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, 13 May 2020 12:53:48 -0700
From:   psodagud@...eaurora.org
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     john.stultz@...aro.org, sboyd@...nel.org, tj@...nel.org,
        linux-kernel@...r.kernel.org, saravanak@...gle.com,
        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

Hi Tglx,

On 2020-05-06 06:28, Thomas Gleixner wrote:
> 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.

I am able to understand all other comments and I will address all those 
comments in the next patch set.
It is not clear to me how to avoid #ifdef's in this case. Could you 
please share an example here?


> 
>> +		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.
Sorry. I will try to address this.  It is not clear to me how to avoid 
#ifdefs in this case too.
Please provide me more information.

> 
> Aside of that accessing the tick internals here open coded is just a
> layering violation.
I will fix this and avoid using referring to tick internals here.

> 
>> +	}
>>  }
>> 
>>  /*
>> @@ -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.

Sure. I will test this CONFIG_DEBUG_PER_CPU_MAPS=y enabled before 
pushing the next patch set.
> 
>>  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.
Got it. I will fix this in the next patch set.

> 
> Thanks,
> 
>         tglx