linux-kernel - Re: Should RCU_BOOST kernels use hrtimers in GP kthread?

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Message-ID: <20210217153253.fy2mhxo3o3ehsuix@linutronix.de>
Date:   Wed, 17 Feb 2021 16:32:53 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        tglx@...utronix.de
Subject: Re: Should RCU_BOOST kernels use hrtimers in GP kthread?

On 2021-02-16 10:36:09 [-0800], Paul E. McKenney wrote:
> Hello, Sebastian,

Hi Paul,

> I punted on this for the moment by making RCU priority boosting testing
> depend on CONFIG_PREEMPT_RT, but longer term I am wondering if RCU's
> various timed delays and timeouts should use hrtimers rather than normal
> timers in kernels built with CONFIG_RCU_BOOST.  As it is, RCU priority
> boosting can be defeated if any of the RCU grace-period kthread's timeouts
> are serviced by the non-realtime ksoftirqd.

I though boosting is accomplished by acquiring a rt_mutex in a
rcu_read() section. Do you have some code to point me to, to see how a
timer is involved here? Or is it the timer saying that *now* boosting is
needed.

If your hrtimer is a "normal" hrtimer then it will be served by
ksoftirqd, too. You would additionally need one of the
HRTIMER_MODE_*_HARD to make it work.

> This might require things like swait_event_idle_hrtimeout_exclusive(),
> either as primitives or just open coded.
> 
> Thoughts?
> 
> 							Thanx, Paul

Sebastian