.\" Copyright (C) 2014 Michael Kerrisk .\" and Copyright (C) 2014 Peter Zijlstra .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .TH sched_setattr 2 2023-03-30 "Linux man-pages 6.05.01" .SH NAME sched_setattr, sched_getattr \- set and get scheduling policy and attributes .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .BR "#include " " /* Definition of " SCHED_* " constants */" .BR "#include " " /* Definition of " SYS_* " constants */" .B #include .PP .BI "int syscall(SYS_sched_setattr, pid_t " pid ", struct sched_attr *" attr , .BI " unsigned int " flags ); .BI "int syscall(SYS_sched_getattr, pid_t " pid ", struct sched_attr *" attr , .BI " unsigned int " size ", unsigned int " flags ); .fi .\" FIXME . Add feature test macro requirements .PP .IR Note : glibc provides no wrappers for these system calls, necessitating the use of .BR syscall (2). .SH DESCRIPTION .SS sched_setattr() The .BR sched_setattr () system call sets the scheduling policy and associated attributes for the thread whose ID is specified in .IR pid . If .I pid equals zero, the scheduling policy and attributes of the calling thread will be set. .PP Currently, Linux supports the following "normal" (i.e., non-real-time) scheduling policies as values that may be specified in .IR policy : .TP 14 .B SCHED_OTHER the standard round-robin time-sharing policy; .\" In the 2.6 kernel sources, SCHED_OTHER is actually called .\" SCHED_NORMAL. .TP .B SCHED_BATCH for "batch" style execution of processes; and .TP .B SCHED_IDLE for running .I very low priority background jobs. .PP 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 .BR sched (7). The real-time policies that may be specified in .I policy are: .TP 14 .B SCHED_FIFO a first-in, first-out policy; and .TP .B SCHED_RR a round-robin policy. .PP Linux also provides the following policy: .TP 14 .B SCHED_DEADLINE a deadline scheduling policy; see .BR sched (7) for details. .PP The .I 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: .PP .in +4n .EX 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; }; .EE .in .PP The fields of the .I sched_attr structure are as follows: .TP .B size This field should be set to the size of the structure in bytes, as in .IR "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, .BR sched_setattr () fails with the error .B E2BIG and updates .I size to contain the size of the kernel structure. .IP The above behavior when the size of the user-space .I 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 .I sched_attr structure is increased. In the future, it could also allow applications that know about a larger user-space .I sched_attr structure to determine whether they are running on an older kernel that does not support the larger structure. .TP .I sched_policy This field specifies the scheduling policy, as one of the .B SCHED_* values listed above. .TP .I sched_flags This field contains zero or more of the following flags that are ORed together to control scheduling behavior: .RS .TP .B SCHED_FLAG_RESET_ON_FORK Children created by .BR fork (2) do not inherit privileged scheduling policies. See .BR sched (7) for details. .TP .BR SCHED_FLAG_RECLAIM " (since Linux 4.13)" .\" 2d4283e9d583a3ee8cfb1cbb9c1270614df4c29d This flag allows a .B SCHED_DEADLINE thread to reclaim bandwidth unused by other real-time threads. .\" Bandwidth reclaim is done via the GRUB algorithm; see .\" Documentation/scheduler/sched-deadline.txt .TP .BR SCHED_FLAG_DL_OVERRUN " (since Linux 4.16)" .\" commit 34be39305a77b8b1ec9f279163c7cdb6cc719b91 This flag allows an application to get informed about run-time overruns in .B 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 .B SIGXCPU signal which is generated on each overrun. .IP This .B SIGXCPU signal is .I process-directed (see .BR signal (7)) rather than thread-directed. This is probably a bug. On the one hand, .BR 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. .RE .TP .I sched_nice This field specifies the nice value to be set when specifying .I sched_policy as .B SCHED_OTHER or .BR SCHED_BATCH . The nice value is a number in the range \-20 (high priority) to +19 (low priority); see .BR sched (7). .TP .I sched_priority This field specifies the static priority to be set when specifying .I sched_policy as .B SCHED_FIFO or .BR SCHED_RR . The allowed range of priorities for these policies can be determined using .BR sched_get_priority_min (2) and .BR sched_get_priority_max (2). For other policies, this field must be specified as 0. .TP .I 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 .B SCHED_DEADLINE scheduling; for further details, see .BR sched (7). .TP .I sched_deadline This field specifies the "Deadline" parameter for deadline scheduling. The value is expressed in nanoseconds. .TP .I sched_period This field specifies the "Period" parameter for deadline scheduling. The value is expressed in nanoseconds. .PP The .I flags argument is provided to allow for future extensions to the interface; in the current implementation it must be specified as 0. .