Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

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On 3/27/21 2:06 PM, guoren@xxxxxxxxxx wrote:
From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>

Some architectures don't have sub-word swap atomic instruction,
they only have the full word's one.

The sub-word swap only improve the performance when:
NR_CPUS < 16K
  *  0- 7: locked byte
  *     8: pending
  *  9-15: not used
  * 16-17: tail index
  * 18-31: tail cpu (+1)

The 9-15 bits are wasted to use xchg16 in xchg_tail.

Please let architecture select xchg16/xchg32 to implement
xchg_tail.

Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Anup Patel <anup@xxxxxxxxxxxxxx>
---
  kernel/Kconfig.locks       |  3 +++
  kernel/locking/qspinlock.c | 44 +++++++++++++++++++++-----------------
  2 files changed, 27 insertions(+), 20 deletions(-)

diff --git a/kernel/Kconfig.locks b/kernel/Kconfig.locks
index 3de8fd11873b..d02f1261f73f 100644
--- a/kernel/Kconfig.locks
+++ b/kernel/Kconfig.locks
@@ -239,6 +239,9 @@ config LOCK_SPIN_ON_OWNER
  config ARCH_USE_QUEUED_SPINLOCKS
  	bool
+config ARCH_USE_QUEUED_SPINLOCKS_XCHG32
+	bool
+
  config QUEUED_SPINLOCKS
  	def_bool y if ARCH_USE_QUEUED_SPINLOCKS
  	depends on SMP
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index cbff6ba53d56..54de0632c6a8 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -163,26 +163,6 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
  	WRITE_ONCE(lock->locked_pending, _Q_LOCKED_VAL);
  }
-/*
- * xchg_tail - Put in the new queue tail code word & retrieve previous one
- * @lock : Pointer to queued spinlock structure
- * @tail : The new queue tail code word
- * Return: The previous queue tail code word
- *
- * xchg(lock, tail), which heads an address dependency
- *
- * p,*,* -> n,*,* ; prev = xchg(lock, node)
- */
-static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
-{
-	/*
-	 * We can use relaxed semantics since the caller ensures that the
-	 * MCS node is properly initialized before updating the tail.
-	 */
-	return (u32)xchg_relaxed(&lock->tail,
-				 tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
-}
-
  #else /* _Q_PENDING_BITS == 8 */
/**
@@ -206,6 +186,30 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
  {
  	atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val);
  }
+#endif
+
+#if _Q_PENDING_BITS == 8 && !defined(CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32)
+/*
+ * xchg_tail - Put in the new queue tail code word & retrieve previous one
+ * @lock : Pointer to queued spinlock structure
+ * @tail : The new queue tail code word
+ * Return: The previous queue tail code word
+ *
+ * xchg(lock, tail), which heads an address dependency
+ *
+ * p,*,* -> n,*,* ; prev = xchg(lock, node)
+ */
+static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
+{
+	/*
+	 * We can use relaxed semantics since the caller ensures that the
+	 * MCS node is properly initialized before updating the tail.
+	 */
+	return (u32)xchg_relaxed(&lock->tail,
+				 tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
+}
+
+#else
/**
   * xchg_tail - Put in the new queue tail code word & retrieve previous one

I don't have any problem adding a CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32 config option to control that.

One minor nit:

#endif /* _Q_PENDING_BITS == 8 */

You should probably remove the comment at the trailing end of the corresponding "#endif" as it is now wrong.

Cheers,
Longman




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