[RFC PATCH 01/13] arm64: preempt: Simplify and optimize __preempt_count_dec_and_test()

[Mark Rutland]

In arm64's __preempt_count_dec_and_test(), the final conditional load of
'ti->preempt_count' is always executed in the common case. The
conditional load leads to unfortunate code generation for
preempt_enable[_notrace](), and it would be better to unconditionally
load 'ti->preempt_count', as described below.

On arm64, struct thread_info contains the following union:

| union {
|         u64             preempt_count;
|         struct {
|                 u32     count;
|                 u32     need_resched;
|         } preempt;
| };

Note: 'need_resched' is encoded so that '0' means a reschedule is
needed, and '1' means a reschedule is NOT needed.

The core logic of __preempt_count_dec_and_test() is:

| static inline bool __preempt_count_dec_and_test(void)
| {
|         struct thread_info *ti = current_thread_info();
|         u64 pc = READ_ONCE(ti->preempt_count);
|
|         WRITE_ONCE(ti->preempt.count, --pc);
|
|         return !pc || !READ_ONCE(ti->preempt_count);
| }

The '!pc' condition can only be true when both:

* The initial value of 'need_resched' was 0, meaning that a reschedule
  is needed. This should be rare.

* The initial value of 'count' was exactly 1. This cannot be true for a
  nested preempt_disable() ... preempt_enable() sequence.

Hence in common cases, '!pc' will be false, and it's necessary to
execute the final READ_ONCE(ti->preempt_count).

This results in a conditional branch in the common case, as can be seen
when __preempt_count_dec_and_test() is outlined:

| <outline___preempt_count_dec_and_test>:
|        mrs     x2, sp_el0
|        ldr     x1, [x2, #8]
|        mov     w0, #0x1
|        sub     x1, x1, #0x1
|        str     w1, [x2, #8]
|        cbz     x1, 1f
|        ldr     x0, [x2, #8]
|        cmp     x0, #0x0
|        cset    w0, eq  // eq = none
| 1:     ret

It would be better to avoid the special case for 'pc == 0', and to
always load 'ti->preempt_count' after decrementing 'ti->preempt.count'.
For the common cases this will remove a conditional branch. For the rare
cases where preemption is needed initially, this only adds a single
load, whose cost should be dominated by other factors.

Remove the special case for 'pc == 0', and always load the combined
'ti->preempt_count' after decrementing 'ti->preempt.count'.

The removal of the conditional branch helps with code generation, as the
compiler can more easily move a dependent slow path out-of-line, as
demonstrated with the following compiled with GCC 15.2.0:

| void outline_preempt_enable_notrace(void)
| {
|        preempt_enable_notrace();
| }

Before this patch:

| <outline_preempt_enable_notrace>:
|        mrs     x1, sp_el0
|        ldr     x0, [x1, #8]
|        sub     x0, x0, #0x1
|        str     w0, [x1, #8]
|        cbz     x0, 1f
|        ldr     x0, [x1, #8]
|        cbnz    x0, 2f
| 1:     paciasp
|        stp     x29, x30, [sp, #-16]!
|        mov     x29, sp
|        bl      preempt_schedule_notrace
|        ldp     x29, x30, [sp], #16
|        autiasp
|        ret
| 2:     ret

After this patch:

| <outline_preempt_enable_notrace>:
|        mrs     x0, sp_el0
|        ldr     w1, [x0, #8]
|        sub     w1, w1, #0x1
|        str     w1, [x0, #8]
|        ldr     x0, [x0, #8]
|        cbz     x0, 1f
|        ret
| 1:     paciasp
|        stp     x29, x30, [sp, #-16]!
|        mov     x29, sp
|        bl      preempt_schedule_notrace
|        ldp     x29, x30, [sp], #16
|        autiasp
|        ret

Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Ada Couprie Diaz <ada.coupriediaz@xxxxxxx>
Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
Cc: Marc Zyngier <maz@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Vladimir Murzin <vladimir.murzin@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Yang Shi <yang@xxxxxxxxxxxxxxxxxxxxxx>
---
 arch/arm64/include/asm/preempt.h | 29 +++++++++++------------------
 1 file changed, 11 insertions(+), 18 deletions(-)

diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
index 932ea4b620428..ca2ad1a8db095 100644
--- a/arch/arm64/include/asm/preempt.h
+++ b/arch/arm64/include/asm/preempt.h
@@ -55,30 +55,23 @@ static inline void __preempt_count_sub(int val)
 	WRITE_ONCE(current_thread_info()->preempt.count, pc);
 }
 
-static inline bool __preempt_count_dec_and_test(void)
-{
-	struct thread_info *ti = current_thread_info();
-	u64 pc = READ_ONCE(ti->preempt_count);
-
-	/* Update only the count field, leaving need_resched unchanged */
-	WRITE_ONCE(ti->preempt.count, --pc);
-
-	/*
-	 * 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;
 }
 
+static inline bool __preempt_count_dec_and_test(void)
+{
+	/*
+	 * We must load the combined 'prempt_count' after decrementing
+	 * 'preempt.count' as an interrupt could modify 'need_resched' before
+	 * __preempt_count_sub() writes back to 'preempt.count'.
+	 */
+	__preempt_count_sub(1);
+	return should_resched(0);
+}
+
 #ifdef CONFIG_PREEMPTION
 
 void preempt_schedule(void);
-- 
2.30.2