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manual: Document lack of conformance of sched_* functions [BZ #14829]

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On Linux, we define _POSIX_PRIORITY_SCHEDULING, but functions such
as sched_setparam and sched_setscheduler apply to individual threads,
not processes.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
This commit is contained in:
Florian Weimer 2019-02-02 14:13:33 +01:00
parent 4dcbbc3b28
commit c70824b9a4
3 changed files with 48 additions and 27 deletions
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10
ChangeLog
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@ -1,3 +1,13 @@
2019-02-02 Florian Weimer <fweimer@redhat.com>
[BZ #14829]
* manual/resource.texi (Basic Scheduling Functions): Add
portability note. Change process to task throughout the section.
Remove incorrect comment about sched_yield as it affects
tasks/threads, not entire processes.
* sysdeps/unix/sysv/linux/bits/posix_opt.h
(_POSIX_PRIORITY_SCHEDULING): Update comment.
2019-02-01 Joseph Myers <joseph@codesourcery.com>
* configure.ac (libc_cv_compiler_ok): Require GCC 6.2 or later.

60
manual/resource.texi
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@ -750,6 +750,14 @@ policy, if anything, only fine tunes the effect of that priority.
The symbols in this section are declared by including file @file{sched.h}.
@strong{Portability Note:} In POSIX, the @code{pid_t} arguments of the
functions below refer to process IDs. On Linux, they are actually
thread IDs, and control how specific threads are scheduled with
regards to the entire system. The resulting behavior does not conform
to POSIX. This is why the following description refers to tasks and
tasks IDs, and not processes and process IDs.
@c https://sourceware.org/bugzilla/show_bug.cgi?id=14829
@deftp {Data Type} {struct sched_param}
@standards{POSIX, sched.h}
This structure describes an absolute priority.
@ -765,11 +773,11 @@ absolute priority value
@c Direct syscall, Linux only.
This function sets both the absolute priority and the scheduling policy
for a process.
for a task.
It assigns the absolute priority value given by @var{param} and the
scheduling policy @var{policy} to the process with Process ID @var{pid},
or the calling process if @var{pid} is zero. If @var{policy} is
scheduling policy @var{policy} to the task with ID @var{pid},
or the calling task if @var{pid} is zero. If @var{policy} is
negative, @code{sched_setscheduler} keeps the existing scheduling policy.
The following macros represent the valid values for @var{policy}:
@ -795,20 +803,20 @@ to this function are:
@item EPERM
@itemize @bullet
@item
The calling process does not have @code{CAP_SYS_NICE} permission and
The calling task does not have @code{CAP_SYS_NICE} permission and
@var{policy} is not @code{SCHED_OTHER} (or it's negative and the
existing policy is not @code{SCHED_OTHER}.
@item
The calling process does not have @code{CAP_SYS_NICE} permission and its
owner is not the target process' owner. I.e., the effective uid of the
calling process is neither the effective nor the real uid of process
The calling task does not have @code{CAP_SYS_NICE} permission and its
owner is not the target task's owner. I.e., the effective uid of the
calling task is neither the effective nor the real uid of task
@var{pid}.
@c We need a cross reference to the capabilities section, when written.
@end itemize
@item ESRCH
There is no process with pid @var{pid} and @var{pid} is not zero.
There is no task with pid @var{pid} and @var{pid} is not zero.
@item EINVAL
@itemize @bullet
@ -835,8 +843,8 @@ tell you what the valid range is.
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
@c Direct syscall, Linux only.
This function returns the scheduling policy assigned to the process with
Process ID (pid) @var{pid}, or the calling process if @var{pid} is zero.
This function returns the scheduling policy assigned to the task with
ID @var{pid}, or the calling task if @var{pid} is zero.
The return value is the scheduling policy. See
@code{sched_setscheduler} for the possible values.
@ -849,7 +857,7 @@ The @code{errno} values specific to this function are:
@table @code
@item ESRCH
There is no process with pid @var{pid} and it is not zero.
