ARM: entry: efficiency cleanups
Make the "fast" syscall return path fast again. The addition of IRQ
tracing and context tracking has made this path grossly inefficient.
We can do much better if these options are enabled if we save the
syscall return code on the stack - we then don't need to save a bunch
of registers around every single callout to C code.
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 92828a1..dd3721d 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -24,35 +24,55 @@
.align 5
+#if !(IS_ENABLED(CONFIG_TRACE_IRQFLAGS) || IS_ENABLED(CONFIG_CONTEXT_TRACKING))
/*
- * This is the fast syscall return path. We do as little as
- * possible here, and this includes saving r0 back into the SVC
- * stack.
+ * This is the fast syscall return path. We do as little as possible here,
+ * such as avoiding writing r0 to the stack. We only use this path if we
+ * have tracing and context tracking disabled - the overheads from those
+ * features make this path too inefficient.
*/
ret_fast_syscall:
UNWIND(.fnstart )
UNWIND(.cantunwind )
- disable_irq @ disable interrupts
+ disable_irq_notrace @ disable interrupts
ldr r1, [tsk, #TI_FLAGS] @ re-check for syscall tracing
- tst r1, #_TIF_SYSCALL_WORK
- bne __sys_trace_return
- tst r1, #_TIF_WORK_MASK
+ tst r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
bne fast_work_pending
- asm_trace_hardirqs_on
/* perform architecture specific actions before user return */
arch_ret_to_user r1, lr
- ct_user_enter
restore_user_regs fast = 1, offset = S_OFF
UNWIND(.fnend )
+ENDPROC(ret_fast_syscall)
-/*
- * Ok, we need to do extra processing, enter the slow path.
- */
+ /* Ok, we need to do extra processing, enter the slow path. */
fast_work_pending:
str r0, [sp, #S_R0+S_OFF]! @ returned r0
-work_pending:
+ /* fall through to work_pending */
+#else
+/*
+ * The "replacement" ret_fast_syscall for when tracing or context tracking
+ * is enabled. As we will need to call out to some C functions, we save
+ * r0 first to avoid needing to save registers around each C function call.
+ */
+ret_fast_syscall:
+ UNWIND(.fnstart )
+ UNWIND(.cantunwind )
+ str r0, [sp, #S_R0 + S_OFF]! @ save returned r0
+ disable_irq_notrace @ disable interrupts
+ ldr r1, [tsk, #TI_FLAGS] @ re-check for syscall tracing
+ tst r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK
+ beq no_work_pending
+ UNWIND(.fnend )
+ENDPROC(ret_fast_syscall)
+
+ /* Slower path - fall through to work_pending */
+#endif
+
+ tst r1, #_TIF_SYSCALL_WORK
+ bne __sys_trace_return_nosave
+slow_work_pending:
mov r0, sp @ 'regs'
mov r2, why @ 'syscall'
bl do_work_pending
@@ -64,16 +84,19 @@
/*
* "slow" syscall return path. "why" tells us if this was a real syscall.
+ * IRQs may be enabled here, so always disable them. Note that we use the
+ * "notrace" version to avoid calling into the tracing code unnecessarily.
+ * do_work_pending() will update this state if necessary.
*/
ENTRY(ret_to_user)
ret_slow_syscall:
- disable_irq @ disable interrupts
+ disable_irq_notrace @ disable interrupts
ENTRY(ret_to_user_from_irq)
ldr r1, [tsk, #TI_FLAGS]
tst r1, #_TIF_WORK_MASK
- bne work_pending
+ bne slow_work_pending
no_work_pending:
- asm_trace_hardirqs_on
+ asm_trace_hardirqs_on save = 0
/* perform architecture specific actions before user return */
arch_ret_to_user r1, lr
@@ -251,6 +274,12 @@
bl syscall_trace_exit
b ret_slow_syscall
+__sys_trace_return_nosave:
+ asm_trace_hardirqs_off save=0
+ mov r0, sp
+ bl syscall_trace_exit
+ b ret_slow_syscall
+
.align 5
#ifdef CONFIG_ALIGNMENT_TRAP
.type __cr_alignment, #object
diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index 423663e..b6cda06 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -562,6 +562,12 @@
asmlinkage int
do_work_pending(struct pt_regs *regs, unsigned int thread_flags, int syscall)
{
+ /*
+ * The assembly code enters us with IRQs off, but it hasn't
+ * informed the tracing code of that for efficiency reasons.
+ * Update the trace code with the current status.
+ */
+ trace_hardirqs_off();
do {
if (likely(thread_flags & _TIF_NEED_RESCHED)) {
schedule();