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

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Date:   Fri, 11 Mar 2022 12:09:20 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     paulmck@...nel.org, frederic@...nel.org, urezki@...il.com,
        quic_neeraju@...cinc.com, josh@...htriplett.org,
        juri.lelli@...hat.com, rcu@...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 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
- If it is not RT prio, what is then? Higher or lower? Afaik it is
  always >= 1.

> 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.

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.

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.

> Suggested-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>

Sebastian