[PATCH v3 3/3] Make finish_task_switch and its subfuncs inline in context switching

[Xie Yuanbin <qq570070308@gmail.com>]

`finish_task_switch` is a hot path in context switching, and due to
possible mitigations inside switch_mm, performance here is greatly
affected by function calls and branch jumps. Make it inline to optimize
the performance.

After `finish_task_switch` is changed to an inline function, the number of
calls to the subfunctions (called by `finish_task_switch`) increases in
this translation unit due to the inline expansion of `finish_task_switch`.
Due to compiler optimization strategies, these functions may transition
from inline functions to non inline functions, which can actually lead to
performance degradation.

Make the subfunctions of finish_task_stwitch inline to prevent
degradation.

Perf test:
Time spent on calling finish_task_switch (rdtsc):
 | compiler && appended cmdline | without patch   | with patch    |
 | gcc + NA                     | 13.93 - 13.94   | 12.39 - 12.44 |
 | gcc + "spectre_v2_user=on"   | 24.69 - 24.85   | 13.68 - 13.73 |
 | clang + NA                   | 13.89 - 13.90   | 12.70 - 12.73 |
 | clang + "spectre_v2_user=on" | 29.00 - 29.02   | 18.88 - 18.97 |

Perf test info:
1. kernel source:
linux-next
commit 9c0826a5d9aa4d52206d ("Add linux-next specific files for 20251107")
2. compiler:
gcc: gcc version 15.2.0 (Debian 15.2.0-7) with
GNU ld (GNU Binutils for Debian) 2.45
clang: Debian clang version 21.1.4 (8) with
Debian LLD 21.1.4 (compatible with GNU linkers)
3. config:
base on default x86_64_defconfig, and setting:
CONFIG_HZ=100
CONFIG_DEBUG_ENTRY=n
CONFIG_X86_DEBUG_FPU=n
CONFIG_EXPERT=y
CONFIG_MODIFY_LDT_SYSCALL=n
CONFIG_CGROUPS=n
CONFIG_BLK_DEV_NVME=y

Size test:
bzImage size:
 | compiler | without patches | with patches  |
 | clang    | 13722624        | 13722624      |
 | gcc      | 12596224        | 12596224      |

Size test info:
1. kernel source && compiler: same as above
2. config:
base on default x86_64_defconfig, and setting:
CONFIG_SCHED_CORE=y
CONFIG_CC_OPTIMIZE_FOR_SIZE=y
CONFIG_NO_HZ_FULL=y

Signed-off-by: Xie Yuanbin <qq570070308@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Rik van Riel <riel@surriel.com>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: H. Peter Anvin (Intel) <hpa@zytor.com>
---
 arch/arm/include/asm/mmu_context.h      |  2 +-
 arch/riscv/include/asm/sync_core.h      |  2 +-
 arch/s390/include/asm/mmu_context.h     |  2 +-
 arch/sparc/include/asm/mmu_context_64.h |  2 +-
 arch/x86/include/asm/sync_core.h        |  2 +-
 include/linux/perf_event.h              |  2 +-
 include/linux/sched/mm.h                | 10 +++++-----
 include/linux/tick.h                    |  4 ++--
 include/linux/vtime.h                   |  8 ++++----
 kernel/sched/core.c                     | 14 +++++++-------
 kernel/sched/sched.h                    | 20 ++++++++++----------
 11 files changed, 34 insertions(+), 34 deletions(-)

diff --git a/arch/arm/include/asm/mmu_context.h b/arch/arm/include/asm/mmu_context.h
index db2cb06aa8cf..bebde469f81a 100644
--- a/arch/arm/include/asm/mmu_context.h
+++ b/arch/arm/include/asm/mmu_context.h
@@ -80,7 +80,7 @@ static inline void check_and_switch_context(struct mm_struct *mm,
 #ifndef MODULE
 #define finish_arch_post_lock_switch \
 	finish_arch_post_lock_switch
-static inline void finish_arch_post_lock_switch(void)
+static __always_inline void finish_arch_post_lock_switch(void)
 {
 	struct mm_struct *mm = current->mm;
 
