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

[Ankur Arora]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> On Sun, Apr 02, 2023 at 10:22:32PM -0700, Ankur Arora wrote:
>> Allow threads marked TIF_ALLOW_RESCHED to be rescheduled in irqexit.
>>
>> This is only necessary under !preempt_model_preemptible() for which
>> we reuse the same logic as irqentry_exit_code_resched().
>>
>> Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
>> ---
>>  kernel/entry/common.c |  8 ++++++++
>>  kernel/sched/core.c   | 36 +++++++++++++++++++++---------------
>>  2 files changed, 29 insertions(+), 15 deletions(-)
>>
>> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
>> index be61332c66b5..f1005595ebe7 100644
>> --- a/kernel/entry/common.c
>> +++ b/kernel/entry/common.c
>> @@ -390,6 +390,9 @@ void raw_irqentry_exit_cond_resched(void)
>>  			preempt_schedule_irq();
>>  	}
>>  }
>> +
>> +void irqentry_exit_allow_resched(void) __alias(raw_irqentry_exit_cond_resched);
>
> Because typing raw_irqentry_exit_cond_resched() was too much effort?

Probably unnecessary but wanted to underscore that irqentry_exit_allow_resched()
handled a separate user path.

>> +
>>  #ifdef CONFIG_PREEMPT_DYNAMIC
>>  #if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
>>  DEFINE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
>> @@ -431,6 +434,11 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
>>  		instrumentation_begin();
>>  		if (IS_ENABLED(CONFIG_PREEMPTION))
>>  			irqentry_exit_cond_resched();
>> +		/*
>> +		 * We care about this clause only in the dynamic !preemptible case.
>> +		 */
>> +		if (unlikely(!preempt_model_preemptible() && resched_allowed()))
>> +			irqentry_exit_allow_resched();
>
> This is wrong, if we have dynamic preemption then we have
> CONFIG_PREEMPTION and we'll have that irqentry_exit_cond_resched() call.
>
> Basically what you've written above is something like:
>
> 	static_call(foo); // raw_foo() when A, NOP if !A

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.

Ignoring the RT case, preempt_model_preemptible() is either:
   IS_ENABLED(CONFIG_PREEMPT)
or:
   preempt_dynamic_mode == preempt_dynamic_full

So, I'm handling both the static and the dynamic case here.

if (!IS_ENABLED(CONFIG_PREEMPTION))
   raw_foo()
or
if (preempt_dynamic_mode != preempt_dynamic_full)
   raw_foo()

> 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.

>>  		/* Covers both tracing and lockdep */
>>  		trace_hardirqs_on();
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 0d18c3969f90..11845a91b691 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>
>> @@ -8597,28 +8599,32 @@ EXPORT_SYMBOL(__cond_resched_rwlock_write);
>>   * SC:preempt_schedule
>>   * SC:preempt_schedule_notrace
>>   * SC:irqentry_exit_cond_resched
>> + * SC:irqentry_exit_allow_resched
>>   *
>>   *
>>   * NONE:
>> - *   cond_resched               <- __cond_resched
>> - *   might_resched              <- RET0
>> - *   preempt_schedule           <- NOP
>> - *   preempt_schedule_notrace   <- NOP
>> - *   irqentry_exit_cond_resched <- NOP
>> + *   cond_resched                <- __cond_resched
>> + *   might_resched               <- RET0
>> + *   preempt_schedule            <- NOP
>> + *   preempt_schedule_notrace    <- NOP
>> + *   irqentry_exit_cond_resched  <- NOP
>> + *   irqentry_exit_allow_resched <- irqentry_exit_allow_resched
>>   *
>>   * VOLUNTARY:
>> - *   cond_resched               <- __cond_resched
>> - *   might_resched              <- __cond_resched
>> - *   preempt_schedule           <- NOP
>> - *   preempt_schedule_notrace   <- NOP
>> - *   irqentry_exit_cond_resched <- NOP
>> + *   cond_resched                <- __cond_resched
>> + *   might_resched               <- __cond_resched
>> + *   preempt_schedule            <- NOP
>> + *   preempt_schedule_notrace    <- NOP
>> + *   irqentry_exit_cond_resched  <- NOP
>> + *   irqentry_exit_allow_resched <- irqentry_exit_allow_resched
>>   *
>>   * FULL:
>> - *   cond_resched               <- RET0
>> - *   might_resched              <- RET0
>> - *   preempt_schedule           <- preempt_schedule
>> - *   preempt_schedule_notrace   <- preempt_schedule_notrace
>> - *   irqentry_exit_cond_resched <- irqentry_exit_cond_resched
>> + *   cond_resched                <- RET0
>> + *   might_resched               <- RET0
>> + *   preempt_schedule            <- preempt_schedule
>> + *   preempt_schedule_notrace    <- preempt_schedule_notrace
>> + *   irqentry_exit_cond_resched  <- irqentry_exit_cond_resched
>> + *   irqentry_exit_allow_resched <- NOP
>>   */
>
> This ^ is all complete nonsense.. irqentry_exit_allow_resched() is not
> a static call. It's an alias of raw_irqentry_exit_cond_resched which
> circumvents the static call entirely.

Yeah you are right. I added the SC:irqentry_exit_allow_resched
prematurely. Will fix.

Thanks for the detailed comments.

--
ankur