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@xxxxxxxxx>
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
>Sebastian
