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Date:   Wed, 5 Apr 2023 22:22:40 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ankur Arora <ankur.a.arora@oracle.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        torvalds@linux-foundation.org, akpm@linux-foundation.org,
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        tglx@linutronix.de, vincent.guittot@linaro.org, jon.grimm@amd.com,
        bharata@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com
Subject: Re: [PATCH 8/9] irqentry: define irqentry_exit_allow_resched()
Message-ID: <20230405202240.GE365912@hirez.programming.kicks-ass.net>
References: <20230403052233.1880567-1-ankur.a.arora@oracle.com>
 <20230403052233.1880567-9-ankur.a.arora@oracle.com>
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In-Reply-To: <20230403052233.1880567-9-ankur.a.arora@oracle.com>
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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@oracle.com>
> ---
>  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?

> +
>  #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
	if (!A)
		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.

What you really care about is the CONFIG_PREEMPTION=n case, but that you
don't actually handle.

>  		/* 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.