linux-kernel - Re: sched: Provide migrate

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Date:   Sun, 9 Feb 2020 20:08:27 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: sched: Provide migrate_disable/enable() inlines


* Thomas Gleixner <tglx@...utronix.de> wrote:

> Code which solely needs to prevent migration of a task uses
> preempt_disable()/enable() pairs. This is the only reliable way to do so
> as setting the task affinity to a single CPU can be undone by a
> setaffinity operation from a different task/process.
> 
> RT provides a seperate migrate_disable/enable() mechanism which does not
> disable preemption to achieve the semantic requirements of a (almost) fully
> preemptible kernel.
> 
> As it is unclear from looking at a given code path whether the intention is
> to disable preemption or migration, introduce migrate_disable/enable()
> inline functions which can be used to annotate code which merely needs to
> disable migration. Map them to preempt_disable/enable() for now. The RT
> substitution will be provided later.
> 
> Code which is annotated that way documents that it has no requirement to
> protect against reentrancy of a preempting task. Either this is not
> required at all or the call sites are already serialized by other means.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Mel Gorman <mgorman@...e.de>
> ---
>  include/linux/preempt.h |   30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -322,4 +322,34 @@ static inline void preempt_notifier_init
>  
>  #endif
>  
> +/**
> + * migrate_disable - Prevent migration of the current task
> + *
> + * Maps to preempt_disable() which also disables preemption. Use
> + * migrate_disable() to annotate that the intent is to prevent migration
> + * but not necessarily preemption.
> + *
> + * Can be invoked nested like preempt_disable() and needs the corresponding
> + * number of migrate_enable() invocations.
> + */
> +static __always_inline void migrate_disable(void)
> +{
> +	preempt_disable();
> +}
> +
> +/**
> + * migrate_enable - Allow migration of the current task
> + *
> + * Counterpart to migrate_disable().
> + *
> + * As migrate_disable() can be invoked nested only the uttermost invocation
> + * reenables migration.
> + *
> + * Currently mapped to preempt_enable().
> + */
> +static __always_inline void migrate_enable(void)
> +{
> +	preempt_enable();
> +}

I've applied this with s/uttermost/outermost, which I suspect was the 
intention?

Thanks,

	Ingo