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Re: sched: Where to queue RT tasks on prio drop
- From: Wanpeng Li <kernellwp at gmail dot com>
- To: "Grochowalski, Matthew (GE Aviation, US)" <MatthewS dot Grochowalski at ge dot com>
- Cc: "linux-kernel at vger dot kernel dot org" <linux-kernel at vger dot kernel dot org>, "tglx at linutronix dot de" <tglx at linutronix dot de>, "parri dot andrea at gmail dot com" <parri dot andrea at gmail dot com>, "libc-alpha at sourceware dot org" <libc-alpha at sourceware dot org>
- Date: Thu, 5 May 2016 10:45:37 +0800
- Subject: Re: sched: Where to queue RT tasks on prio drop
- Authentication-results: sourceware.org; auth=none
- References: <4F5135A853DDCC4BAAF325E04E1419C8077E8C46 at ALPMBAPA10 dot e2k dot ad dot ge dot com>
2016-05-05 4:51 GMT+08:00 Grochowalski, Matthew (GE Aviation, US)
> It looks like commit 81a44c5 (sched: Queue RT tasks to head when prio drop) made the behavior on dropping (userspace view) more sensible but I believe the behavior is still incorrect according to POSIX.
> POSIX (in volume 2 section 2.8.4 Process Scheduling) specifies two different semantics for where the task is placed in the thread list for the new priority
> 8. If a thread whose policy or priority has been modified by pthread_setschedprio() is a running thread or is runnable, the effect on its position in the thread list depends on the direction of the modification, as follows:
> a. If the priority is raised, the thread becomes the tail of the thread list.
> b. If the priority is unchanged, the thread does not change position in the thread list.
> c. If the priority is lowered, the thread becomes the head of the thread list.
> 7. If a thread whose policy or priority has been modified other than by pthread_setschedprio() is a running thread or is runnable, it then becomes the tail of the thread list for its new priority.
> Commit 81a44c5 made all of the priority change functions behave according to the pthread_setschedprio semantics.
> It appears commit ff77e46 (sched/rt: Fix PI handling vs. sched_setscheduler()) causes changing a task's priority to its existing priority to requeue it at the tail.
So this is almost follow 7, right?
> So a task settings its own priority to its current priority would be the same as a sched_yield().
> I believe the correct behavior is to have the existing priority change syscalls (sched_setscheduler and sched_setparam) always move the changed task to the back of the queue for the new priority.
> But as far as I can tell the kernel provides no way to implement pthread_setschedprio with the correct semantics.
> It seems the best way to implement this would be adding a flag (SCHED_SETSCHEDPRIO) to the existing sched_setattr syscall.
> Any thoughts?
> --Matt Grochowalski