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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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
