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

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Message-ID: <87frltcgpj.fsf@oracle.com>
Date: Wed, 08 Jan 2025 10:18:16 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: paulmck@...nel.org
Cc: Ankur Arora <ankur.a.arora@...cle.com>, 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


Paul E. McKenney <paulmck@...nel.org> writes:

> 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!

Great. Thanks Paul!

Ankur

>> 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
>>


--
ankur