scheduler (sched_4bsd) questions

[Julian Elischer]

Stephan Uphoff wrote:

>Hi,
>
>I finally found some time to take a peek at the sched_4bsd scheduler
>and found two things I could not explain:
>
>It looks like the thread priority of a thread in the kernel is normally
>its user priority.
>( Unless it slept with priority change or due to mutex priority
>inheritance)
>

exactly.

>
>If this is true kernel threads can be preempted while holding
>for example the root vnode lock (or other important kernel
>resources) while not getting a chance to run until there are no more
>user processes with better priority. 
>

This is also true,  though it is a slightly more complicated thing than 
that.
Preempting threads are usually interrupt threads and are thus usually 
short lived,.


The theory is however that as long as the CPU is doing something, teh 
throughput is
still being maintianed. (For this reason we  have not worked on the 
problem you mention yet, though
it will eventually get to the top of someone's list :-)
.

>I am probably missing a priority adjustment on kernel entry somewhere
>but seem to be unable to find it :-(
>

>
>I am also stomped by the special case of adding a thread X with better
>priority than the current thread to the runqueue if they belong to the
>same ksegroup. In this case both kg_last_assigned and kg_avail_opennings
>might be zero and setrunqueue() will not call sched_add().
>Because of this it looks like the current thread will neither be
>preempted not will TDF_NEEDRESCHED be set to force rescheduling at the
>kernel boundary.
>This situation should resolve itself at the next sched_switch - however
>this might take a long time. (Especially if essential interrupt threads
>are blocked by mutexes held by thread X)
>

you are correct. I am not yet  preempting a running thread with a lesser 
priority if they are siblings
(unless there is a slot available) Thsi is not becasue I don't want to 
do it, but simply because it has not been done yet..
we did have NO preemption, so having "some" preemption is still better 
than where we were.
 Special case code to check curthread for a preemption could be done but 
at the moment  the decision code for
whether to preempt or not is in maybe_preempt() and I don't want  to 
duplicate that. it is on th edrawing board though.
The other thing is, that even if we should be able to preempt a running 
thread, there is no guarantee that it is on THIS
CPU.  It may be on another CPU and that gets nasty in a hurry.



>
>Thanks in advance for any hints.
>
>	Stephan
>
>PS: I am impressed how clean and easy to read the scheduler sources are.
>    Looks like a lot of hard work went into this.
>

There is a lot to clean up yet..
what version are you reading? -current?


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