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Date:   Wed, 29 Aug 2018 15:22:14 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     linux-kernel@vger.kernel.org
Cc:     mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
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        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org
Subject: Re: [PATCH tip/core/rcu 0/19] RCU flavor-consolidation changes for
 v4.20/v5.0
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On Wed, Aug 29, 2018 at 03:20:21PM -0700, Paul E. McKenney wrote:
> Hello!
> 
> This series contains the RCU flavor consolidation, along with some initial
> cleanup work enabled by that consolidation (and some that became apparent
> while cleaning up):
> 
> 1.	Refactor rcu_{nmi,irq}_{enter,exit}(), saving a branch on the
> 	idle entry/exit hotpaths, courtesy of Byungchul Park,
> 
> 2.	Defer reporting RCU-preempt quiescent states when disabled.
> 	This is the key commit that consolidates the RCU-bh and RCU-sched
> 	flavors into RCU, however, the RCU-bh and RCU-sched flavors
> 	still exist independently as well at this point.
> 
> 3.	Test extended "rcu" read-side critical sections.  This commit
> 	causes rcutorture to test RCU's new-found ability to act as
> 	the combination of RCU, RCU-bh, and RCU-sched.
> 
> 4.	Allow processing deferred QSes for exiting RCU-preempt readers.
> 	This is a optimization.
> 
> 5.	Remove now-unused ->b.exp_need_qs field from the rcu_special union.
> 
> 6.	Add warning to detect half-interrupts.  Test the claim that
> 	the Linux kernel no longer does half-interrupts.
> 
> 7.	Apply RCU-bh QSes to RCU-sched and RCU-preempt when safe, that is,
> 	make the consolidated RCU inherit RCU-bh's denial-of-service
> 	avoidance mechanism.
> 
> 8.	Report expedited grace periods at context-switch time.  This is
> 	an optimization enabled by the RCU flavor consolidation.
> 
> 9.	Define RCU-bh update API in terms of RCU.  This commit gets rid
> 	of the RCU-bh update mechanism.
> 
> 10.	Update comments and help text to account for the removal of
> 	the RCU-bh update mechanism.
> 
> 11.	Drop "wake" parameter from rcu_report_exp_rdp().
> 
> 12.	Fix typo in rcu_get_gp_kthreads_prio() header comment.
> 
> 13.	Define RCU-sched API in terms of RCU for Tree RCU PREEMPT builds.
> 	Although this commit gets rid of the RCU-sched update mechanism
> 	from PREEMPT builds, it of course remains as the sole RCU flavor
> 	for !PREEMPT && !SMP builds.
> 
> 14.	Express Tiny RCU updates in terms of RCU rather than RCU-sched.
> 	This will enable additional cleanups and code savings.
> 
> 15.	Remove RCU_STATE_INITIALIZER() in favor of just using an open-coded
> 	initializer for the sole remaining rcu_state structure.
> 
> 16.	Eliminate rcu_state structure's ->call field, as it is now always
> 	just call_rcu().
> 
> 17.	Remove rcu_state structure's ->rda field, as there is now only one
> 	set of per-CPU rcu_data structures.

And last, but perhaps not least:

18.	Remove rcu_state_p pointer to default rcu_state structure, now
	that there is only one rcu_state structure.

19.	Remove rcu_data_p pointer to default rcu_data structure, now that
	there is only one set of per-CPU rcu_data structures.

							Thanx, Paul

> ------------------------------------------------------------------------
> 
>  Documentation/RCU/Design/Requirements/Requirements.html |   50 -
>  include/linux/rcupdate.h                                |   48 -
>  include/linux/rcupdate_wait.h                           |    6 
>  include/linux/rcutiny.h                                 |   61 +
>  include/linux/rcutree.h                                 |   31 
>  include/linux/sched.h                                   |    6 
>  kernel/rcu/Kconfig                                      |   10 
>  kernel/rcu/rcutorture.c                                 |    1 
>  kernel/rcu/tiny.c                                       |  163 +--
>  kernel/rcu/tree.c                                       |  655 +++++-----------
>  kernel/rcu/tree.h                                       |   44 -
>  kernel/rcu/tree_exp.h                                   |  256 +++---
>  kernel/rcu/tree_plugin.h                                |  523 ++++++------
>  kernel/rcu/update.c                                     |    2 
>  kernel/softirq.c                                        |    3 
>  15 files changed, 836 insertions(+), 1023 deletions(-)