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    SubjectRE: [PATCH v3] rcu: Only boost rcu reader tasks with lower priority than boost kthreads
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    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.

    Thanks,
    Zqiang

    >
    > 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@gmail.com>
    > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
    >
    > >Sebastian

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