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* [PATCH v2 0/5] locking/rwsem: Rework reader optimistic spinning
@ 2020-11-21  4:14 Waiman Long
  2020-11-21  4:14 ` [PATCH v2 1/5] locking/rwsem: Pass the current atomic count to rwsem_down_read_slowpath() Waiman Long
                   ` (5 more replies)
  0 siblings, 6 replies; 21+ messages in thread
From: Waiman Long @ 2020-11-21  4:14 UTC (permalink / raw)
  To: Peter Zijlstra, Ingo Molnar, Will Deacon
  Cc: linux-kernel, Davidlohr Bueso, Phil Auld, Waiman Long

 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


^ permalink raw reply	[flat|nested] 21+ messages in thread

end of thread, other threads:[~2020-12-09 18:43 UTC | newest]

Thread overview: 21+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-11-21  4:14 [PATCH v2 0/5] locking/rwsem: Rework reader optimistic spinning Waiman Long
2020-11-21  4:14 ` [PATCH v2 1/5] locking/rwsem: Pass the current atomic count to rwsem_down_read_slowpath() Waiman Long
2020-12-09 18:38   ` [tip: locking/core] " tip-bot2 for Waiman Long
2020-11-21  4:14 ` [PATCH v2 2/5] locking/rwsem: Prevent potential lock starvation Waiman Long
2020-11-26  8:12   ` [locking/rwsem] 25d0c60b0e: vm-scalability.throughput 316.2% improvement kernel test robot
2020-12-09 18:38   ` [tip: locking/core] locking/rwsem: Prevent potential lock starvation tip-bot2 for Waiman Long
2020-11-21  4:14 ` [PATCH v2 3/5] locking/rwsem: Enable reader optimistic lock stealing Waiman Long
2020-12-09 18:38   ` [tip: locking/core] " tip-bot2 for Waiman Long
2020-11-21  4:14 ` [PATCH v2 4/5] locking/rwsem: Wake up all waiting readers if RWSEM_WAKE_READ_OWNED Waiman Long
2020-11-24  3:15   ` [locking/rwsem] c9847a7f94: aim7.jobs-per-min -91.8% regression kernel test robot
2020-11-21  4:14 ` [PATCH v2 5/5] locking/rwsem: Remove reader optimistic spinning Waiman Long
2020-11-23 15:53   ` [locking/rwsem] 10a59003d2: unixbench.score -25.5% regression kernel test robot
2020-11-23 19:28     ` Waiman Long
2020-12-08  3:56   ` [PATCH v2 5/5] locking/rwsem: Remove reader optimistic spinning Davidlohr Bueso
2020-12-08 10:07   ` Peter Zijlstra
2020-12-08 15:29     ` Waiman Long
2020-12-09 18:38   ` [tip: locking/core] " tip-bot2 for Waiman Long
2020-12-08 14:57 ` [PATCH v2 0/5] locking/rwsem: Rework " Peter Zijlstra
2020-12-08 16:33   ` Waiman Long
2020-12-08 17:02     ` Peter Zijlstra
2020-12-08 17:30       ` Waiman Long

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