Currently, kernel context and interrupts are handled using a single
kernel stack navigated by sp_el1. This forces a system to use 16KB
stack, not 8KB one. This restriction makes low memory platforms suffer
from memory pressure accompanied by performance degradation.
This patch addresses the issue as introducing a separate percpu IRQ
stack to handle both hard and soft interrupts with two ground rules:
- Utilize sp_el0 in EL1 context, which is not used currently
- Do not complicate current_thread_info calculation
It is a core concept to directly retrieve struct thread_info from
sp_el0. This approach helps to prevent text section size from being
increased largely as removing masking operation using THREAD_SIZE
in tons of places.
[Thanks to James Morse for his valuable feedbacks which greatly help
to figure out a better implementation. - Jungseok]
Tested-by: James Morse <james.mo...@arm.com>
Signed-off-by: Jungseok Lee <jungseokle...@gmail.com>
---
Changes since v2:
- Optmised current_thread_info function as removing masking operation
and volatile keyword per James and Catalin
- Reworked irq re-enterance check logic using top-bit comparison of
stacks per James
- Added sp_el0 update in cpu_resume per James
- Selected HAVE_IRQ_EXIT_ON_IRQ_STACK to expose this feature explicitly
- Added a Tested-by tag from James
- Added comments on sp_el0 as a helper messeage
Changes since v1:
- Rebased on top of v4.3-rc1
- Removed Kconfig about IRQ stack, per James
- Used PERCPU for IRQ stack, per James
- Tried to allocate IRQ stack when CPU is about to start up, per James
- Moved sp_el0 update into kernel_entry macro, per James
- Dropped S_SP removal patch, per Mark and James
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/irq.h | 2 +
arch/arm64/include/asm/thread_info.h | 10 ++++-
arch/arm64/kernel/entry.S | 35 +++++++++++++---
arch/arm64/kernel/head.S | 5 +++
arch/arm64/kernel/irq.c | 21 ++++++++++
arch/arm64/kernel/sleep.S | 2 +
arch/arm64/kernel/smp.c | 6 +++
8 files changed, 75 insertions(+), 7 deletions(-)
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 7d95663..829de2b 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -67,6 +67,7 @@ config ARM64
select HAVE_FUNCTION_GRAPH_TRACER
select HAVE_GENERIC_DMA_COHERENT
select HAVE_HW_BREAKPOINT if PERF_EVENTS
+ select HAVE_IRQ_EXIT_ON_IRQ_STACK
select HAVE_MEMBLOCK
select HAVE_PATA_PLATFORM
select HAVE_PERF_EVENTS
diff --git a/arch/arm64/include/asm/irq.h b/arch/arm64/include/asm/irq.h
index bbb251b..ba12725 100644
--- a/arch/arm64/include/asm/irq.h
+++ b/arch/arm64/include/asm/irq.h
@@ -10,6 +10,8 @@ struct pt_regs;
extern void migrate_irqs(void);
extern void set_handle_irq(void (*handle_irq)(struct pt_regs *));
+extern int alloc_irq_stack(unsigned int cpu);
+
static inline void acpi_irq_init(void)
{
/*
diff --git a/arch/arm64/include/asm/thread_info.h
b/arch/arm64/include/asm/thread_info.h
index dcd06d1..fa014df 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -71,10 +71,16 @@ register unsigned long current_stack_pointer asm ("sp");
*/
static inline struct thread_info *current_thread_info(void)
__attribute_const__;
+/*
+ * struct thread_info can be accessed directly via sp_el0.
+ */
static inline struct thread_info *current_thread_info(void)
{
- return (struct thread_info *)
- (current_stack_pointer & ~(THREAD_SIZE - 1));
+ unsigned long sp_el0;
+
+ asm ("mrs %0, sp_el0" : "=r" (sp_el0));
+
+ return (struct thread_info *)sp_el0;
}
#define thread_saved_pc(tsk) \
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 4306c93..7b4943b 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -88,7 +88,8 @@
.if \el == 0
mrs x21, sp_el0
- get_thread_info tsk // Ensure MDSCR_EL1.SS is clear,
+ mov tsk, sp
+ and tsk, tsk, #~(THREAD_SIZE - 1) // Ensure MDSCR_EL1.SS is clear,
ldr x19, [tsk, #TI_FLAGS] // since we can unmask debug
disable_step_tsk x19, x20 // exceptions when scheduling.
