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 13:55:31 -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

On 2020-05-13 13:28, Thomas Gleixner wrote:
> psodagud@...eaurora.org writes:
>> On 2020-05-06 06:28, Thomas Gleixner wrote:
>>>>  #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?
> 
> The answer is further down already:

Thanks Tglx for quick response.

I think, you are referring stub functions. Yes. I can reduce some of the 
#ifdefs with stub functions as you mentioned and not all the cases 
right?
I have introduced two variables timer_base_deferrable and 
deferrable_pending and I can put stub function where ever is possible. 
But it may not be appropriate to have stub function for all the 
references of these variables right? Correct me if my understanding is 
wrong.

-Thanks, Prasad

> 
>>> Stub functions exist to avoid this unreadable #ifdef garbage.