Messages in this thread |  | | | Subject | RE: SCHED_RR and kernel threads | | Date | Mon, 8 Nov 2004 17:56:46 -0800 | | From | "Stephen Warren" <> |
| |
> From: Con Kolivas [mailto:kernel@kolivas.org] > Stephen Warren writes: >> I guess we could have most threads stay at SCHED_NORMAL, and just make >> the few critical threads SCHED_RR, but I'm getting a lot of push-back on >> 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.org 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 kernel...
-- Stephen Warren, Software Engineer, NVIDIA, Fort Collins, CO swarren@nvidia.com http://www.nvidia.com/ swarren@wwwdotorg.org http://www.wwwdotorg.org/pgp.html - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
|  |