Mailing List Archive

From: Lai Jiangshan <jiangshan.ljs@antgroup.com>

Implement PCPU_RCU_PREEMPT_COUNT for x86.
Mainly copied from asm/preempt.h

Make rcu_read_[un]lock() inlined for rcu-preempt.
Make rcu_read_lock() only one instruction.
Make rcu_read_unlock() only two instructions in the fast path.

Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
---
arch/x86/Kconfig | 1 +
arch/x86/include/asm/current.h | 3 +
arch/x86/include/asm/rcu_preempt.h | 107 +++++++++++++++++++++++++++++
arch/x86/kernel/cpu/common.c | 7 +-
4 files changed, 115 insertions(+), 3 deletions(-)
create mode 100644 arch/x86/include/asm/rcu_preempt.h

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 78050d5d7fac..7eb17c12f7b7 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -257,6 +257,7 @@ config X86
select HAVE_OBJTOOL if X86_64
select HAVE_OPTPROBES
select HAVE_PAGE_SIZE_4KB
+ select HAVE_PCPU_RCU_PREEMPT_COUNT
select HAVE_PCSPKR_PLATFORM
select HAVE_PERF_EVENTS
select HAVE_PERF_EVENTS_NMI
diff --git a/arch/x86/include/asm/current.h b/arch/x86/include/asm/current.h
index bf5953883ec3..dcc2ef784120 100644
--- a/arch/x86/include/asm/current.h
+++ b/arch/x86/include/asm/current.h
@@ -24,6 +24,9 @@ struct pcpu_hot {
unsigned long top_of_stack;
void *hardirq_stack_ptr;
u16 softirq_pending;
+#ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
+ int rcu_preempt_count;
+#endif // #ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
#ifdef CONFIG_X86_64
bool hardirq_stack_inuse;
#else
diff --git a/arch/x86/include/asm/rcu_preempt.h b/arch/x86/include/asm/rcu_preempt.h
new file mode 100644
index 000000000000..cb25ebe038a5
--- /dev/null
+++ b/arch/x86/include/asm/rcu_preempt.h
@@ -0,0 +1,107 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_RCU_PREEMPT_H
+#define __ASM_RCU_PREEMPT_H
+
+#include <asm/rmwcc.h>
+#include <asm/percpu.h>
+#include <asm/current.h>
+
+#ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
+
+/* We use the MSB mostly because its available */
+#define RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED 0x80000000
+
+/*
+ * We use the RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED bit as an inverted
+ * current->rcu_read_unlock_special.s such that a decrement hitting 0
+ * means we can and should call rcu_read_unlock_special().
+ */
+#define RCU_PREEMPT_INIT (0 + RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED)
+
+/*
+ * We mask the RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED bit so as not to
+ * confuse all current users that think a non-zero value indicates we
+ * are in a critical section.
+ */
+static inline int pcpu_rcu_preempt_count(void)
+{
+ return raw_cpu_read_4(pcpu_hot.rcu_preempt_count) & ~RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED;
+}
+
+static inline void pcpu_rcu_preempt_count_set(int count)
+{
+ int old, new;
+
+ old = raw_cpu_read_4(pcpu_hot.rcu_preempt_count);
+ do {
+ new = (old & RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED) |
+ (count & ~RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED);
+ } while (!raw_cpu_try_cmpxchg_4(pcpu_hot.rcu_preempt_count, &old, new));
+}
+
+/*
+ * We fold the RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED bit into the RCU
+ * preempt count such that rcu_read_unlock() can decrement and test for
+ * the need of unlock-special handling with a single instruction.
+ *
+ * We invert the actual bit, so that when the decrement hits 0 we know
+ * we both reach a quiescent state (no rcu preempt count) and need to
+ * handle unlock-special (the bit is cleared), normally to report the
+ * quiescent state immediately.
+ */
+
+static inline void pcpu_rcu_preempt_special_set(void)
+{
+ raw_cpu_and_4(pcpu_hot.rcu_preempt_count, ~RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED);
+}
+
+static inline void pcpu_rcu_preempt_special_clear(void)
+{
+ raw_cpu_or_4(pcpu_hot.rcu_preempt_count, RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED);
+}
+
+static inline bool pcpu_rcu_preempt_special_test(void)
+{
+ return !(raw_cpu_read_4(pcpu_hot.rcu_preempt_count) & RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED);
+}
+
+static inline void pcpu_rcu_preempt_switch(int count, bool special)
+{
+ if (likely(!special))
+ raw_cpu_write(pcpu_hot.rcu_preempt_count, count | RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED);
+ else
+ raw_cpu_write(pcpu_hot.rcu_preempt_count, count);
+}
+
+/*
+ * The various rcu_preempt_count add/sub methods
+ */
+
+static __always_inline void pcpu_rcu_preempt_count_add(int val)
+{
+ raw_cpu_add_4(pcpu_hot.rcu_preempt_count, val);
+}
+
+static __always_inline void pcpu_rcu_preempt_count_sub(int val)
+{
+ raw_cpu_add_4(pcpu_hot.rcu_preempt_count, -val);
+}
+
+/*
+ * Because we keep RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED set when we do
+ * _not_ need to handle unlock-special for a fast-path decrement.
+ */
+static __always_inline bool pcpu_rcu_preempt_count_dec_and_test(void)
+{
+ return GEN_UNARY_RMWcc("decl", __my_cpu_var(pcpu_hot.rcu_preempt_count), e,
+ __percpu_arg([var]));
+}
+
+#define pcpu_rcu_read_unlock_special() \
+do { \
+ rcu_read_unlock_special(); \
+} while (0)
+
+#endif // #ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
+
+#endif /* __ASM_RCU_PREEMPT_H */
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index ba8cf5e9ce56..0b204a649442 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1992,9 +1992,10 @@ static __init int setup_clearcpuid(char *arg)
__setup("clearcpuid=", setup_clearcpuid);

DEFINE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot) = {
- .current_task = &init_task,
- .preempt_count = INIT_PREEMPT_COUNT,
- .top_of_stack = TOP_OF_INIT_STACK,
+ .current_task = &init_task,
+ .preempt_count = INIT_PREEMPT_COUNT,
+ .top_of_stack = TOP_OF_INIT_STACK,
+ .rcu_preempt_count = RCU_PREEMPT_INIT,
};
EXPORT_PER_CPU_SYMBOL(pcpu_hot);
EXPORT_PER_CPU_SYMBOL(const_pcpu_hot);
--
2.19.1.6.gb485710b