Realtime thread priorities

[Kostik Belousov]

On Fri, Dec 10, 2010 at 10:50:45AM -0500, John Baldwin wrote:
> So I finally had a case today where I wanted to use rtprio but it doesn't seem 
> very useful in its current state.  Specifically, I want to be able to tag 
> certain user processes as being more important than any other user processes 
> even to the point that if one of my important processes blocks on a mutex, the 
> owner of that mutex should be more important than sshd being woken up from 
> sbwait by new data (for example).  This doesn't work currently with rtprio due 
> to the way the priorities are laid out (and I believe I probably argued for 
> the current layout back when it was proposed).
> 
> The current layout breaks up the global thread priority space (0 - 255) into a 
> couple of bands:
> 
>   0 -  63 : interrupt threads
>  64 - 127 : kernel sleep priorities (PSOCK, etc.)
> 128 - 159 : real-time user threads (rtprio)
> 160 - 223 : time-sharing user threads
> 224 - 255 : idle threads (idprio and kernel idle procs)
> 
> The problem I am running into is that when a time-sharing thread goes to sleep 
> in the kernel (waiting on select, socket data, tty, etc.) it actually ends up 
> in the kernel priorities range (64 - 127).  This means when it wakes up it 
> will trump (and preempt) a real-time user thread even though these processes 
> nominally have a priority down in the 160 - 223 range.  We do drop the kernel 
> sleep priority during userret(), but we don't recheck the scheduler queues to 
> see if we should preempt the thread during userret(), so it effectively runs 
> with the kernel sleep priority for the rest of the quantum while it is in 
> userland.
> 
> My first question is if this behavior is the desired behavior?  Originally I 
> think I preferred the current layout because I thought a thread in the kernel 
> should always have priority so it can release locks, etc.  However, priority 
> propagation should actually handle the case of some very important thread 
> needing a lock.  In my use case today where I actually want to use rtprio I 
> think I want different behavior where the rtprio thread is more important than 
> the thread waking up with PSOCK, etc.
> 
> If we decide to change the behavior I see two possible fixes:
> 
> 1) (easy) just move the real-time priority range above the kernel sleep 
> priority range
> 
> 2) (harder) make sched_userret() check the run queue to see if it should 
> preempt when dropping the kernel sleep priority.  I think bde@ has suggested 
> that we should do this for correctness previously (and I've had some old, 
> unfinished patches to do this in a branch in p4 for several years).

Would not doing #2 allow e.g. two threads that perform ping-pong with
a single byte read/write into a socket to usurp the CPU ? The threads
could try to also do some CPU-intensive calculations for some time
during the quantum too.

Such threads are arguably "interactive", but I think that the gain is
priority is too unfair.
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