linux-kernel - Re: [PATCH v3] rcu: Only boost rcu reader tasks with lower priority than boost kthreads

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Message-ID: <20220316165931.GI4285@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 16 Mar 2022 09:59:31 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Zhang, Qiang1" <qiang1.zhang@...el.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "frederic@...nel.org" <frederic@...nel.org>,
        "urezki@...il.com" <urezki@...il.com>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "josh@...htriplett.org" <josh@...htriplett.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [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@...il.com>
> > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> 
> >Sebastian