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

Vincent Chen
2019-01-24T00:31:00Z

On Wed, Jan 23, 2019 at 02:58:51AM +0800, Palmer Dabbelt wrote:
> On Wed, 02 Jan 2019 21:45:55 PST (-0800), linux@roeck-us.net wrote:
> > 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
> >> +
>
> I don't like preprocessor stuff if we can avoid it, are you OK if I squash in
> the following diff:
>
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index cfbad2f689c3..fd9b57c8b4ce 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -145,7 +145,7 @@ _save_context:
> .endm
>
> #if !IS_ENABLED(CONFIG_PREEMPT)
> -#define resume_kernel restore_all
> +.set resume_kernel, restore_all
> #endif
>
> ENTRY(handle_exception)
>
> I think that should do the same thing, but at link time instead of in the
> preprocessor -- that makes it a bit less likely to bit us in the future.
>
> >> 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
>
> I'm just going to assume you're OK with the squash and drop this into my plans
> for the next RC, let me know if that's not OK.
>
> Thanks for fixing this!

OK, it's fine for me.

Vincent