On Sat, Mar 12, 2022 at 03:11:04AM +0000, Zhang, Qiang1 wrote: > On 2022-03-11 10:22:26 [+0800], Zqiang wrote: > > When RCU_BOOST is enabled, the boost kthreads will boosting readers > > who are blocking a given grace period, if the current reader tasks > ^ Period. > > > have a higher priority than boost kthreads(the boost kthreads priority > > not always 1, if the kthread_prio is set), > > >>This confuses me: > >>- Why does this matter > > In preempt-rt system, if the kthread_prio is not set, it prio is 1. > the boost kthreads can preempt almost rt task, It will affect > the real-time performance of some user rt tasks. In preempt-rt systems, > in most scenarios, this kthread_prio will be configured. Just following up... These questions might have been answered, but I am not seeing those answers right off-hand. Is the grace-period latency effect of choosing not to boost high-priority tasks visible at the system level in any actual workload? Suppose that a SCHED_DEADLINE task has exhausted its time quantum, and has thus been preempted within an RCU read-side critical section. Can priority boosting from a SCHED_FIFO prio-1 task cause it to start running? Do delays in RCU priority boosting cause excessive grace-period latencies on real workloads, even when all the to-be-boosted tasks are SCHED_OTHER? Thoughts? Thanx, Paul > Thanks > Zqiang > > >>- If it is not RT prio, what is then? Higher or lower? Afaik it is > >> always >= 1. > > >>>If it is not RT prio, the sanitize_kthread_prio() will limit RT prio > > > boosting is useless, skip > > current task and select next task to boosting, reduce the time for a > > given grace period. > > >>So if the task, that is stuck in a rcu_read() section, has a higher > >>priority than the boosting thread then boosting is futile. Understood. > >> > >>Please correct me if I'm wrong but this is intended for !SCHED_OTHER > >>tasks since there shouldn't a be PI chain on boost_mtx so that its > >>default RT priority is boosted above what has been configured. > > >>>Yes, you are right. If the boosting task which itself already boosted due to PI chain, > >>>and Its priority may only be temporarily higher than boost kthreads, once that > >>>PI boost is lifted the task may still be in a RCU section, but if we have been skipped it, > >>>this task have been missed the opportunity to be boosted. > > >> > >>You skip boosting tasks which are itself already boosted due to a PI > >>chain. Once that PI boost is lifted the task may still be in a RCU > >>section. But if I understand you right, your intention is skip boosting > >>tasks with a higher priority and concentrate and those which are in > >>need. This shouldn't make a difference unless the scheduler is able to > >>move the rcu-boosted task to another CPU. > >> > > >>>Yes, It make sense when the rcu-boosted kthreads and task which to be boosting > >>>should run difference CPU . > > >>Am I right so far? If so this should be part of the commit message (the > >>intention and the result). Also, please add that part with > >>boost_exp_tasks. The comment above boost_mtx is now above > >>boost_exp_tasks with a space so it looks (at least to me) like these two > >>don't belong together. > > >>>Yes, I will add your description to the commit information. > > > > Suggested-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx> > > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx> > > >Sebastian
