Subject: [PATCH v3 00/17] powerpc: alternate queued spinlock implementation

From: Nicholas Piggin <[email-masked]>
Date: Sat, 26 Nov 2022 19:59:15 +1000

This replaces the generic queued spinlock code (like s390 does) with
our own implementation. There is an extra shim patch 1a to get the
series to apply.

Generic PV qspinlock code is causing latency / starvation regressions on
large systems that are resulting in hard lockups reported (mostly in
pathoogical cases).  The generic qspinlock code has a number of issues
important for powerpc hardware and hypervisors that aren't easily solved
without changing code that would impact other architectures. Follow
s390's lead and implement our own for now.

Issues for powerpc using generic qspinlocks:
- The previous lock value should not be loaded with simple loads, and
  need not be passed around from previous loads or cmpxchg results,
  because powerpc uses ll/sc-style atomics which can perform more
  complex operations that do not require this. powerpc implementations
  tend to prefer loads use larx for improved coherency performance.
- The queueing process should absolutely minimise the number of stores
  to the lock word to reduce exclusive coherency probes, important for
  large system scalability. The pending logic is counter productive
  here.
- Non-atomic unlock for paravirt locks is important (atomic instructions
  tend to still be more expensive than x86 CPUs).
- Yielding to the lock owner is important in the oversubscribed paravirt
  case, which requires storing the owner CPU in the lock word.
- More control of lock stealing for the paravirt case is important to
  keep latency down on large systems.
- The lock acquisition operation should always be made with a special
  variant of atomic instructions with the lock hint bit set, including
  (especially) in the queueing paths. This is more a matter of adding
  more arch lock helpers so not an insurmountable problem for generic
  code.

Thanks,
Nick

Since v2:
- Rebase the series on upstream and remove the 1a shim patch.
- Squash in the RFC patches that avoid a few more cmpxchg patterns in
  favour of more optimal larx/stcx implementations and allows the
  non-stealing queueing case to be removed, significantly reducing
  the queuing code.
- Reword some changelogs.

Since v1:
- Change most 'if (cond) return 1 ; return 0;'
- Bug fix: was testing count == MAX, but reentrant NMIs could bring that
  > MAX and crash.
- Fix missing memory barrier lost in asm conversion patch.
- Seperate the release barrier in publish_tail from the acquire barrier
  in get_tail_qnode.
- Moving a few minor things into their logically correct change.
- Make encode_tail_cpu take a cpu argument to match get_tail_cpu.
- Rename get_tail_cpu to decode_tail_cpu to match encode_tail_cpu.
- Rename lock_set_locked to set_locked.
- IS_ENABLED(x) ? 1 : 0 -> IS_ENABLED(x)
- Fix some comments inside inline asm.
- Change tunable names to lowercase.
- Consolidate asm for trylock_clear_tail_cpu and trylock_with_tail_cpu
- Restructure steal/wait loops to be more readable
- Count a failed cmpxchg as an iteration in steal/wait loops to avoid
  theoretical livelock/latency concern.

Nicholas Piggin (17):
  powerpc/qspinlock: add mcs queueing for contended waiters
  powerpc/qspinlock: use a half-word store to unlock to avoid larx/stcx.
  powerpc/qspinlock: convert atomic operations to assembly
  powerpc/qspinlock: allow new waiters to steal the lock before queueing
  powerpc/qspinlock: theft prevention to control latency
  powerpc/qspinlock: store owner CPU in lock word
  powerpc/qspinlock: paravirt yield to lock owner
  powerpc/qspinlock: implement option to yield to previous node
  powerpc/qspinlock: allow stealing when head of queue yields
  powerpc/qspinlock: allow propagation of yield CPU down the queue
  powerpc/qspinlock: add ability to prod new queue head CPU
  powerpc/qspinlock: allow lock stealing in trylock and lock fastpath
  powerpc/qspinlock: use spin_begin/end API
  powerpc/qspinlock: reduce remote node steal spins
  powerpc/qspinlock: allow indefinite spinning on a preempted owner
  powerpc/qspinlock: provide accounting and options for sleepy locks
  powerpc/qspinlock: add compile-time tuning adjustments

 arch/powerpc/include/asm/qspinlock.h       | 130 ++-
 arch/powerpc/include/asm/qspinlock_types.h |  63 +-
 arch/powerpc/lib/qspinlock.c               | 985 ++++++++++++++++++++-
 3 files changed, 1167 insertions(+), 11 deletions(-)

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2.37.2