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Subject[PATCH] RISC-V: Add _TIF_NEED_RESCHED check for kernel thread when CONFIG_PREEMPT=y
Date
The cond_resched() can be used to yield the CPU resource if
CONFIG_PREEMPT is not defined. Otherwise, cond_resched() is a dummy
function. In order to avoid kernel thread occupying entire CPU,
when CONFIG_PREEMPT=y, the kernel thread needs to follow the
rescheduling mechanism like a user thread.

Signed-off-by: Vincent Chen <vincentc@andestech.com>
---
arch/riscv/kernel/asm-offsets.c | 1 +
arch/riscv/kernel/entry.S | 18 +++++++++++++++++-
2 files changed, 18 insertions(+), 1 deletions(-)

diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index 6a92a2f..dac9834 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -39,6 +39,7 @@ void asm_offsets(void)
OFFSET(TASK_STACK, task_struct, stack);
OFFSET(TASK_TI, task_struct, thread_info);
OFFSET(TASK_TI_FLAGS, task_struct, thread_info.flags);
+ OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
OFFSET(TASK_TI_USER_SP, task_struct, thread_info.user_sp);
OFFSET(TASK_TI_CPU, task_struct, thread_info.cpu);
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 13d4826..728b72d 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -144,6 +144,10 @@ _save_context:
REG_L x2, PT_SP(sp)
.endm

+#if !IS_ENABLED(CONFIG_PREEMPT)
+#define resume_kernel restore_all
+#endif
+
ENTRY(handle_exception)
SAVE_ALL

@@ -228,7 +232,7 @@ ret_from_exception:
REG_L s0, PT_SSTATUS(sp)
csrc sstatus, SR_SIE
andi s0, s0, SR_SPP
- bnez s0, restore_all
+ bnez s0, resume_kernel

resume_userspace:
/* Interrupts must be disabled here so flags are checked atomically */
@@ -250,6 +254,18 @@ restore_all:
RESTORE_ALL
sret

+#if IS_ENABLED(CONFIG_PREEMPT)
+resume_kernel:
+ REG_L s0, TASK_TI_PREEMPT_COUNT(tp)
+ bnez s0, restore_all
+need_resched:
+ REG_L s0, TASK_TI_FLAGS(tp)
+ andi s0, s0, _TIF_NEED_RESCHED
+ beqz s0, restore_all
+ call preempt_schedule_irq
+ j need_resched
+#endif
+
work_pending:
/* Enter slow path for supplementary processing */
la ra, ret_from_exception
--
1.7.1
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