Re: [PATCH 8/9] irqentry: define irqentry_exit_allow_resched()

[Ankur Arora <ankur.a.arora@oracle.com>]

Peter Zijlstra <peterz@infradead.org> writes:

> On Thu, Apr 06, 2023 at 09:56:07AM -0700, Ankur Arora wrote:
>
>>
>> Right, so:
>>
>> CONFIG_PREEMPTION: raw_foo()
>> CONFIG_PREEMPTION && (preempt_dynamic_mode == preempt_dynamic_full): raw_foo()
>>
>> This is NOP if CONFIG_PREEMPTION && preempt_dynamic_mode != preempt_dynamic_full.
>>
>> > 	if (!A)
>> > 		raw_foo();
>>
>> So I would call raw_foo() for the CONFIG_PREEMPTION=n case.
>>
>> > What you really care about is the CONFIG_PREEMPTION=n case, but that you
>> > don't actually handle.
>>
>> I don't see that. The CONFIG_PREEMPTION=n (or its dynamic version)
>> is being handled here.
>
> Bah, I overlooked we have multiple definitions of the
> preempt_model_foo() things. For some reason I thought all that was only
> for DYNAMIC_PREEMPT.
>
>> > And yeah, you've got the extra resched_allowed() thing in there, but
>> > that doesn't make it any better -- this is all quite horrible.
>>
>> I don't disagree. There's a quite a lot of static/dynamic config options
>> here and adding this clause didn't improve things.
>>
>> I think just going with a static call here for the allow-resched case
>> might have been cleaner. Alternately I'll see if it can be cleanly
>> folded in with the irqentry_exit_cond_resched() logic.
>
> Something like the below perhaps?
>
> ---
>  include/linux/entry-common.h |  6 ++++++
>  kernel/entry/common.c        | 23 +++++++++++++++++++++--
>  2 files changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
> index d95ab85f96ba..0c365dc1f1c2 100644
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -415,10 +415,16 @@ irqentry_state_t noinstr irqentry_enter(struct pt_regs *regs);
>   * Conditional reschedule with additional sanity checks.
>   */
>  void raw_irqentry_exit_cond_resched(void);
> +void irqentry_exit_cond_resched_tif(void);
> +
>  #ifdef CONFIG_PREEMPT_DYNAMIC
>  #if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
>  #define irqentry_exit_cond_resched_dynamic_enabled	raw_irqentry_exit_cond_resched
> +#ifdef TIF_RESCHED_ALLOW
> +#define irqentry_exit_cond_resched_dynamic_disabled	irqentry_exit_cond_resched_tif
> +#else
>  #define irqentry_exit_cond_resched_dynamic_disabled	NULL
> +#endif

So this is clever. Essentially this would toggle between the two kinds
for the preempt_model_preemptible()/!preempt_model_preemptible() dynamic
case. Do I have that right?

>  DECLARE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
>  #define irqentry_exit_cond_resched()	static_call(irqentry_exit_cond_resched)()
>  #elif defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index be61332c66b5..211d118aa672 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -390,6 +390,21 @@ void raw_irqentry_exit_cond_resched(void)
>  			preempt_schedule_irq();
>  	}
>  }
> +
> +void irqentry_exit_cond_resched_tif(void)
> +{
> +#ifdef TIF_RESCHED_ALLOW
> +	if (resched_allowed()) {
> +		/* Sanity check RCU and thread stack */
> +		rcu_irq_exit_check_preempt();
> +		if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
> +			WARN_ON_ONCE(!on_thread_stack());
> +		if (need_resched())
> +			preempt_schedule_irq();
> +	}
> +#endif
> +}
> +
>  #ifdef CONFIG_PREEMPT_DYNAMIC
>  #if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
>  DEFINE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
> @@ -397,8 +412,10 @@ DEFINE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
>  DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
>  void dynamic_irqentry_exit_cond_resched(void)
>  {
> -	if (!static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched))
> -		return;
> +	if (!static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched)) {
> +		irqentry_exit_cond_resched_tif();
> +		return
> +	}
>  	raw_irqentry_exit_cond_resched();
>  }
>  #endif
> @@ -431,6 +448,8 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
>  		instrumentation_begin();
>  		if (IS_ENABLED(CONFIG_PREEMPTION))
>  			irqentry_exit_cond_resched();
> +		else
> +			irqentry_exit_cond_resched_tif();

And, if we choose between the two resched modes at compile time then this
would work.

Might massage the names a little but this should work as is. Okay if I
use your Codeveloped-by/Suggested-by on this patch?

--
ankur