linux-kernel - [PATCH v2 0/6] RCU changes for PREEMPT

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Message-Id: <20241106201758.428310-1-ankur.a.arora@oracle.com>
Date: Wed,  6 Nov 2024 12:17:52 -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 v2 0/6] RCU changes for PREEMPT_LAZY

This series adds RCU and some leftover scheduler bits for lazy
preemption.

The main problem addressed in the RCU related patches is that before
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 and 2 are cleanup patches:
  "rcu: fix header guard for rcu_all_qs()"
  "rcu: rename PREEMPT_AUTO to PREEMPT_LAZY"

Patch 3, "rcu: limit PREEMPT_RCU configurations", explicitly limits
PREEMPT_RCU=y to the PREEMPT_DYNAMIC or the latency oriented models.

Patches 4 and 5,
  "rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y"
  "osnoise: handle quiescent states for PREEMPT_RCU=n, PREEMPTION=y"

handle quiescent states for the (PREEMPT_LAZY=y, PREEMPT_RCU=n)
configuration.

And, finally patch-6
  "sched: warn for high latency with TIF_NEED_RESCHED_LAZY"
adds high latency warning for TIF_NEED_RESCHED_LAZY.

Goes on top of PeterZ's tree:

 git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git sched/core

Changelog:
  - 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 (6):
  rcu: fix header guard for rcu_all_qs()
  rcu: rename PREEMPT_AUTO to PREEMPT_LAZY
  rcu: limit PREEMPT_RCU configurations
  rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y
  osnoise: handle quiescent states for PREEMPT_RCU=n, PREEMPTION=y
  sched: warn for high latency with TIF_NEED_RESCHED_LAZY

 include/linux/rcutree.h      |  2 +-
 include/linux/srcutiny.h     |  2 +-
 kernel/rcu/Kconfig           |  4 ++--
 kernel/rcu/srcutiny.c        | 14 +++++++-------
 kernel/rcu/tree_plugin.h     | 11 +++++++----
 kernel/sched/core.c          |  3 ++-
 kernel/sched/debug.c         |  7 +++++--
 kernel/trace/trace_osnoise.c | 22 ++++++++++++----------
 8 files changed, 37 insertions(+), 28 deletions(-)

-- 
2.43.5