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Date:   Tue, 24 Nov 2020 17:13:35 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
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        Alexandre Chartre <alexandre.chartre@oracle.com>,
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        Tom Lendacky <thomas.lendacky@amd.com>,
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        "Paul E. McKenney" <paulmck@kernel.org>,
        Tim Chen <tim.c.chen@intel.com>
Subject: Re: [PATCH -tip 19/32] entry/idle: Enter and exit kernel protection
 during idle entry and exit
Message-ID: <20201124161335.GB3021@hirez.programming.kicks-ass.net>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
 <20201117232003.3580179-20-joel@joelfernandes.org>
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On Tue, Nov 17, 2020 at 06:19:49PM -0500, Joel Fernandes (Google) wrote:
> Add a generic_idle_{enter,exit} helper function to enter and exit kernel
> protection when entering and exiting idle, respectively.
> 
> While at it, remove a stale RCU comment.
> 
> Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
> Tested-by: Julien Desfossez <jdesfossez@digitalocean.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  include/linux/entry-common.h | 18 ++++++++++++++++++
>  kernel/sched/idle.c          | 11 ++++++-----
>  2 files changed, 24 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
> index 022e1f114157..8f34ae625f83 100644
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -454,4 +454,22 @@ static inline bool entry_kernel_protected(void)
>  	return IS_ENABLED(CONFIG_SCHED_CORE) && sched_core_kernel_protected()
>  		&& _TIF_UNSAFE_RET != 0;
>  }
> +
> +/**
> + * generic_idle_enter - General tasks to perform during idle entry.
> + */
> +static inline void generic_idle_enter(void)
> +{
> +	/* Entering idle ends the protected kernel region. */
> +	sched_core_unsafe_exit();
> +}
> +
> +/**
> + * generic_idle_exit  - General tasks to perform during idle exit.
> + */
> +static inline void generic_idle_exit(void)
> +{
> +	/* Exiting idle (re)starts the protected kernel region. */
> +	sched_core_unsafe_enter();
> +}
>  #endif

That naming is terrible..

> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 8bdb214eb78f..ee4f91396c31 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -8,6 +8,7 @@
>   */
>  #include "sched.h"
>  
> +#include <linux/entry-common.h>
>  #include <trace/events/power.h>
>  
>  /* Linker adds these: start and end of __cpuidle functions */
> @@ -54,6 +55,7 @@ __setup("hlt", cpu_idle_nopoll_setup);
>  
>  static noinline int __cpuidle cpu_idle_poll(void)
>  {
> +	generic_idle_enter();
>  	trace_cpu_idle(0, smp_processor_id());
>  	stop_critical_timings();
>  	rcu_idle_enter();
> @@ -66,6 +68,7 @@ static noinline int __cpuidle cpu_idle_poll(void)
>  	rcu_idle_exit();
>  	start_critical_timings();
>  	trace_cpu_idle(PWR_EVENT_EXIT, smp_processor_id());
> +	generic_idle_exit();
>  
>  	return 1;
>  }
> @@ -156,11 +159,7 @@ static void cpuidle_idle_call(void)
>  		return;
>  	}
>  
> -	/*
> -	 * The RCU framework needs to be told that we are entering an idle
> -	 * section, so no more rcu read side critical sections and one more
> -	 * step to the grace period
> -	 */
> +	generic_idle_enter();
>  
>  	if (cpuidle_not_available(drv, dev)) {
>  		tick_nohz_idle_stop_tick();
> @@ -225,6 +224,8 @@ static void cpuidle_idle_call(void)
>  	 */
>  	if (WARN_ON_ONCE(irqs_disabled()))
>  		local_irq_enable();
> +
> +	generic_idle_exit();
>  }

I'm confused.. arch_cpu_idle_{enter,exit}() weren't conveniently placed
for you?