linux-kernel - [PATCH] riscv: Turn has_fpu into a static key if FPU

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Message-ID: <20210512020152.200a182d@xhacker>
Date:   Wed, 12 May 2021 02:01:52 +0800
From:   Jisheng Zhang <jszhang3@...l.ustc.edu.cn>
To:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH] riscv: Turn has_fpu into a static key if FPU

From: Jisheng Zhang <jszhang@...nel.org>

The has_fpu check sits at hot code path: switch_to(). Currently, has_fpu
is a bool variable if FPU, switch_to() checks it each time, we can
optimize out this check by turning the has_fpu into a static key.

Before the patch:
	...
	ccc7c783		lbu     a5,-820(a5) # ffffffff80cab098 <has_fpu>
	e3b5			bnez    a5,ffffffff8031a438 <__schedule+0x232>
	...

After the patch:
If FPU isn't supported:
	...
	00000013		nop
	...

If FPU is supported:
	...
	0540006f                j       ffffffff8031a418 <__schedule+0x21a>
	...

Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
---
 arch/riscv/include/asm/switch_to.h | 11 ++++++++---
 arch/riscv/kernel/cpufeature.c     |  4 ++--
 arch/riscv/kernel/process.c        |  2 +-
 arch/riscv/kernel/signal.c         |  4 ++--
 4 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
index 407bcc96a710..ea25ccb7785e 100644
--- a/arch/riscv/include/asm/switch_to.h
+++ b/arch/riscv/include/asm/switch_to.h
@@ -6,6 +6,7 @@
 #ifndef _ASM_RISCV_SWITCH_TO_H
 #define _ASM_RISCV_SWITCH_TO_H
 
+#include <linux/jump_label.h>
 #include <linux/sched/task_stack.h>
 #include <asm/processor.h>
 #include <asm/ptrace.h>
@@ -55,9 +56,13 @@ static inline void __switch_to_aux(struct task_struct *prev,
 	fstate_restore(next, task_pt_regs(next));
 }
 
-extern bool has_fpu;
+extern struct static_key_false cpu_hwcap_fpu;
+static __always_inline bool has_fpu(void)
+{
+	return static_branch_unlikely(&cpu_hwcap_fpu);
+}
 #else
-#define has_fpu false
+static __always_inline bool has_fpu(void) { return false; }
 #define fstate_save(task, regs) do { } while (0)
 #define fstate_restore(task, regs) do { } while (0)
 #define __switch_to_aux(__prev, __next) do { } while (0)
@@ -70,7 +75,7 @@ extern struct task_struct *__switch_to(struct task_struct *,
 do {							\
 	struct task_struct *__prev = (prev);		\
 	struct task_struct *__next = (next);		\
-	if (has_fpu)					\
+	if (has_fpu())					\
 		__switch_to_aux(__prev, __next);	\
 	((last) = __switch_to(__prev, __next));		\
 } while (0)
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index ac202f44a670..a2848dc36927 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -19,7 +19,7 @@ unsigned long elf_hwcap __read_mostly;
 static DECLARE_BITMAP(riscv_isa, RISCV_ISA_EXT_MAX) __read_mostly;
 
 #ifdef CONFIG_FPU
-bool has_fpu __read_mostly;
+__ro_after_init DEFINE_STATIC_KEY_FALSE(cpu_hwcap_fpu);
 #endif
 
 /**
@@ -146,6 +146,6 @@ void riscv_fill_hwcap(void)
 
 #ifdef CONFIG_FPU
 	if (elf_hwcap & (COMPAT_HWCAP_ISA_F | COMPAT_HWCAP_ISA_D))
-		has_fpu = true;
+		static_branch_enable(&cpu_hwcap_fpu);
 #endif
 }
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index f9cd57c9c67d..03ac3aa611f5 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -87,7 +87,7 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
 	unsigned long sp)
 {
 	regs->status = SR_PIE;
-	if (has_fpu) {
+	if (has_fpu()) {
 		regs->status |= SR_FS_INITIAL;
 		/*
 		 * Restore the initial value to the FP register
diff --git a/arch/riscv/kernel/signal.c b/arch/riscv/kernel/signal.c
index 65942b3748b4..c2d5ecbe5526 100644
--- a/arch/riscv/kernel/signal.c
+++ b/arch/riscv/kernel/signal.c
@@ -90,7 +90,7 @@ static long restore_sigcontext(struct pt_regs *regs,
 	/* sc_regs is structured the same as the start of pt_regs */
 	err = __copy_from_user(regs, &sc->sc_regs, sizeof(sc->sc_regs));
 	/* Restore the floating-point state. */
-	if (has_fpu)
+	if (has_fpu())
 		err |= restore_fp_state(regs, &sc->sc_fpregs);
 	return err;
 }
@@ -143,7 +143,7 @@ static long setup_sigcontext(struct rt_sigframe __user *frame,
 	/* sc_regs is structured the same as the start of pt_regs */
 	err = __copy_to_user(&sc->sc_regs, regs, sizeof(sc->sc_regs));
 	/* Save the floating-point state. */
-	if (has_fpu)
+	if (has_fpu())
 		err |= save_fp_state(regs, &sc->sc_fpregs);
 	return err;
 }
-- 
2.31.0