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

From: "Zhang, Qiang1" <qiang1.zhang@intel.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "paulmck@kernel.org" <paulmck@kernel.org>,
	"frederic@kernel.org" <frederic@kernel.org>,
	"urezki@gmail.com" <urezki@gmail.com>,
	"quic_neeraju@quicinc.com" <quic_neeraju@quicinc.com>,
	"josh@joshtriplett.org" <josh@joshtriplett.org>,
	"juri.lelli@redhat.com" <juri.lelli@redhat.com>,
	"rcu@vger.kernel.org" <rcu@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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	[thread overview]
Message-ID: <PH0PR11MB58807AD9A1BAA122218B92DBDA0D9@PH0PR11MB5880.namprd11.prod.outlook.com> (raw)
In-Reply-To: <PH0PR11MB58802B8804EDFFC73BA676F2DA0D9@PH0PR11MB5880.namprd11.prod.outlook.com>

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