linux-kernel - [PATCH v2 0/5] locking/rwsem: Rework reader optimistic spinning

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Message-Id: <20201121041416.12285-1-longman@redhat.com>
Date:   Fri, 20 Nov 2020 23:14:11 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Phil Auld <pauld@...hat.com>, Waiman Long <longman@...hat.com>
Subject: [PATCH v2 0/5] locking/rwsem: Rework reader optimistic spinning

 v2:
  - Update some commit logs to incorporate review comments.
  - Patch 2: remove unnecessary comment.
  - Patch 3: rename osq_is_empty() to rwsem_no_spinners() as suggested.
  - Patch 4: correctly handle HANDOFF clearing.
  - Patch 5: fix !CONFIG_RWSEM_SPIN_ON_OWNER compilation errors.

A recent report of SAP certification failure caused by increased system
time due to rwsem reader optimistic spinning led me to reexamine the
code to see the pro and cons of doing it. This led me to discover a
potential lock starvation scenario as explained in patch 2. That patch
does reduce reader spinning to avoid this potential problem. Patches
3 and 4 are further optimizations of the current code.

Then there is the issue of reader fragmentation that can potentially
reduce performance in some heavily contented cases. Two different
approaches are attempted:
 1) further reduce reader optimistic spinning
 2) disable reader spinning

See the performance shown in patch 5.

This patch series adopts the second approach by dropping reader spinning
for now as it simplifies the code. However, writers are still allowed
to spin on a reader-owned rwsem for a limited time.

Waiman Long (5):
  locking/rwsem: Pass the current atomic count to
    rwsem_down_read_slowpath()
  locking/rwsem: Prevent potential lock starvation
  locking/rwsem: Enable reader optimistic lock stealing
  locking/rwsem: Wake up all waiting readers if RWSEM_WAKE_READ_OWNED
  locking/rwsem: Remove reader optimistic spinning

 kernel/locking/lock_events_list.h |   6 +-
 kernel/locking/rwsem.c            | 293 ++++++++----------------------
 2 files changed, 82 insertions(+), 217 deletions(-)

-- 
2.18.1