From: "Christoph Lameter (Ampere)" <cl@xxxxxxxxxx>
Some architectures support load acquire which can save us a memory
barrier and save some cycles.
A typical sequence
do {
seq = read_seqcount_begin(&s);
<something>
} while (read_seqcount_retry(&s, seq);
requires 13 cycles on ARM64 for an empty loop. Two read memory barriers are
needed. One for each of the seqcount_* functions.
We can replace the first read barrier with a load acquire of
the seqcount which saves us one barrier.
On ARM64 doing so reduces the cycle count from 13 to 8.
Signed-off-by: Christoph Lameter (Ampere) <cl@xxxxxxxxxx>
---
arch/Kconfig | 5 +++++
arch/arm64/Kconfig | 1 +
include/linux/seqlock.h | 41 +++++++++++++++++++++++++++++++++++++++++
3 files changed, 47 insertions(+)
diff --git a/arch/Kconfig b/arch/Kconfig
index 975dd22a2dbd..3f8867110a57 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1600,6 +1600,11 @@ config ARCH_HAS_KERNEL_FPU_SUPPORT
Architectures that select this option can run floating-point code in
the kernel, as described in Documentation/core-api/floating-point.rst.
+config ARCH_HAS_ACQUIRE_RELEASE
+ bool
+ help
+ Architectures that support acquire / release can avoid memory fences
+
source "kernel/gcov/Kconfig"
source "scripts/gcc-plugins/Kconfig"
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index a2f8ff354ca6..19e34fff145f 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -39,6 +39,7 @@ config ARM64
select ARCH_HAS_PTE_DEVMAP
select ARCH_HAS_PTE_SPECIAL
select ARCH_HAS_HW_PTE_YOUNG
+ select ARCH_HAS_ACQUIRE_RELEASE
select ARCH_HAS_SETUP_DMA_OPS
select ARCH_HAS_SET_DIRECT_MAP
select ARCH_HAS_SET_MEMORY
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index d90d8ee29d81..353fcf32b800 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -176,6 +176,28 @@ __seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \
return seq; \
} \
\
+static __always_inline unsigned \
+__seqprop_##lockname##_sequence_acquire(const seqcount_##lockname##_t *s) \
+{ \
+ unsigned seq = smp_load_acquire(&s->seqcount.sequence); \
+ \
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
+ return seq; \
+ \
+ if (preemptible && unlikely(seq & 1)) { \
+ __SEQ_LOCK(lockbase##_lock(s->lock)); \
+ __SEQ_LOCK(lockbase##_unlock(s->lock)); \
+ \
+ /* \
+ * Re-read the sequence counter since the (possibly \
+ * preempted) writer made progress. \
+ */ \
+ seq = smp_load_acquire(&s->seqcount.sequence); \
+ } \
+ \
+ return seq; \
+} \
+ \
static __always_inline bool \
__seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s) \
{ \
@@ -211,6 +233,11 @@ static inline unsigned __seqprop_sequence(const seqcount_t *s)
return READ_ONCE(s->sequence);
}
+static inline unsigned __seqprop_sequence_acquire(const seqcount_t *s)
+{
+ return smp_load_acquire(&s->sequence);
+}
+
static inline bool __seqprop_preemptible(const seqcount_t *s)
{
return false;
@@ -259,6 +286,7 @@ SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex)
#define seqprop_ptr(s) __seqprop(s, ptr)(s)
#define seqprop_const_ptr(s) __seqprop(s, const_ptr)(s)
#define seqprop_sequence(s) __seqprop(s, sequence)(s)
+#define seqprop_sequence_acquire(s) __seqprop(s, sequence_acquire)(s)
#define seqprop_preemptible(s) __seqprop(s, preemptible)(s)
#define seqprop_assert(s) __seqprop(s, assert)(s)
@@ -293,6 +321,18 @@ SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex)
*
* Return: count to be passed to read_seqcount_retry()
*/
+#ifdef CONFIG_ARCH_HAS_ACQUIRE_RELEASE
+#define raw_read_seqcount_begin(s) \
+({ \
+ unsigned _seq; \
+ \
+ while ((_seq = seqprop_sequence_acquire(s)) & 1) \
+ cpu_relax(); \
+ \
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
+ _seq; \
+})
+#else
#define raw_read_seqcount_begin(s) \
({ \
unsigned _seq = __read_seqcount_begin(s); \
@@ -300,6 +340,7 @@ SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex)
smp_rmb(); \
_seq; \
})
+#endif
/**
* read_seqcount_begin() - begin a seqcount_t read critical section
---
base-commit: 6b4aa469f04999c3f244515fa7491b4d093c5167
change-id: 20240813-seq_optimize-68c48696c798
Best regards,
--
Christoph Lameter <cl@xxxxxxxxxx>
