Re: [PATCH v4 5/7] sched/preempt: add PREEMPT_DYNAMIC using static keys

From: Sven Schnelle <svens@linux.ibm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, ardb@kernel.org,
	bp@alien8.de, catalin.marinas@arm.com,
	dave.hansen@linux.intel.com, frederic@kernel.org,
	james.morse@arm.com, joey.gouly@arm.com, juri.lelli@redhat.com,
	linux-kernel@vger.kernel.org, luto@kernel.org, mingo@redhat.com,
	peterz@infradead.org, tglx@linutronix.de,
	valentin.schneider@arm.com, will@kernel.org
Subject: Re: [PATCH v4 5/7] sched/preempt: add PREEMPT_DYNAMIC using static keys
Date: Fri, 18 Mar 2022 09:49:25 +0100	[thread overview]
Message-ID: <yt9dk0crr6ga.fsf@linux.ibm.com> (raw)
In-Reply-To: <20220214165216.2231574-6-mark.rutland@arm.com> (Mark Rutland's message of "Mon, 14 Feb 2022 16:52:14 +0000")

Hi Mark,

Mark Rutland <mark.rutland@arm.com> writes:

> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index 1739ca79613b..b145249ad91a 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -3,6 +3,7 @@
>  #include <linux/context_tracking.h>
>  #include <linux/entry-common.h>
>  #include <linux/highmem.h>
> +#include <linux/jump_label.h>
>  #include <linux/livepatch.h>
>  #include <linux/audit.h>
>  #include <linux/tick.h>
> @@ -392,7 +393,17 @@ void raw_irqentry_exit_cond_resched(void)
>  	}
>  }
>  #ifdef CONFIG_PREEMPT_DYNAMIC
> +#if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
>  DEFINE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
> +#elif defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
> +DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
> +void dynamic_irqentry_exit_cond_resched(void)
> +{
> +	if (!static_key_unlikely(&sk_dynamic_irqentry_exit_cond_resched))
> +		return;

I think that should be static_branch_unlikely()?

Also can we make dynamic_irqentry_exit_cond_resched static so that the
compiler will inline it? I played a bit with your changes on s390 and
noticed the the compiler will generate an empty function. Declaring it
static inlines it and saves one unnecessary function call.

Thanks!
Sven