There is no task with pid @var{pid} and it is not zero.
@item EINVAL
@var{pid} is negative.
@ -869,7 +877,7 @@ absolute priority, use @code{sched_getparam}.
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
@c Direct syscall, Linux only.
This function sets a process' absolute priority.
This function sets a task's absolute priority.
It is functionally identical to @code{sched_setscheduler} with
@var{policy} = @code{-1}.
@ -883,13 +891,13 @@ It is functionally identical to @code{sched_setscheduler} with
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
@c Direct syscall, Linux only.
This function returns a process' absolute priority.
This function returns a task's absolute priority.
@var{pid} is the Process ID (pid) of the process whose absolute priority
you want to know.
@var{pid} is the task ID of the task whose absolute priority you want
to know.
@var{param} is a pointer to a structure in which the function stores the
absolute priority of the process.
absolute priority of the task.
On success, the return value is @code{0}. Otherwise, it is @code{-1}
and @code{errno} is set accordingly. The @code{errno} values specific
@ -898,7 +906,7 @@ to this function are:
@table @code
@item ESRCH
There is no process with pid @var{pid} and it is not zero.
There is no task with ID @var{pid} and it is not zero.
@item EINVAL
@var{pid} is negative.
@ -914,7 +922,7 @@ There is no process with pid @var{pid} and it is not zero.
@c Direct syscall, Linux only.
This function returns the lowest absolute priority value that is
allowable for a process with scheduling policy @var{policy}.
allowable for a task with scheduling policy @var{policy}.
On Linux, it is 0 for SCHED_OTHER and 1 for everything else.
@ -935,7 +943,7 @@ to this function are:
@c Direct syscall, Linux only.
This function returns the highest absolute priority value that is
allowable for a process that with scheduling policy @var{policy}.
allowable for a task that with scheduling policy @var{policy}.
On Linux, it is 0 for SCHED_OTHER and 99 for everything else.
@ -956,8 +964,8 @@ to this function are:
@c Direct syscall, Linux only.
This function returns the length of the quantum (time slice) used with
the Round Robin scheduling policy, if it is used, for the process with
Process ID @var{pid}.
the Round Robin scheduling policy, if it is used, for the task with
task ID @var{pid}.
It returns the length of time as @var{interval}.
@c We need a cross-reference to where timespec is explained. But that
@ -980,18 +988,18 @@ function, so there are no specific @code{errno} values.
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
@c Direct syscall on Linux; alias to swtch on HURD.
This function voluntarily gives up the process' claim on the CPU.
This function voluntarily gives up the task's claim on the CPU.
Technically, @code{sched_yield} causes the calling process to be made
Technically, @code{sched_yield} causes the calling task to be made
immediately ready to run (as opposed to running, which is what it was
before). This means that if it has absolute priority higher than 0, it
gets pushed onto the tail of the queue of processes that share its
gets pushed onto the tail of the queue of tasks that share its
absolute priority and are ready to run, and it will run again when its
turn next arrives. If its absolute priority is 0, it is more
complicated, but still has the effect of yielding the CPU to other
processes.
tasks.
If there are no other processes that share the calling process' absolute
If there are no other tasks that share the calling task's absolute
priority, this function doesn't have any effect.
To the extent that the containing program is oblivious to what other

5
sysdeps/unix/sysv/linux/bits/posix_opt.h
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@ -25,7 +25,10 @@
/* Processes have a saved set-user-ID and a saved set-group-ID. */
#define _POSIX_SAVED_IDS 1
/* Priority scheduling is supported. */
/* Priority scheduling is not supported with the correct semantics,
but GNU/Linux applications expect that the corresponding interfaces
are available, even though the semantics do not meet the POSIX
requirements. See glibc bug 14829. */
#define _POSIX_PRIORITY_SCHEDULING 200809L
/* Synchronizing file data is supported. */