RISC-V: Add _TIF_NEED_RESCHED check for kernel thread when CONFIG_PREEMPT=y

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>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

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);

arch/riscv/kernel/entry.S

@@ -144,6 +144,10 @@ _save_context:
 	REG_L x2,  PT_SP(sp)
 	.endm
 
+#if !IS_ENABLED(CONFIG_PREEMPT)
+.set 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