> On Fri, Mar 18, 2022 at 05:50:35AM +0000, Zhang, Qiang1 wrote: > > 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? > > > > I have tested this modification these days, I originally planned to generate a Kconfig option to control > > whether to skip tasks with higher priority than boost kthreads. but it doesn't seem necessary > > because I find it's optimization is not particularly > > obvious in the actual scene, I find that tasks with higher priority than boost kthreads > > will quickly exit the rcu critical area , even if be preempted in the rcu critical area. > > sorry for the noise. > > Thank you for getting back with this information, and no need to > apologize. We all get excited about a potential change from time to time. > Part of us maintainers' jobs is to ask hard questions when that appears > to be happening. ;-) > > If you have continued interest in this area, it would be good to keep > looking. After all, neither RCU expedited grace periods nor RCU priority > boosting were designed with these new use cases in mind, so it is quite > likely that there is a useful change to be made in there somewhere. > > You see, RCU expedited grace periods were designed for throughput rather > than latency. The original use case was an old networking API that > needed to wait for a grace period on each and every one of a series of > some tens of thousands of system calls. If one or two of those system > calls took a few hundred milliseconds, but the rest completed in less than > a millisecond, no harm done. (Yes, there are now newer APIs that allow > many changes to be made with only the one grace-period wait. But the > kernel must continue to support the old API: Never Break Userspace.) > > For its part, RCU priority boosting was originally designed for > debuggging. The point was to avoid OOMing the system when someone > misconfigured their application's real-time priorities. As you know, > such misconfiguration can easily prevent low-priority RCU readers from > ever completing. > > So it is reasonably likely that some change or another is needed. After > all, new use cases require new functionality and new fixes. The trick > is figuring out which change makes sense amongst the huge group of other > possible changes that each add much more complexity than improvement. > But part of the process of finding that change that makes sense is trying > out quite a few changes that don't help all that much. ;-) > Sorry for the late response, but i think i should comment on it since i have tried to simulate and test this patch on Android device. Basically we do have RT tasks in Android and i do not see that the patch that is in question makes any difference. Actually i was not able to trigger its functionality at all.
