Corinna Vinschen wrote:
On Nov 25 21:20, Christian Franke wrote:
Hi Corinna,
Corinna Vinschen wrote:
Hi Christian,
On Nov 25 15:00, Christian Franke wrote:
Corinna Vinschen wrote:
Fixes: ...?
... the very first commit (cgf 2001) of sched.cc :-)
New patch attached.
From e95fc1aceb5287f9ad65c6c078125fecba6c6de9 Mon Sep 17 00:00:00 2001
From: Christian Franke <christian.fra...@t-online.de>
Date: Mon, 25 Nov 2024 14:51:04 +0100
Subject: [PATCH] Cygwin: sched_setscheduler: allow changes of the priority
Behave like sched_setparam() if the requested policy is identical
to the fixed value (SCHED_FIFO) returned by sched_getscheduler().
Fixes: 6b2a2aa4af1e ("Add missing files.")
Huh, yeah, this is spot on. I wonder if it would make sense to change
that to 9a08b2c02eea ("* sched.cc: New file. Implement sched*.")
though, given that was the patch intended to add sched.cc :)))
Sorry, but I have to ask two more questions:
- Isn't returning SCHED_FIFO sched_getscheduler() just as wrong?
Definitly. SCHED_FIFO is a non-preemptive(!) real-time policy. Windows does
not offer anything like that to userland (AFAIK).
https://man7.org/linux/man-pages/man7/sched.7.html
I wonder whether there was a use case for this emulation when this module
was introduced in 2001.
Just guessing here, but using one of the RT schedulers was the only way
to enable changing the priority from user space and using SCHED_FIFO was
maybe in error.
Shouldn't that be SCHED_OTHER, and sched_setscheduler() should check
for that instead? Cygwin in a real-time scenario sounds a bit
far-fetched...
Agree.
Note that SCHED_OTHER requires sched_priority == 0, so most of the
sched_get/set*() priority related code would no longer make sense then.
This is the other problem. Changing this to SCHED_OTHER sounds like
dropping potentially used functionality. Maybe we should just switch to
SCHED_RR?
Yes, it at least would be closer to what windows does. It is still
non-preemptive from the point of view of lower priorities. I don't know
what the Win32 *_PRIORITY_CLASSes actually do.
As far as I understand the related documentation, a more sophisticated
emulation (aka fake) of SCHED_* would be:
- Allow to switch between SCHED_OTHER (default) and SCHED_RR with
sched_setscheduler().
- If SCHED_OTHER is selected, change PRIORITY_CLASS with setpriority()
and ignore (or fail on?) attempts to change sched_priority with
sched_setparam().
- If SCHED_RR is selected, ignore setpriority() and change
PRIORITY_CLASS with sched_setparam().
Possibly not worth the effort...
--
Regards,
Christian
