linux-kernel - Review of RCU-related patches in -rt

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Date:   Tue, 28 May 2019 13:50:30 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     bigeasy@...utronix.de
Cc:     linux-kernel@...r.kernel.org
Subject: Review of RCU-related patches in -rt

Hello, Sebastian,

Finally getting around to taking another look:

c7e07056a108 EXP rcu: skip the workqueue path on RT

	This one makes sense given the later commit setting the
	rcu_normal_after_boot kernel parameter.  Otherwise, it is
	slowing down expedited grace periods for no reason.  But
	should the check also include rcu_normal_after_boot and
	rcu_normal?  For example:

		if ((IS_ENABLED(CONFIG_PREEMPT_RT_FULL) &&
		     (rcu_normal || rcu_normal_after_boot) ||
		    !READ_ONCE(rcu_par_gp_wq) ||
		    rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
		    rcu_is_last_leaf_node(rnp)) {

	Alternatively, one approach would be to take the kernel
	parameters out in -rt:

		static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
		#ifndef CONFIG_PREEMPT_RT_FULL
		module_param(rcu_normal_after_boot, int, 0);
		#endif

	And similar for rcu_normal and rcu_expedited.

	Or is there some reason to allow run-time expedited grace
	periods in CONFIG_PREEMPT_RT_FULL=y kernels?

d1f52391bd8a rcu: Disable RCU_FAST_NO_HZ on RT

	Looks good.  More complexity could be added if too many people
	get themselves in trouble via "select RCU_FAST_NO_HZ".

42b346870326 rcu: make RCU_BOOST default on RT

	To avoid complaints about this showing up when people don't
	expected, could you please instead "select RCU_BOOST" in
	the Kconfig definition of PREEMPT_RT_FULL?

	Or do people really want to be able to disable boosting?

457c1b0d9c0e sched: Do not account rcu_preempt_depth on RT in might_sleep()

	The idea behind this one is to avoid false-positive complaints
	about -rt's sleeping spinlocks, correct?

7ee13e640b01 rbtree: don't include the rcu header
c9b0c9b87081 rtmutex: annotate sleeping lock context

	No specific comments.

7912d002ebf9 rcu: Eliminate softirq processing from rcutree

	This hasn't caused any problems in -rcu from what I can see.
	I am therefore planning to submit the -rcu variant of this to
	mainline during the next merge window.

f06d34ebdbbb srcu: Remove srcu_queue_delayed_work_on()

	Looks plausible.  I will check more carefully for mainline.

aeb04e894cc9 srcu: replace local_irqsave() with a locallock
e48989b033ad irqwork: push most work into softirq context

	These look to still be -rt only.

							Thanx, Paul