sched_setattr(2) — Linux manual page

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sched_setattr(2)           System Calls Manual          sched_setattr(2)

NAME         top

       sched_setattr, sched_getattr - set and get scheduling policy and
       attributes

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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         top

   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         top

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

ERRORS         top

       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         top

       Linux.

HISTORY         top

       Linux 3.14.

NOTES         top

       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         top

       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.

SEE ALSO         top

       chrt(1), nice(2), sched_get_priority_max(2),
       sched_get_priority_min(2), sched_getaffinity(2),
       sched_getparam(2), sched_getscheduler(2),
       sched_rr_get_interval(2), sched_setaffinity(2),
       sched_setparam(2), sched_setscheduler(2), sched_yield(2),
       setpriority(2), pthread_getschedparam(3),
       pthread_setschedparam(3), pthread_setschedprio(3),
       capabilities(7), cpuset(7), sched(7)

COLOPHON         top

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Linux man-pages 6.9.1          2024-06-13               sched_setattr(2)

Pages that refer to this page: uclampset(1)openat2(2)sched_setparam(2)sched_setscheduler(2)syscalls(2)capabilities(7)credentials(7)sched(7)