functionality of sched_setscheduler(2), sched_setparam(2),
       nice(2), and (other than the ability to set the priority of all processes belonging to a  specified  user
       or  all processes in a specified group) setpriority(2).  Analogously, sched_getattr() provides a superset
       of the functionality of sched_getscheduler(2), sched_getparam(2), and (partially) getpriority(2).

BUGS

       In Linux versions up to 3.15, sched_setattr() failed with the error EFAULT instead of E2BIG for the  case
       described in ERRORS.

       Up  to  Linux  5.3, sched_getattr() failed with the error EFBIG if the in-kernel sched_attr structure was
       larger than the size passed by user space.