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

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) */
/* Remaining fields are for SCHED_DEADLINE */
u64 sched_runtime;
u64 sched_deadline;
u64 sched_period;
};

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_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.

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; attr.sched_flags contains
              a flag other than SCHED_FLAG_RESET_ON_FORK; or attr.sched_priority is  invalid;  or
              attr.sched_policy  is SCHED_DEADLINE and the deadline scheduling parameters in attr
              are invalid.

       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.