RE: SCHED_RR and kernel threads

From: Stephen Warren
Date: Mon Nov 08 2004 - 21:05:02 EST

> From: Con Kolivas [mailto:kernel@xxxxxxxxxxx]
> Stephen Warren writes:
>> I guess we could have most threads stay at SCHED_NORMAL, and just
>> the few critical threads SCHED_RR, but I'm getting a lot of push-back
>> this, since it makes our thread API a lot more complex.
>Your workaround is not suitable for the kernel at large.

You mean the official kernel? I wasn't implying that the
patch should be part of that!

In our system we have literally EVERY single thread (kernel, user-space
daemons, and user-space applications) all setup as SCHED_RR with
identical priority at present, except a couple higher priority threads.
We did this initially for user-space by replacing /sbin/init with a
wrapper that set the scheduler policy and default priority, and verified
that this was inherited by all daemons & application threads. Then, we
found that the kernel threads could get starved in some situations,
hence the kernel change.

Our threading model dictates that every thread have a priority (so that
the thread model is portable between Linux, embedded RTOSs etc.), and in
Linux AFAIK, the only way to implement priorities is to use a real-time
scheduling policy. Some threads do a lot of calculation. We want to make
them equal (or probably, lower) priority to the kernel threads, so
therefore the kernel threads must then be SCHED_RR.

Can you elaborate on specific conditions that would cause the kernel
threads to suck up unusual amounts of CPU time?

In our application, keyboard processing is a real-time requirement, so
if that is performed in a kernel thread, that kernel thread should be
real-time. We basically want the control to insert e.g. the keyboard
processing kernel thread into the middle of our priority hierarchy,
rather than having it forced as the lowest possible priority.

I get the impression you're implying that scheduling doesn't work
correctly in this situation - that if kernel threads are set to
SCHED_RR, they can still lock out user-space threads of the same or
higher priority? Is this what you're saying, or do you mean that the
kernel threads can lock out user-space threads of *lower* priority,
which is to be expected. In all the RTOS's I've seen, all threads are
SCHED_RR, thus mimicking the situation we've creating by patching our

Stephen Warren, Software Engineer, NVIDIA, Fort Collins, CO
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