2012/12/21 Steven Rostedt <rost...@goodmis.org>: > On Thu, 2012-12-20 at 19:32 +0100, Frederic Weisbecker wrote: >> Let's imagine you have 4 CPUs. We keep the CPU 0 to offline RCU callbacks >> there and to >> handle the timekeeping. We set the rest as full dynticks. So you need the >> following kernel >> parameters: >> >> rcu_nocbs=1-3 full_nohz=1-3 >> >> (Note rcu_nocbs value must always be the same as full_nohz). > > Why? You can't have: rcu_nocbs=1-4 full_nohz=1-3
That should be allowed. > or: rcu_nocbs=1-3 full_nohz=1-4 ? But that not. You need to have: rcu_nocbs & full_nohz == full_nohz. This is because the tick is not there to maintain the local RCU callbacks anymore. So this must be offloaded to the rcu_nocb threads. I just have a doubt with rcu_nocb. Do we still need the tick to complete the grace period for local rcu callbacks? I need to discuss that with Paul. > > That needs to be fixed. Either with a warning, and/or to force the two > to be the same. That is, if they specify: > > rcu_nocbs=1-3 full_nohz=1-4 > > Then set rcu_nocbs=1-4 with a warning about it. Or simply set > full_nohz=1-3. Yep, will do. Thanks! > > -- Steve > >> >> Now if you want proper isolation you need to: >> >> * Migrate your processes adequately >> * Migrate your irqs to CPU 0 >> * Migrate the RCU nocb threads to CPU 0. Example with the above >> configuration: >> >> for p in $(ps -o pid= -C rcuo1,rcuo2,rcuo3) >> do >> taskset -cp 0 $p >> done >> >> Then run what you want on the full dynticks CPUs. For best results, run 1 >> task >> per CPU, mostly in userspace and mostly CPU bound (otherwise more IO = more >> kernel >> mode execution = more chances to get IPIs, tick restarted, workqueues, >> kthreads, etc...) >> >> This page contains a good reminder for those interested in CPU isolation: >> https://github.com/gby/linux/wiki >> >> But keep in mind that my tree is not yet ready for serious production. >> > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
