arm64: Only call into preempt_schedule() if need_resched()

The asm-generic/preempt.h implementation doesn't make use of the PREEMPT_NEED_RESCHED flag, since this can interact badly with load/store architectures which rely on the preempt_count word being unchanged across an interrupt.

However, since we're a 64-bit architecture and the preempt count is only 32 bits wide, we can simply pack it next to the resched flag and load the whole thing in one go, so that a dec-and-test operation doesn't need to load twice.

Signed-off-by Will Deacon <[email protected]>

---
 arch/arm64/include/asm/Kbuild        |  1 -
 arch/arm64/include/asm/preempt.h     | 88 ++++++++++++++++++++++++++++++++++++
 arch/arm64/include/asm/thread_info.h | 13 +++++-
 3 files changed, 100 insertions(+), 2 deletions(-)
 create mode 100644 arch/arm64/include/asm/preempt.h

arch/arm64/include/asm/Kbuild +0 -1

index 6cd5d77b6b44..33498f900390 100644
@@ -14,7 +14,6 @@ generic-y += local64.h
 generic-y += mcs_spinlock.h
 generic-y += mm-arch-hooks.h
 generic-y += msi.h
-generic-y += preempt.h
 generic-y += qrwlock.h
 generic-y += qspinlock.h
 generic-y += rwsem.h

arch/arm64/include/asm/preempt.h new file +53 -0

new file mode 100644
index 000000000000..f1c1398cf065
@@ -0,0 +1,88 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_PREEMPT_H
+#define __ASM_PREEMPT_H
+
+#include <linux/thread_info.h>
+
+#define PREEMPT_NEED_RESCHED   BIT(32)
+#define PREEMPT_ENABLED        (PREEMPT_NEED_RESCHED)
+
+static inline int preempt_count(void)
+{
+       return READ_ONCE(current_thread_info()->preempt.count);
+}
+
+static inline void preempt_count_set(u64 pc)
+{
+       /* Preserve existing value of PREEMPT_NEED_RESCHED */
+       WRITE_ONCE(current_thread_info()->preempt.count, pc);
+}
+
+#define init_task_preempt_count(p) do { \
+       task_thread_info(p)->preempt_count = FORK_PREEMPT_COUNT; \
+} while (0)
+
+#define init_idle_preempt_count(p, cpu) do { \
+       task_thread_info(p)->preempt_count = PREEMPT_ENABLED; \
+} while (0)
+
+static inline void set_preempt_need_resched(void)
+{
+       current_thread_info()->preempt.need_resched = 0;
+}
+
+static inline void clear_preempt_need_resched(void)
+{
+       current_thread_info()->preempt.need_resched = 1;
+}
+
+static inline bool test_preempt_need_resched(void)
+{
+       return !current_thread_info()->preempt.need_resched;
+}
+
+static inline void __preempt_count_add(int val)
+{
+       u32 pc = READ_ONCE(current_thread_info()->preempt.count);
+       pc += val;
+       WRITE_ONCE(current_thread_info()->preempt.count, pc);
+}
+
+static inline void __preempt_count_sub(int val)
+{
+       u32 pc = READ_ONCE(current_thread_info()->preempt.count);

arch/arm64/include/asm/preempt.h new file +25 -0

+
+       /*
+        * If we wrote back all zeroes, then we're preemptible and in
+        * need of a reschedule. Otherwise, we need to reload the
+        * preempt_count in case the need_resched flag was cleared by an
+        * interrupt occurring between the non-atomic READ_ONCE/WRITE_ONCE
+        * pair.
+        */
+       return !pc || !READ_ONCE(ti->preempt_count);
+}
+
+static inline bool should_resched(int preempt_offset)
+{
+       u64 pc = READ_ONCE(current_thread_info()->preempt_count);
+       return pc == preempt_offset;
+}
+
+#ifdef CONFIG_PREEMPT
+void preempt_schedule(void);
+#define __preempt_schedule() preempt_schedule()
+void preempt_schedule_notrace(void);
+#define __preempt_schedule_notrace() preempt_schedule_notrace()
+#endif /* CONFIG_PREEMPT */
+
+#endif /* __ASM_PREEMPT_H */

arch/arm64/include/asm/thread_info.h +12 -2

index cb2c10a8f0a8..bbca68b54732 100644
@@ -42,7 +42,18 @@ struct thread_info {
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
        u64                     ttbr0;          /* saved TTBR0_EL1 */
 #endif
-       int                     preempt_count;  /* 0 => preemptable, <0 => bug */
+       union {
+               u64             preempt_count;  /* 0 => preemptible, <0 => bug */
+               struct {
+#ifdef CONFIG_CPU_BIG_ENDIAN
+                       u32     need_resched;
+                       u32     count;
+#else
+                       u32     count;
+                       u32     need_resched;
+#endif
+               } preempt;
+       };
 };
 
 #define thread_saved_pc(tsk)   \
--

2.1.4

Hi all,

This is version two of the patches I originally posted here:

http://lkml.kernel.org/r/[email protected]

The only change since v1 is that __preempt_count_dec_and_test() now reloads the need_resched flag if it initially saw that it was set. This resolves the issue spotted by Peter, where an IRQ coming in during the decrement can cause a reschedule to be missed.

Quoted 4 lines · Will Deacon

This is version two of the patches I originally posted here:

http://lkml.kernel.org/r/[email protected]

Quoted 4 lines

The only change since v1 is that __preempt_count_dec_and_test() now reloads the need_resched flag if it initially saw that it was set. This resolves the issue spotted by Peter, where an IRQ coming in during the decrement can cause a reschedule to be missed.

Yes, I think this one will work, so:

Acked-by Peter Zijlstra (Intel) <[email protected]>

However, this leaves me wondering if the sequence is actually much better than what you had?

I suppose there's a win due to cache locality -- you only have to load a single line -- but I'm thinking that on pure instruction count, you're not actually winning much.