[PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in another pointer

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	npiggin@gmail.com, msuchanek@suse.de
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in another pointer
Date: Mon,  8 Feb 2021 15:10:40 +0000 (UTC)	[thread overview]
Message-ID: <24804747098369ebcdac38970b8f7a1260bdd248.1612796617.git.christophe.leroy@csgroup.eu> (raw)
In-Reply-To: <cover.1612796617.git.christophe.leroy@csgroup.eu>

By saving the pointer pointing to thread_info.flags, gcc copies r2
in a non-volatile register.

We know 'current' doesn't change, so avoid that intermediaite pointer.

Reduces null_syscall benchmark by 2 cycles (322 => 320 cycles)

On PPC64, gcc seems to know that 'current' is not changing, and it keeps
it in a non volatile register to avoid multiple read of 'current' in paca.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
v5: Also in interrupt exit prepare
---
 arch/powerpc/kernel/interrupt.c | 28 +++++++++++-----------------
 1 file changed, 11 insertions(+), 17 deletions(-)

diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index 8c38e8c95be2..c89a8eac3e24 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -223,7 +223,6 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
 					   struct pt_regs *regs,
 					   long scv)
 {
-	unsigned long *ti_flagsp = &current_thread_info()->flags;
 	unsigned long ti_flags;
 	unsigned long ret = 0;
 
@@ -241,7 +240,7 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
 	/* Check whether the syscall is issued inside a restartable sequence */
 	rseq_syscall(regs);
 
-	ti_flags = *ti_flagsp;
+	ti_flags = current_thread_info()->flags;
 
 	if (unlikely(r3 >= (unsigned long)-MAX_ERRNO) && !scv) {
 		if (likely(!(ti_flags & (_TIF_NOERROR | _TIF_RESTOREALL)))) {
@@ -255,7 +254,7 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
 			ret = _TIF_RESTOREALL;
 		else
 			regs->gpr[3] = r3;
-		clear_bits(_TIF_PERSYSCALL_MASK, ti_flagsp);
+		clear_bits(_TIF_PERSYSCALL_MASK, &current_thread_info()->flags);
 	} else {
 		regs->gpr[3] = r3;
 	}
@@ -268,7 +267,7 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
 	local_irq_disable();
 
 again:
-	ti_flags = READ_ONCE(*ti_flagsp);
+	ti_flags = READ_ONCE(current_thread_info()->flags);
 	while (unlikely(ti_flags & (_TIF_USER_WORK_MASK & ~_TIF_RESTORE_TM))) {
 		local_irq_enable();
 		if (ti_flags & _TIF_NEED_RESCHED) {
@@ -284,7 +283,7 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
 			do_notify_resume(regs, ti_flags);
 		}
 		local_irq_disable();
-		ti_flags = READ_ONCE(*ti_flagsp);
+		ti_flags = READ_ONCE(current_thread_info()->flags);
 	}
 
 	if (IS_ENABLED(CONFIG_PPC_BOOK3S) && IS_ENABLED(CONFIG_PPC_FPU)) {
@@ -339,10 +338,6 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
 #ifndef CONFIG_PPC_BOOK3E_64 /* BOOK3E not yet using this */
 notrace unsigned long interrupt_exit_user_prepare(struct pt_regs *regs, unsigned long msr)
 {
-#ifdef CONFIG_PPC_BOOK3E
-	struct thread_struct *ts = &current->thread;
-#endif
-	unsigned long *ti_flagsp = &current_thread_info()->flags;
 	unsigned long ti_flags;
 	unsigned long flags;
 	unsigned long ret = 0;
@@ -365,7 +360,7 @@ notrace unsigned long interrupt_exit_user_prepare(struct pt_regs *regs, unsigned
 	local_irq_save(flags);
 
 again:
-	ti_flags = READ_ONCE(*ti_flagsp);
+	ti_flags = READ_ONCE(current_thread_info()->flags);
 	while (unlikely(ti_flags & (_TIF_USER_WORK_MASK & ~_TIF_RESTORE_TM))) {
 		local_irq_enable(); /* returning to user: may enable */
 		if (ti_flags & _TIF_NEED_RESCHED) {
@@ -376,7 +371,7 @@ notrace unsigned long interrupt_exit_user_prepare(struct pt_regs *regs, unsigned
 			do_notify_resume(regs, ti_flags);
 		}
 		local_irq_disable();
-		ti_flags = READ_ONCE(*ti_flagsp);
+		ti_flags = READ_ONCE(current_thread_info()->flags);
 	}
 
 	if (IS_ENABLED(CONFIG_PPC_BOOK3S) && IS_ENABLED(CONFIG_PPC_FPU)) {
@@ -407,13 +402,13 @@ notrace unsigned long interrupt_exit_user_prepare(struct pt_regs *regs, unsigned
 	}
 
 #ifdef CONFIG_PPC_BOOK3E
-	if (unlikely(ts->debug.dbcr0 & DBCR0_IDM)) {
+	if (unlikely(current->thread.debug.dbcr0 & DBCR0_IDM)) {
 		/*
 		 * Check to see if the dbcr0 register is set up to debug.
 		 * Use the internal debug mode bit to do this.
 		 */
 		mtmsr(mfmsr() & ~MSR_DE);
-		mtspr(SPRN_DBCR0, ts->debug.dbcr0);
+		mtspr(SPRN_DBCR0, current->thread.debug.dbcr0);
 		mtspr(SPRN_DBSR, -1);
 	}
 #endif
@@ -438,7 +433,6 @@ void preempt_schedule_irq(void);
 
 notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs, unsigned long msr)
 {
-	unsigned long *ti_flagsp = &current_thread_info()->flags;
 	unsigned long flags;
 	unsigned long ret = 0;
 #ifdef CONFIG_PPC64
@@ -461,8 +455,8 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs, unsign
 	amr = kuap_get_and_check_amr();
 #endif
 
-	if (unlikely(*ti_flagsp & _TIF_EMULATE_STACK_STORE)) {
-		clear_bits(_TIF_EMULATE_STACK_STORE, ti_flagsp);
+	if (unlikely(current_thread_info()->flags & _TIF_EMULATE_STACK_STORE)) {
+		clear_bits(_TIF_EMULATE_STACK_STORE, &current_thread_info()->flags);
 		ret = 1;
 	}
 
@@ -474,7 +468,7 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs, unsign
 again:
 		if (IS_ENABLED(CONFIG_PREEMPT)) {
 			/* Return to preemptible kernel context */
-			if (unlikely(*ti_flagsp & _TIF_NEED_RESCHED)) {
+			if (unlikely(current_thread_info()->flags & _TIF_NEED_RESCHED)) {
 				if (preempt_count() == 0)
 					preempt_schedule_irq();
 			}
-- 
2.25.0