.else
@@ -105,6 +106,7 @@
.if \el == 0
mvn x21, xzr
str x21, [sp, #S_SYSCALLNO]
+ msr sp_el0, tsk
.endif
/*
@@ -164,8 +166,28 @@ alternative_endif
.endm
.macro get_thread_info, rd
- mov \rd, sp
- and \rd, \rd, #~(THREAD_SIZE - 1) // top of stack
+ mrs \rd, sp_el0
+ .endm
+
+ .macro irq_stack_entry
+ adr_l x21, irq_stack
+ mrs x22, tpidr_el1
+ add x21, x21, x22
+ ldr x22, [x21]
+ and x21, x22, #~(THREAD_SIZE - 1)
+ mov x23, sp
+ and x23, x23, #~(THREAD_SIZE - 1)
+ cmp x21, x23 // check irq re-enterance
+ mov x21, sp // x21 = task stack
+ csel x23, x21, x22, eq // x22 = irq stack
+ mov x23, sp
+ .endm
+
+ /*
+ * x21 is preserved between irq_stack_entry and irq_stack_exit
+ */
+ .macro irq_stack_exit
+ mov sp, x21 // x21 = task stack
.endm
/*
@@ -183,10 +205,11 @@ tsk .req x28 // current thread_info
* Interrupt handling.
*/
.macro irq_handler
- adrp x1, handle_arch_irq
- ldr x1, [x1, #:lo12:handle_arch_irq]
+ ldr_l x1, handle_arch_irq
mov x0, sp
+ irq_stack_entry
blr x1
+ irq_stack_exit
.endm
.text
@@ -597,6 +620,8 @@ ENTRY(cpu_switch_to)
ldp x29, x9, [x8], #16
ldr lr, [x8]
mov sp, x9
+ and x9, x9, #~(THREAD_SIZE - 1)
+ msr sp_el0, x9
ret
ENDPROC(cpu_switch_to)
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index a055be6..265253d 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -441,6 +441,9 @@ __mmap_switched:
b 1b
2:
adr_l sp, initial_sp, x4
+ mov x4, sp
+ and x4, x4, #~(THREAD_SIZE - 1)
+ msr sp_el0, x4 // Save thread_info
str_l x21, __fdt_pointer, x5 // Save FDT pointer
str_l x24, memstart_addr, x6 // Save PHYS_OFFSET
mov x29, #0
@@ -613,6 +616,8 @@ ENDPROC(secondary_startup)
ENTRY(__secondary_switched)
ldr x0, [x21] // get secondary_data.stack
mov sp, x0
+ and x0, x0, #~(THREAD_SIZE - 1)
+ msr sp_el0, x0 // save thread_info
mov x29, #0
b secondary_start_kernel
ENDPROC(__secondary_switched)
diff --git a/arch/arm64/kernel/irq.c b/arch/arm64/kernel/irq.c
index 11dc3fd..5272bfd 100644
--- a/arch/arm64/kernel/irq.c
+++ b/arch/arm64/kernel/irq.c
@@ -31,6 +31,8 @@
unsigned long irq_err_count;
+DEFINE_PER_CPU(void *, irq_stack);
+
int arch_show_interrupts(struct seq_file *p, int prec)
{
show_ipi_list(p, prec);
@@ -50,6 +52,9 @@ void __init set_handle_irq(void (*handle_irq)(struct pt_regs
*))
void __init init_IRQ(void)
{
+ if (alloc_irq_stack(smp_processor_id()))
+ panic("Failed to allocate IRQ stack for boot cpu");
+
irqchip_init();
if (!handle_arch_irq)
panic("No interrupt controller found.");
@@ -115,3 +120,19 @@ void migrate_irqs(void)
local_irq_restore(flags);
}
#endif /* CONFIG_HOTPLUG_CPU */
+
+int alloc_irq_stack(unsigned int cpu)
+{
+ void *stack;
+
+ if (per_cpu(irq_stack, cpu))
+ return 0;
+
+ stack = (void *)__get_free_pages(THREADINFO_GFP, THREAD_SIZE_ORDER);
+ if (!stack)
+ return -ENOMEM;
+
+ per_cpu(irq_stack, cpu) = stack + THREAD_START_SP;
+
+ return 0;
+}
diff --git a/arch/arm64/kernel/sleep.S b/arch/arm64/kernel/sleep.S
index f586f7c..87d9453 100644
--- a/arch/arm64/kernel/sleep.S
+++ b/arch/arm64/kernel/sleep.S
@@ -173,6 +173,8 @@ ENTRY(cpu_resume)
/* load physical address of identity map page table in x1 */
adrp x1, idmap_pg_dir
mov sp, x2
+ and x2, x2, #~(THREAD_SIZE - 1)
+ msr sp_el0, x2
/*
* cpu_do_resume expects x0 to contain context physical address
* pointer and x1 to contain physical address of 1:1 page tables
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index dbdaacd..0bd7049 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -97,6 +97,12 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
secondary_data.stack = task_stack_page(idle) + THREAD_START_SP;
__flush_dcache_area(&secondary_data, sizeof(secondary_data));
+ ret = alloc_irq_stack(cpu);
+ if (ret) {
+ pr_crit("CPU%u: failed to allocate IRQ stack\n", cpu);
+ return ret;
+ }
+
/*
* Now bring the CPU into our world.
*/
--
2.5.0
--
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