diff --git a/arch/riscv/include/asm/sync_core.h b/arch/riscv/include/asm/sync_core.h
index 9153016da8f1..2fe6b7fe6b12 100644
--- a/arch/riscv/include/asm/sync_core.h
+++ b/arch/riscv/include/asm/sync_core.h
@@ -6,7 +6,7 @@
  * RISC-V implements return to user-space through an xRET instruction,
  * which is not core serializing.
  */
-static inline void sync_core_before_usermode(void)
+static __always_inline void sync_core_before_usermode(void)
 {
 	asm volatile ("fence.i" ::: "memory");
 }
diff --git a/arch/s390/include/asm/mmu_context.h b/arch/s390/include/asm/mmu_context.h
index d9b8501bc93d..c124ef6a01b3 100644
--- a/arch/s390/include/asm/mmu_context.h
+++ b/arch/s390/include/asm/mmu_context.h
@@ -97,7 +97,7 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 }
 
 #define finish_arch_post_lock_switch finish_arch_post_lock_switch
-static inline void finish_arch_post_lock_switch(void)
+static __always_inline void finish_arch_post_lock_switch(void)
 {
 	struct task_struct *tsk = current;
 	struct mm_struct *mm = tsk->mm;
diff --git a/arch/sparc/include/asm/mmu_context_64.h b/arch/sparc/include/asm/mmu_context_64.h
index 78bbacc14d2d..d1967214ef25 100644
--- a/arch/sparc/include/asm/mmu_context_64.h
+++ b/arch/sparc/include/asm/mmu_context_64.h
@@ -160,7 +160,7 @@ static inline void arch_start_context_switch(struct task_struct *prev)
 }
 
 #define finish_arch_post_lock_switch	finish_arch_post_lock_switch
-static inline void finish_arch_post_lock_switch(void)
+static __always_inline void finish_arch_post_lock_switch(void)
 {
 	/* Restore the state of MCDPER register for the new process
 	 * just switched to.
diff --git a/arch/x86/include/asm/sync_core.h b/arch/x86/include/asm/sync_core.h
index 96bda43538ee..4b55fa353bb5 100644
--- a/arch/x86/include/asm/sync_core.h
+++ b/arch/x86/include/asm/sync_core.h
@@ -93,7 +93,7 @@ static __always_inline void sync_core(void)
  * to user-mode. x86 implements return to user-space through sysexit,
  * sysrel, and sysretq, which are not core serializing.
  */
-static inline void sync_core_before_usermode(void)
+static __always_inline void sync_core_before_usermode(void)
 {
 	/* With PTI, we unconditionally serialize before running user code. */
 	if (static_cpu_has(X86_FEATURE_PTI))
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 9870d768db4c..d9de20c20f38 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1624,7 +1624,7 @@ static inline void perf_event_task_migrate(struct task_struct *task)
 		task->sched_migrated = 1;
 }
 
-static inline void perf_event_task_sched_in(struct task_struct *prev,
+static __always_inline void perf_event_task_sched_in(struct task_struct *prev,
 					    struct task_struct *task)
 {
 	if (static_branch_unlikely(&perf_sched_events))
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 0e1d73955fa5..e7787a6e7d22 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -44,7 +44,7 @@ static inline void smp_mb__after_mmgrab(void)
 
 extern void __mmdrop(struct mm_struct *mm);
 
-static inline void mmdrop(struct mm_struct *mm)
+static __always_inline void mmdrop(struct mm_struct *mm)
 {
 	/*
 	 * The implicit full barrier implied by atomic_dec_and_test() is
@@ -71,14 +71,14 @@ static inline void __mmdrop_delayed(struct rcu_head *rhp)
  * Invoked from finish_task_switch(). Delegates the heavy lifting on RT
  * kernels via RCU.
  */
-static inline void mmdrop_sched(struct mm_struct *mm)
+static __always_inline void mmdrop_sched(struct mm_struct *mm)
 {
 	/* Provides a full memory barrier. See mmdrop() */
 	if (atomic_dec_and_test(&mm->mm_count))
 		call_rcu(&mm->delayed_drop, __mmdrop_delayed);
 }
 #else
-static inline void mmdrop_sched(struct mm_struct *mm)
+static __always_inline void mmdrop_sched(struct mm_struct *mm)
 {
 	mmdrop(mm);
 }
@@ -104,7 +104,7 @@ static inline void mmdrop_lazy_tlb(struct mm_struct *mm)
 	}
 }
 
