linux-kernel - Re: Kernel broken on processors without performance counters

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Message-ID: <20150721082107.GE18673@twins.programming.kicks-ass.net>
Date:	Tue, 21 Jul 2015 10:21:07 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Jason Baron <jasonbaron0@...il.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Vince Weaver <vince@...ter.net>,
	"hillf.zj" <hillf.zj@...baba-inc.com>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Kernel broken on processors without performance counters

On Fri, Jul 10, 2015 at 04:13:59PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 08, 2015 at 05:36:43PM -0700, Andy Lutomirski wrote:
> > >> In what universe is "static_key_false" a reasonable name for a
> > >> function that returns true if a static key is true?
> 
> > I think the current naming is almost maximally bad.  The naming would
> > be less critical if it were typesafe, though.
> 
> How about something like so on top? It will allow us to slowly migrate
> existing and new users over to the hopefully saner interface?

Anybody? (Adding more Cc's)

> ---
>  include/linux/jump_label.h | 67 +++++++++++++++++++++++++++++++++++++++++++++-
>  kernel/sched/core.c        | 18 ++++++-------
>  2 files changed, 74 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
> index f4de473f226b..98ed3c2ec78d 100644
> --- a/include/linux/jump_label.h
> +++ b/include/linux/jump_label.h
> @@ -213,6 +213,71 @@ static inline bool static_key_enabled(struct static_key *key)
>  	return static_key_count(key) > 0;
>  }
>  
> -#endif	/* _LINUX_JUMP_LABEL_H */
> +static inline void static_key_enable(struct static_key *key)
> +{
> +	int count = static_key_count(key);
> +
> +	WARN_ON_ONCE(count < 0 || count > 1);
> +
> +	if (!count)
> +		static_key_slow_inc(key);
> +}
> +
> +static inline void static_key_disable(struct static_key *key)
> +{
> +	int count = static_key_count(key);
> +
> +	WARN_ON_ONCE(count < 0 || count > 1);
> +
> +	if (count)
> +		static_key_slow_dec(key);
> +}
> +
> +/* -------------------------------------------------------------------------- */
> +
> +/*
> + * likely -- default enabled, puts the branch body in-line
> + */
> +
> +struct static_likely_key {
> +	struct static_key key;
> +};
> +
> +#define STATIC_LIKELY_KEY_INIT	(struct static_likely_key){ .key = STATIC_KEY_INIT_TRUE, }
> +
> +static inline bool static_likely_branch(struct static_likely_key *key)
> +{
> +	return static_key_true(&key->key);
> +}
> +
> +/*
> + * unlikely -- default disabled, puts the branch body out-of-line
> + */
> +
> +struct static_unlikely_key {
> +	struct static_key key;
> +};
> +
> +#define STATIC_UNLIKELY_KEY_INIT (struct static_unlikely_key){ .key = STATIC_KEY_INIT_FALSE, }
> +
> +static inline bool static_unlikely_branch(struct static_unlikely_key *key)
> +{
> +	return static_key_false(&key->key);
> +}
> +
> +/*
> + * Advanced usage; refcount, branch is enabled when: count != 0
> + */
> +
> +#define static_branch_inc(_k)	static_key_slow_inc(&(_k)->key)
> +#define static_branch_dec(_k)	static_key_slow_dec(&(_k)->key)
> +
> +/*
> + * Normal usage; boolean enable/disable.
> + */
> +
> +#define static_branch_enable(_k)	static_key_enable(&(_k)->key)
> +#define static_branch_disable(_k)	static_key_disable(&(_k)->key)
>  
>  #endif /* __ASSEMBLY__ */
> +#endif	/* _LINUX_JUMP_LABEL_H */
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 78b4bad10081..22ba92297375 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -164,14 +164,12 @@ struct static_key sched_feat_keys[__SCHED_FEAT_NR] = {
>  
>  static void sched_feat_disable(int i)
>  {
> -	if (static_key_enabled(&sched_feat_keys[i]))
> -		static_key_slow_dec(&sched_feat_keys[i]);
> +	static_key_enable(&sched_feat_keys[i]);
>  }
>  
>  static void sched_feat_enable(int i)
>  {
> -	if (!static_key_enabled(&sched_feat_keys[i]))
> -		static_key_slow_inc(&sched_feat_keys[i]);
> +	static_key_disable(&sched_feat_keys[i]);
>  }
>  #else
>  static void sched_feat_disable(int i) { };
> @@ -2318,17 +2316,17 @@ void wake_up_new_task(struct task_struct *p)
>  
>  #ifdef CONFIG_PREEMPT_NOTIFIERS
>  
> -static struct static_key preempt_notifier_key = STATIC_KEY_INIT_FALSE;
> +static struct static_unlikely_key preempt_notifier_key = STATIC_UNLIKELY_KEY_INIT;
>  
>  void preempt_notifier_inc(void)
>  {
> -	static_key_slow_inc(&preempt_notifier_key);
> +	static_branch_inc(&preempt_notifier_key);
>  }
>  EXPORT_SYMBOL_GPL(preempt_notifier_inc);
>  
>  void preempt_notifier_dec(void)
>  {
> -	static_key_slow_dec(&preempt_notifier_key);
> +	static_branch_dec(&preempt_notifier_key);
>  }
>  EXPORT_SYMBOL_GPL(preempt_notifier_dec);
>  
> @@ -2338,7 +2336,7 @@ EXPORT_SYMBOL_GPL(preempt_notifier_dec);
>   */
>  void preempt_notifier_register(struct preempt_notifier *notifier)
>  {
> -	if (!static_key_false(&preempt_notifier_key))
> +	if (!static_unlikely_branch(&preempt_notifier_key))
>  		WARN(1, "registering preempt_notifier while notifiers disabled\n");
>  
>  	hlist_add_head(&notifier->link, &current->preempt_notifiers);
> @@ -2367,7 +2365,7 @@ static void __fire_sched_in_preempt_notifiers(struct task_struct *curr)
>  
>  static __always_inline void fire_sched_in_preempt_notifiers(struct task_struct *curr)
>  {
> -	if (static_key_false(&preempt_notifier_key))
> +	if (static_unlikely_branch(&preempt_notifier_key))
>  		__fire_sched_in_preempt_notifiers(curr);
>  }
>  
> @@ -2385,7 +2383,7 @@ static __always_inline void
>  fire_sched_out_preempt_notifiers(struct task_struct *curr,
>  				 struct task_struct *next)
>  {
> -	if (static_key_false(&preempt_notifier_key))
> +	if (static_unlikely_branch(&preempt_notifier_key))
>  		__fire_sched_out_preempt_notifiers(curr, next);
>  }
>  
--
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