\" .\" .SS sched_getattr() The .BR sched_getattr () system call fetches the scheduling policy and the associated attributes for the thread whose ID is specified in .IR pid . If .I pid equals zero, the scheduling policy and attributes of the calling thread will be retrieved. .PP The .I size argument should be set to the size of the .I sched_attr structure as known to user space. The value must be at least as large as the size of the initially published .I sched_attr structure, or the call fails with the error .BR EINVAL . .PP The retrieved scheduling attributes are placed in the fields of the .I sched_attr structure pointed to by .IR attr . The kernel sets .I attr.size to the size of its .I sched_attr structure. .PP If the caller-provided .I attr buffer is larger than the kernel's .I sched_attr structure, the additional bytes in the user-space structure are not touched. If the caller-provided structure is smaller than the kernel .I sched_attr structure, the kernel will silently not return any values which would be stored outside the provided space. As with .BR sched_setattr (), these semantics allow for future extensibility of the interface. .PP The .I flags argument is provided to allow for future extensions to the interface; in the current implementation it must be specified as 0. .SH RETURN VALUE On success, .BR sched_setattr () and .BR sched_getattr () return 0. On error, \-1 is returned, and .I errno is set to indicate the error. .SH ERRORS .BR sched_getattr () and .BR sched_setattr () can both fail for the following reasons: .TP .B EINVAL .I attr is NULL; or .I pid is negative; or .I flags is not zero. .TP .B ESRCH The thread whose ID is .I pid could not be found. .PP In addition, .BR sched_getattr () can fail for the following reasons: .TP .B E2BIG The buffer specified by .I size and .I attr is too small. .TP .B EINVAL .I size is invalid; that is, it is smaller than the initial version of the .I sched_attr structure (48 bytes) or larger than the system page size. .PP In addition, .BR sched_setattr () can fail for the following reasons: .TP .B E2BIG The buffer specified by .I size and .I attr is larger than the kernel structure, and one or more of the excess bytes is nonzero. .TP .B EBUSY .B SCHED_DEADLINE admission control failure, see .BR sched (7). .TP .B EINVAL .I attr.sched_policy is not one of the recognized policies; .I attr.sched_flags contains a flag other than .BR SCHED_FLAG_RESET_ON_FORK ; or .I attr.sched_priority is invalid; or .I attr.sched_policy is .B SCHED_DEADLINE and the deadline scheduling parameters in .I attr are invalid. .TP .B EPERM The caller does not have appropriate privileges. .TP .B EPERM The CPU affinity mask of the thread specified by .I pid does not include all CPUs in the system (see .BR sched_setaffinity (2)). .SH STANDARDS Linux. .SH HISTORY Linux 3.14. .\" FIXME . Add glibc version .SH NOTES glibc does not provide wrappers for these system calls; call them using .BR syscall (2). .PP .BR sched_setattr () provides a superset of the functionality of .BR sched_setscheduler (2), .BR sched_setparam (2), .BR 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) .BR setpriority (2). Analogously, .BR sched_getattr () provides a superset of the functionality of .BR sched_getscheduler (2), .BR sched_getparam (2), and (partially) .BR getpriority (2). .SH BUGS In Linux versions up to .\" FIXME . patch sent to Peter Zijlstra 3.15, .BR sched_setattr () failed with the error .B EFAULT instead of .B E2BIG for the case described in ERRORS. .PP Up to Linux 5.3, .BR sched_getattr () failed with the error .B EFBIG if the in-kernel .I sched_attr structure was larger than the .I size passed by user space. .\" In Linux versions up to up 3.15, .\" FIXME . patch from Peter Zijlstra pending .\" .BR sched_setattr () .\" allowed a negative .\" .I attr.sched_policy .\" value. .SH SEE ALSO .ad l .nh .BR chrt (1), .BR nice (2), .BR sched_get_priority_max (2), .BR sched_get_priority_min (2), .BR sched_getaffinity (2), .BR sched_getparam (2), .BR sched_getscheduler (2), .BR sched_rr_get_interval (2), .BR sched_setaffinity (2), .BR sched_setparam (2), .BR sched_setscheduler (2), .BR sched_yield (2), .BR setpriority (2), .BR pthread_getschedparam (3), .BR pthread_setschedparam (3), .BR pthread_setschedprio (3), .BR capabilities (7), .BR cpuset (7), .BR sched (7) .ad