-static inline void mmdrop_lazy_tlb_sched(struct mm_struct *mm)
+static __always_inline void mmdrop_lazy_tlb_sched(struct mm_struct *mm)
 {
 	if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT))
 		mmdrop_sched(mm);
@@ -531,7 +531,7 @@ enum {
 #include <asm/membarrier.h>
 #endif
 
-static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
+static __always_inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
 {
 	/*
 	 * The atomic_read() below prevents CSE. The following should
diff --git a/include/linux/tick.h b/include/linux/tick.h
index ac76ae9fa36d..fce16aa10ba2 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -175,7 +175,7 @@ extern cpumask_var_t tick_nohz_full_mask;
 #ifdef CONFIG_NO_HZ_FULL
 extern bool tick_nohz_full_running;
 
-static inline bool tick_nohz_full_enabled(void)
+static __always_inline bool tick_nohz_full_enabled(void)
 {
 	if (!context_tracking_enabled())
 		return false;
@@ -299,7 +299,7 @@ static inline void __tick_nohz_task_switch(void) { }
 static inline void tick_nohz_full_setup(cpumask_var_t cpumask) { }
 #endif
 
-static inline void tick_nohz_task_switch(void)
+static __always_inline void tick_nohz_task_switch(void)
 {
 	if (tick_nohz_full_enabled())
 		__tick_nohz_task_switch();
diff --git a/include/linux/vtime.h b/include/linux/vtime.h
index 29dd5b91dd7d..428464bb81b3 100644
--- a/include/linux/vtime.h
+++ b/include/linux/vtime.h
@@ -67,24 +67,24 @@ static __always_inline void vtime_account_guest_exit(void)
  * For now vtime state is tied to context tracking. We might want to decouple
  * those later if necessary.
  */
-static inline bool vtime_accounting_enabled(void)
+static __always_inline bool vtime_accounting_enabled(void)
 {
 	return context_tracking_enabled();
 }
 
-static inline bool vtime_accounting_enabled_cpu(int cpu)
+static __always_inline bool vtime_accounting_enabled_cpu(int cpu)
 {
 	return context_tracking_enabled_cpu(cpu);
 }
 
-static inline bool vtime_accounting_enabled_this_cpu(void)
+static __always_inline bool vtime_accounting_enabled_this_cpu(void)
 {
 	return context_tracking_enabled_this_cpu();
 }
 
 extern void vtime_task_switch_generic(struct task_struct *prev);
 
-static inline void vtime_task_switch(struct task_struct *prev)
+static __always_inline void vtime_task_switch(struct task_struct *prev)
 {
 	if (vtime_accounting_enabled_this_cpu())
 		vtime_task_switch_generic(prev);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0e50ef3d819a..78d2c90bc73a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4868,7 +4868,7 @@ static inline void prepare_task(struct task_struct *next)
 	WRITE_ONCE(next->on_cpu, 1);
 }
 
-static inline void finish_task(struct task_struct *prev)
+static __always_inline void finish_task(struct task_struct *prev)
 {
 	/*
 	 * This must be the very last reference to @prev from this CPU. After
@@ -4884,7 +4884,7 @@ static inline void finish_task(struct task_struct *prev)
 	smp_store_release(&prev->on_cpu, 0);
 }
 
-static void do_balance_callbacks(struct rq *rq, struct balance_callback *head)
+static __always_inline void do_balance_callbacks(struct rq *rq, struct balance_callback *head)
 {
 	void (*func)(struct rq *rq);
 	struct balance_callback *next;
@@ -4919,7 +4919,7 @@ struct balance_callback balance_push_callback = {
 	.func = balance_push,
 };
 
-static inline struct balance_callback *
+static __always_inline struct balance_callback *
 __splice_balance_callbacks(struct rq *rq, bool split)
 {
 	struct balance_callback *head = rq->balance_callback;
@@ -4949,7 +4949,7 @@ struct balance_callback *splice_balance_callbacks(struct rq *rq)
 	return __splice_balance_callbacks(rq, true);
 }
 
-static void __balance_callbacks(struct rq *rq)
+static __always_inline void __balance_callbacks(struct rq *rq)
 {
 	do_balance_callbacks(rq, __splice_balance_callbacks(rq, false));
 }
@@ -4982,7 +4982,7 @@ prepare_lock_switch(struct rq *rq, struct task_struct *next, struct rq_flags *rf
 #endif
 }
 
