linux-kernel - Re: [PATCH v3 0/7] RCU changes for PREEMPT

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Message-ID: <c38ebc69-33cc-4848-bcd6-a07404d8daa2@paulmck-laptop>
Date: Tue, 7 Jan 2025 17:14:33 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org, tglx@...utronix.de,
	mingo@...nel.org, bigeasy@...utronix.de, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, frederic@...nel.org, efault@....de,
	sshegde@...ux.ibm.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH v3 0/7] RCU changes for PREEMPT_LAZY

On Thu, Dec 12, 2024 at 08:06:51PM -0800, Ankur Arora wrote:
> This series adds RCU bits for lazy preemption.
> 
> The problem addressed is that pre-PREEMPT_LAZY, PREEMPTION=y implied
> PREEMPT_RCU=y. With PREEMPT_LAZY, that's no longer true. 
> 
> That's because PREEMPT_RCU makes some trade-offs to optimize for
> latency as opposed to throughput, and configurations with limited
> preemption might prefer the stronger forward-progress guarantees of
> PREEMPT_RCU=n.
> 
> Accordingly, with standalone PREEMPT_LAZY (much like PREEMPT_NONE,
> PREEMPT_VOLUNTARY) we want to use PREEMPT_RCU=n. And, when used in
> conjunction with PREEMPT_DYNAMIC, we continue to use PREEMPT_RCU=y.
> 
> Patches 1-3  are cleanup patches:
>   "rcu: fix header guard for rcu_all_qs()"
>   "rcu: rename PREEMPT_AUTO to PREEMPT_LAZY"
>   "sched: update __cond_resched comment about RCU quiescent states"
> 
> Patch 4,
>   "rcu: handle unstable rdp in rcu_read_unlock_strict()"
> 
> handles a latent RCU bug rcu_report_qs_rdp() could be called with
> an unstable rdp.
> 
> Patches 5 and 6,
>   "rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y"
>   "osnoise: provide quiescent states"
> 
> handle quiescent states for the (PREEMPT_LAZY=y, PREEMPT_RCU=n)
> configuration.
> 
> And, finally patch 7, "rcu: limit PREEMPT_RCU configurations",
> explicitly limits PREEMPT_RCU=y to the PREEMPT_DYNAMIC or the latency
> oriented models.

Pulled into my -rcu tree for further review and testing, with initial
tests passing.  Apologies for the delay!

							Thanx, Paul

> Changelog:
> 
> v3:
>  - moved patch-3 to be the last one in the series (suggested by Sebastian)
>  - added "rcu: handle unstable rdp in rcu_read_unlock_strict()"
>    (suggested by Frederic Weisbecker).
>  - switched to a more robust check in rcu_flavor_sched_clock_irq()
>    (suggested by Frederic Weisbecker).
>  - simplified check in osnoise (suggested by Frederic Weisbecker).
>  - dropped an unrelated scheduler patch.
> 
> v2:
>   - fixup incorrect usage of tif_need_resched_lazy() (comment from
>     from Sebastian Andrzej Siewior)
>   - massaged the commit messages a bit
>   - drops the powerpc support for PREEMPT_LAZY as that was orthogonal
>     to this series (Shrikanth will send that out separately.)
> 
> Please review.
> 
> Ankur Arora (7):
>   rcu: fix header guard for rcu_all_qs()
>   rcu: rename PREEMPT_AUTO to PREEMPT_LAZY
>   sched: update __cond_resched comment about RCU quiescent states
>   rcu: handle unstable rdp in rcu_read_unlock_strict()
>   rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y
>   osnoise: provide quiescent states
>   rcu: limit PREEMPT_RCU configurations
> 
>  include/linux/rcupdate.h     |  2 +-
>  include/linux/rcutree.h      |  2 +-
>  include/linux/srcutiny.h     |  2 +-
>  kernel/rcu/Kconfig           |  4 ++--
>  kernel/rcu/srcutiny.c        | 14 +++++++-------
>  kernel/rcu/tree_plugin.h     | 22 +++++++++++++++++-----
>  kernel/sched/core.c          |  4 +++-
>  kernel/trace/trace_osnoise.c | 32 +++++++++++++++-----------------
>  8 files changed, 47 insertions(+), 35 deletions(-)
> 
> -- 
> 2.43.5
>