Re: [PATCH] RISC-V: Add _TIF_NEED_RESCHED check for kernel thread when CONFIG_PREEMPT=y

Guenter Roeck
2019-01-03T05:46:00Z

On Thu, Jan 03, 2019 at 11:32:33AM +0800, Vincent Chen wrote:
> 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>

This patch seems to do the trick. I no longer see a problem with
CONFIG_PREEMPT=y and the various lock torture tests enabled, as
previously reported.

Nice catch and fix.

Tested-by: Guenter Roeck <linux@roeck-us.net>

Guenter

> ---
> 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