linux-kernel - Re: [PATCH v3 1/2] timer: make deferrable cpu unbound timers really not bound to a cpu

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Date:   Wed, 06 May 2020 15:28:46 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Prasad Sodagudi <psodagud@...eaurora.org>, john.stultz@...aro.org,
        sboyd@...nel.org, tj@...nel.org
Cc:     linux-kernel@...r.kernel.org, saravanak@...gle.com,
        psodagud@...eaurora.org, pkondeti@...eaurora.org,
        Joonwoo Park <joonwoop@...eaurora.org>
Subject: Re: [PATCH v3 1/2] timer: make deferrable cpu unbound timers really not bound to a cpu

Prasad Sodagudi <psodagud@...eaurora.org> writes:
> To make all cpu unbound deferrable timers are scalable, introduce a common
> timer base which is only for cpu unbound deferrable timers to make those
> are indeed cpu unbound so that can be scheduled by any of non idle cpus.
> This common timer fixes scalability issue of delayed work and all other cpu
> unbound deferrable timer using implementations.

Scalability? That's really the wrong term here. A global timer base is
the opposite and you really want to explain why this is not creating a
scalability problem on large systems.

>  #ifdef CONFIG_SMP
> +struct timer_base timer_base_deferrable;
>  unsigned int sysctl_timer_migration = 1;
>  
>  DEFINE_STATIC_KEY_FALSE(timers_migration_enabled);
> @@ -841,8 +842,14 @@ static inline struct timer_base *get_timer_cpu_base(u32 tflags, u32 cpu)
>  	 * If the timer is deferrable and NO_HZ_COMMON is set then we need
>  	 * to use the deferrable base.
>  	 */
> -	if (IS_ENABLED(CONFIG_NO_HZ_COMMON) && (tflags & TIMER_DEFERRABLE))
> -		base = per_cpu_ptr(&timer_bases[BASE_DEF], cpu);
> +	if (IS_ENABLED(CONFIG_NO_HZ_COMMON) && (tflags & TIMER_DEFERRABLE)) {
> +#ifdef CONFIG_SMP
> +		base = &timer_base_deferrable;
> +#endif

There are definitely smarter ways of solving this than sprinkling
#ifdef's around the code.

> +		if (tflags & TIMER_PINNED)
> +			base = per_cpu_ptr(&timer_bases[BASE_DEF], cpu);
> +	}
> +
>  	return base;
>  }
> @@ -1785,8 +1798,14 @@ static __latent_entropy void run_timer_softirq(struct softirq_action *h)
>  	struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
>  
>  	__run_timers(base);
> -	if (IS_ENABLED(CONFIG_NO_HZ_COMMON))
> +	if (IS_ENABLED(CONFIG_NO_HZ_COMMON)) {
>  		__run_timers(this_cpu_ptr(&timer_bases[BASE_DEF]));
> +#ifdef CONFIG_SMP
> +		if (tick_do_timer_cpu == TICK_DO_TIMER_NONE ||
> +				tick_do_timer_cpu == smp_processor_id())
> +			__run_timers(&timer_base_deferrable);
> +#endif

Again, this can be solved in readable ways. Just slapping #ifdefs all
over the place is sloppy and lazy.

Aside of that accessing the tick internals here open coded is just a
layering violation.

> +	}
>  }
>  
>  /*
> @@ -2025,6 +2044,16 @@ static void __init init_timer_cpu(int cpu)
>  	}
>  }
>  
> +#if defined(CONFIG_NO_HZ_COMMON) && defined(CONFIG_SMP)
> +static void __init init_timer_deferrable_global(void)
> +{
> +	timer_base_deferrable.cpu = nr_cpu_ids;

This was obviously never tested with CONFIG_DEBUG_PER_CPU_MAPS=y as this
will simply result in out of bounds accesses.

>  static void __init init_timer_cpus(void)
>  {
>  	int cpu;
> @@ -2036,6 +2065,9 @@ static void __init init_timer_cpus(void)
>  void __init init_timers(void)
>  {
>  	init_timer_cpus();
> +#if defined(CONFIG_NO_HZ_COMMON) && defined(CONFIG_SMP)
> +	init_timer_deferrable_global();
> +#endif

Stub functions exist to avoid this unreadable #ifdef garbage.

Thanks,

        tglx