* [PATCH 0/3] Discourage sched_yield(2)
@ 2023-05-03 17:03 Alejandro Colomar
2023-05-03 17:03 ` [PATCH 1/3] sched_yield.2: HISTORY: POSIX.1-2008 makes this non-optional Alejandro Colomar
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: Alejandro Colomar @ 2023-05-03 17:03 UTC (permalink / raw)
To: linux-man, a.clayton; +Cc: Alejandro Colomar, andrew
Hi Andrew,
Here's a patch set for discouraging sched_yield(2). See the formatted
page at the bottom. I also updated some POSIX historic detail.
Cheers,
Alex
Alejandro Colomar (3):
sched_yield.2: HISTORY: POSIX.1-2008 makes this non-optional
sched_yield.2: NOTES: Remove misleading sentence
sched_yield.2: Rename NOTES to CAVEATS, and reorder contents
man2/sched_yield.2 | 41 +++++++++++++++++++----------------------
1 file changed, 19 insertions(+), 22 deletions(-)
---
$ MANWIDTH=72 man ./man2/sched_yield.2 | cat
sched_yield(2) System Calls Manual sched_yield(2)
NAME
sched_yield - yield the processor
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sched.h>
int sched_yield(void);
DESCRIPTION
sched_yield() causes the calling thread to relinquish the CPU.
The thread is moved to the end of the queue for its static pri‐
ority and a new thread gets to run.
RETURN VALUE
On success, sched_yield() returns 0. On error, -1 is returned,
and errno is set to indicate the error.
ERRORS
In the Linux implementation, sched_yield() always succeeds.
STANDARDS
POSIX.1‐2008.
HISTORY
POSIX.1‐2001 (but optional). POSIX.1‐2008.
Before POSIX.1‐2008, systems on which sched_yield() is avail‐
able defined _POSIX_PRIORITY_SCHEDULING in <unistd.h>.
CAVEATS
sched_yield() is intended for use with real‐time scheduling
policies (i.e., SCHED_FIFO or SCHED_RR). Use of sched_yield()
with nondeterministic scheduling policies such as SCHED_OTHER
is unspecified and very likely means your application design is
broken.
If the calling thread is the only thread in the highest prior‐
ity list at that time, it will continue to run after a call to
sched_yield().
Avoid calling sched_yield() unnecessarily or inappropriately
(e.g., when resources needed by other schedulable threads are
still held by the caller), since doing so will result in unnec‐
essary context switches, which will degrade system performance.
SEE ALSO
sched(7)
Linux man‐pages (unreleased) (date) sched_yield(2)
--
2.40.1
^ permalink raw reply [flat|nested] 4+ messages in thread
* [PATCH 1/3] sched_yield.2: HISTORY: POSIX.1-2008 makes this non-optional
2023-05-03 17:03 [PATCH 0/3] Discourage sched_yield(2) Alejandro Colomar
@ 2023-05-03 17:03 ` Alejandro Colomar
2023-05-03 17:03 ` [PATCH 2/3] sched_yield.2: NOTES: Remove misleading sentence Alejandro Colomar
2023-05-03 17:03 ` [PATCH 3/3] sched_yield.2: Rename NOTES to CAVEATS, and reorder contents Alejandro Colomar
2 siblings, 0 replies; 4+ messages in thread
From: Alejandro Colomar @ 2023-05-03 17:03 UTC (permalink / raw)
To: linux-man, a.clayton; +Cc: Alejandro Colomar, andrew
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---
man2/sched_yield.2 | 18 ++++++++++--------
1 file changed, 10 insertions(+), 8 deletions(-)
diff --git a/man2/sched_yield.2 b/man2/sched_yield.2
index f1024762a..bab0f9569 100644
--- a/man2/sched_yield.2
+++ b/man2/sched_yield.2
@@ -38,20 +38,22 @@ .SH ERRORS
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
-POSIX.1-2001.
+POSIX.1-2001 (but optional).
+POSIX.1-2008.
+.PP
+Before POSIX.1-2008,
+systems on which
+.BR sched_yield ()
+is available defined
+.B _POSIX_PRIORITY_SCHEDULING
+in
+.IR <unistd.h> .
.SH NOTES
If the calling thread is the only thread in the highest
priority list at that time,
it will continue to run after a call to
.BR sched_yield ().
.PP
-POSIX systems on which
-.BR sched_yield ()
-is available define
-.B _POSIX_PRIORITY_SCHEDULING
-in
-.IR <unistd.h> .
