Subject: Re: [linux-audio-dev] priority inversion & inheritance
From: Kai Vehmanen (kai.vehmanen_AT_wakkanet.fi)
Date: Fri Jul 12 2002 - 00:39:16 EEST
On Thu, 11 Jul 2002, Martijn Sipkema wrote:
>> CPU. If we have two SCHED_FIFO threads communicating with a mutex
>> protected queue, different priorities are not a problem as for priority
>> inversion to happen, one of them needs to block on the shared mutex. And
>> if the thread is blocked, it then isn\'t in the run queue, and thus cannot
>> get the CPU.
> Not true as far as I know. FIFO only means that the thread won\'t be preempted
> after a certain time in case a thread of the same priority is ready to run.
> AFAIK all threads running either SCHED_FIFO or SCHED_RR will always be
> preempted by higher priority SCHED_* threads. Please correct me if I am wrong
> in this.
Let's say we have threads A (SCHED_FIFO/50) and B (SCHED_FIFO/90). If both
threads are runnable, B will always be selected and it will run until
(quoting the man page) "... either it is blocked by an I/O request, it is
preempted by a higher priority process, or it calls sched_yield".
But if B blocks on a mutex currently owned by A, B will then become
blocked and A will be selected to run. A can now run until it unlocks
the mutex, B becomes unblocked and once the next scheduling decision
occurs, B will again be chosen to run.
But it's true that if we have more threads the situation gets complex
pretty quickly. Let's say we have C (SCHED_FIFO/70) that holds a resource
needed by B, and is currently blocked by a long i/o operation. Then all
three threads will be stuck for an unbounded time.
Do correct me if this seems wrong to you.
Btw; great day today on lad - lots of very interesting discussion! ;)
-- http://www.eca.cx Audio software for Linux!
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