Thank you Valdis. Yes, I'm basically getting what I want - the RT proc never ever gives up to the system. There are a plenty of cores left to run non-rt tasks on the machine. The question is why this old 2.6 kernel decide that it needs per-cpu events and kblockd tasks. Maybe someone can give a hint in what subsystem's documentation can I find anything about workqueue tasks. /Documentation/kernel-per-CPU-kthreads.txt is great, but described controls appeared only in 3.10.x :) On 06/28/2017 01:04 PM, valdis.kletnieks@xxxxxx wrote: > On Wed, 28 Jun 2017 08:39:07 -0500, Andrei Hurynovich said: >> We set sysctl kernel.sched_rt_runtime_us = -1 so realtime threads are >> NEVER interrupted. >> According to /proc/sched_debug, it seems that kernel still schedules >> some SCHED_OTHER(e.g. non-realtime) kernel tasks to isolated cpus - for >> example cpu 18 get tasks events/18 and kblockd/18 that are stuck in >> runnable(but not running state), so those kernel processes never got a >> single time slice because our realtime process hogs 100% of cpu. > This is what happens when you have a priority inversion - when you tell the system > to give 100% to a process, you shouldn't be surprised when other tasks don't > get any service. > >> The question is: Is it possible to never schedule kernel tasks on >> selected cpus? > Only if the userspace process on that CPU never makes system calls - which is > very unlikely if the process has actual real-time requirements. > > Also, if your "real-time" process is taking 100% of the CPU, you have a disaster > waiting to happen. You have zero headroom for dealing with unexpected events. > Thought experiment: What happens if your real-world part of the system has > an unexpected error, that requires 1% of a CPU for error recovery? You are > forced to either ignore the error or miss a real-time deadline. > > You might want to think about dividing up your process into 2 parts - one that > handles the *actual* real-time work and only uses (for example) 20-30% of a > CPU, and the parts that don't have actual real-time constraints that can then > run with the rest of the available CPU, but allow other threads (such as kernel) > to execute as well. > -- Thanks, Andrei Hurynovich Charlesworth Research LLC. http://www.charlesworthresearch.com/ _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