-static inline void finish_lock_switch(struct rq *rq)
+static __always_inline void finish_lock_switch(struct rq *rq)
 {
 	/*
 	 * If we are tracking spinlock dependencies then we have to
@@ -5014,7 +5014,7 @@ static inline void kmap_local_sched_out(void)
 #endif
 }
 
-static inline void kmap_local_sched_in(void)
+static __always_inline void kmap_local_sched_in(void)
 {
 #ifdef CONFIG_KMAP_LOCAL
 	if (unlikely(current->kmap_ctrl.idx))
@@ -5067,7 +5067,7 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev,
  * past. 'prev == current' is still correct but we need to recalculate this_rq
  * because prev may have moved to another CPU.
  */
-static struct rq *finish_task_switch(struct task_struct *prev)
+static __always_inline struct rq *finish_task_switch(struct task_struct *prev)
 	__releases(rq->lock)
 {
 	struct rq *rq = this_rq();
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 7d305ec10374..ec301a91cb43 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1374,12 +1374,12 @@ static inline struct cpumask *sched_group_span(struct sched_group *sg);
 
 DECLARE_STATIC_KEY_FALSE(__sched_core_enabled);
 
-static inline bool sched_core_enabled(struct rq *rq)
+static __always_inline bool sched_core_enabled(struct rq *rq)
 {
 	return static_branch_unlikely(&__sched_core_enabled) && rq->core_enabled;
 }
 
-static inline bool sched_core_disabled(void)
+static __always_inline bool sched_core_disabled(void)
 {
 	return !static_branch_unlikely(&__sched_core_enabled);
 }
@@ -1388,7 +1388,7 @@ static inline bool sched_core_disabled(void)
  * Be careful with this function; not for general use. The return value isn't
  * stable unless you actually hold a relevant rq->__lock.
  */
-static inline raw_spinlock_t *rq_lockp(struct rq *rq)
+static __always_inline raw_spinlock_t *rq_lockp(struct rq *rq)
 {
 	if (sched_core_enabled(rq))
 		return &rq->core->__lock;
@@ -1396,7 +1396,7 @@ static inline raw_spinlock_t *rq_lockp(struct rq *rq)
 	return &rq->__lock;
 }
 
-static inline raw_spinlock_t *__rq_lockp(struct rq *rq)
+static __always_inline raw_spinlock_t *__rq_lockp(struct rq *rq)
 {
 	if (rq->core_enabled)
 		return &rq->core->__lock;
@@ -1487,12 +1487,12 @@ static inline bool sched_core_disabled(void)
 	return true;
 }
 
-static inline raw_spinlock_t *rq_lockp(struct rq *rq)
+static __always_inline raw_spinlock_t *rq_lockp(struct rq *rq)
 {
 	return &rq->__lock;
 }
 
-static inline raw_spinlock_t *__rq_lockp(struct rq *rq)
+static __always_inline raw_spinlock_t *__rq_lockp(struct rq *rq)
 {
 	return &rq->__lock;
 }
@@ -1542,23 +1542,23 @@ static inline void lockdep_assert_rq_held(struct rq *rq)
 extern void raw_spin_rq_lock_nested(struct rq *rq, int subclass);
 extern bool raw_spin_rq_trylock(struct rq *rq);
 
-static inline void raw_spin_rq_lock(struct rq *rq)
+static __always_inline void raw_spin_rq_lock(struct rq *rq)
 {
 	raw_spin_rq_lock_nested(rq, 0);
 }
 
-static inline void raw_spin_rq_unlock(struct rq *rq)
+static __always_inline void raw_spin_rq_unlock(struct rq *rq)
 {
 	raw_spin_unlock(rq_lockp(rq));
 }
 
-static inline void raw_spin_rq_lock_irq(struct rq *rq)
+static __always_inline void raw_spin_rq_lock_irq(struct rq *rq)
 {
 	local_irq_disable();
 	raw_spin_rq_lock(rq);
 }
 
-static inline void raw_spin_rq_unlock_irq(struct rq *rq)
+static __always_inline void raw_spin_rq_unlock_irq(struct rq *rq)
 {
 	raw_spin_rq_unlock(rq);
 	local_irq_enable();
-- 
2.51.0