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

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Message-Id: <20241213040658.2282640-1-ankur.a.arora@oracle.com>
Date: Thu, 12 Dec 2024 20:06:51 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: linux-kernel@...r.kernel.org
Cc: peterz@...radead.org, tglx@...utronix.de, paulmck@...nel.org,
        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, ankur.a.arora@...cle.com,
        efault@....de, sshegde@...ux.ibm.com, boris.ostrovsky@...cle.com
Subject: [PATCH v3 0/7] RCU changes for PREEMPT_LAZY

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.

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