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SubjectRe: [PATCH v3] rcu: Only boost rcu reader tasks with lower priority than boost kthreads
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@gmail.com>
> > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
>
> >Sebastian

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