On Wed, Oct 05, 2022 at 06:03:39PM +0200, Daniel Bristot de Oliveira wrote: > On 10/5/22 18:01, Florent Carli wrote: > > Hello, > > > > I'm trying to isolate some cores to run a CPU-bound real-time task and > > even though I'm using rcu_nocbs and rcu_nocb_poll, I can see the rcuc > > threads wake up, and I get RCU stall warnings on the isolated core. Isolating a CPU does not mean that RCU activity is forbidden on it, even with nohz_full, if the CPU enters the kernel (say syscall or interrupt), then it may enter a read-side critical section so RCU will be watching it, and such CPU will have to report quiescent state. > > As per the documentation, if I use rcutree.kthread_prio with a > > priority > my RT task, then the rcu stall does not happen. Yes, it sounds like the main RCU GP thread (mostly called rcu_preempt thread) in your system is competing with your RT task. Increase kthread_prio is standard procedure to resolve this issue, as you did. > > However I find it confusing: why are the rcuc threads woken up on the > > isolated cores despite using rcu_nocbs and rcu_nocb_poll? In my (very > > likely erroneous) understanding, I shouldn't have to fiddle with rcu > > priority... In other words, how come I get rcu stalls on cores that > > have no rcu callbacks? It is not possible to provide further help without more info, in particular are you using nohz_full and isolcpus options? Can you provide kernel and #CPU configuration? Happy to look further! Also adding some more folks who know a lot about this stuff and +rcu list for archives. thanks, - Joel > > Adding Joel because we had a chat about it during lpc... > > > Thanks. > > > -- Daniel >