-.PP
Strategic calls to
.BR sched_yield ()
can improve performance by giving other threads or processes
--
2.40.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* [PATCH 2/3] sched_yield.2: NOTES: Remove misleading sentence
2023-05-03 17:03 [PATCH 0/3] Discourage sched_yield(2) Alejandro Colomar
2023-05-03 17:03 ` [PATCH 1/3] sched_yield.2: HISTORY: POSIX.1-2008 makes this non-optional Alejandro Colomar
@ 2023-05-03 17:03 ` Alejandro Colomar
2023-05-03 17:03 ` [PATCH 3/3] sched_yield.2: Rename NOTES to CAVEATS, and reorder contents Alejandro Colomar
2 siblings, 0 replies; 4+ messages in thread
From: Alejandro Colomar @ 2023-05-03 17:03 UTC (permalink / raw)
To: linux-man, a.clayton; +Cc: Alejandro Colomar, andrew, Alejandro Colomar
sched_yield(2) is not the right thing for heavily contended resources.
The right thing to do is to call functions that wake the waiting threads.
Link: <https://www.realworldtech.com/forum/?threadid=189711&curpostid=189752>
Cc: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>
---
man2/sched_yield.2 | 5 -----
1 file changed, 5 deletions(-)
diff --git a/man2/sched_yield.2 b/man2/sched_yield.2
index bab0f9569..5e5b45a48 100644
--- a/man2/sched_yield.2
+++ b/man2/sched_yield.2
@@ -54,11 +54,6 @@ .SH NOTES
it will continue to run after a call to
.BR sched_yield ().
.PP
-Strategic calls to
-.BR sched_yield ()
-can improve performance by giving other threads or processes
-a chance to run when (heavily) contended resources (e.g., mutexes)
-have been released by the caller.
Avoid calling
.BR sched_yield ()
unnecessarily or inappropriately
--
2.40.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* [PATCH 3/3] sched_yield.2: Rename NOTES to CAVEATS, and reorder contents
2023-05-03 17:03 [PATCH 0/3] Discourage sched_yield(2) Alejandro Colomar
2023-05-03 17:03 ` [PATCH 1/3] sched_yield.2: HISTORY: POSIX.1-2008 makes this non-optional Alejandro Colomar
2023-05-03 17:03 ` [PATCH 2/3] sched_yield.2: NOTES: Remove misleading sentence Alejandro Colomar
@ 2023-05-03 17:03 ` Alejandro Colomar
2 siblings, 0 replies; 4+ messages in thread
From: Alejandro Colomar @ 2023-05-03 17:03 UTC (permalink / raw)
To: linux-man, a.clayton; +Cc: Alejandro Colomar, andrew, Alejandro Colomar
Put the last paragraph at the top of the CAVEATS section, since it's
probably the most important for readers. This system call is likely not
the right one for most programs; let's discourage its use.
Link: <https://www.realworldtech.com/forum/?threadid=189711&curpostid=189752>
Cc: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>
---
man2/sched_yield.2 | 24 ++++++++++++------------
1 file changed, 12 insertions(+), 12 deletions(-)
diff --git a/man2/sched_yield.2 b/man2/sched_yield.2
index 5e5b45a48..4eb858018 100644
--- a/man2/sched_yield.2
+++ b/man2/sched_yield.2
@@ -48,7 +48,18 @@ .SH HISTORY
.B _POSIX_PRIORITY_SCHEDULING
in
.IR <unistd.h> .
-.SH NOTES
+.SH CAVEATS
+.BR sched_yield ()
+is intended for use with real-time scheduling policies (i.e.,
+.B SCHED_FIFO
+or
+.BR SCHED_RR ).
+Use of
+.BR sched_yield ()
+with nondeterministic scheduling policies such as
+.B SCHED_OTHER
+is unspecified and very likely means your application design is broken.
+.PP
If the calling thread is the only thread in the highest
priority list at that time,
it will continue to run after a call to
@@ -61,16 +72,5 @@ .SH NOTES
schedulable threads are still held by the caller),
since doing so will result in unnecessary context switches,
which will degrade system performance.
-.PP
-.BR sched_yield ()
-is intended for use with real-time scheduling policies (i.e.,
-.B SCHED_FIFO
-or
-.BR SCHED_RR ).
-Use of
-.BR sched_yield ()
-with nondeterministic scheduling policies such as
-.B SCHED_OTHER
-is unspecified and very likely means your application design is broken.
.SH SEE ALSO
.BR sched (7)
--
2.40.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
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2023-05-03 17:03 [PATCH 0/3] Discourage sched_yield(2) Alejandro Colomar
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