Messages in this thread | | | From | "Zhang, Qiang1" <> | Subject | RE: [PATCH v3] rcu: Only boost rcu reader tasks with lower priority than boost kthreads | Date | Sat, 12 Mar 2022 03:11:04 +0000 |
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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.
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@gmail.com> > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
>Sebastian
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