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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) */
               /* 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.

SEE ALSO

       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)