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>

diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index 6a92a2f..dac9834 100644 (file)
--- 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 355166f..fd9b57c 100644 (file)
--- 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)
+.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