linux-kernel - Re: [PATCH 1/3] timer: Allocate per-cpu tvec

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Date:	Tue, 31 Mar 2015 09:45:53 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] timer: Allocate per-cpu tvec_base's statically


* Viresh Kumar <viresh.kumar@...aro.org> wrote:

> From: Peter Zijlstra <peterz@...radead.org>
> 
> Memory for tvec_base is allocated separately for boot CPU (statically) and
> non-boot CPUs (dynamically).
> 
> The reason is because __TIMER_INITIALIZER() needs to set ->base to a valid
> pointer (because we've made NULL special, hint: lock_timer_base()) and we cannot
> get a compile time pointer to per-cpu entries because we don't know where we'll
> map the section, even for the boot cpu.
> 
> This can be simplified a bit by statically allocating per-cpu memory. The only
> disadvantage is that memory for one of the structures will stay unused, i.e. for
> the boot CPU, which uses boot_tvec_bases.
> 
> This will also guarantee that tvec_base is cacheline aligned. Even though
> tvec_base has ____cacheline_aligned stuck on, kzalloc_node() does not actually
> respect that (but guarantees a minimum u64 alignment).
> 
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  kernel/time/timer.c | 36 ++++++++----------------------------
>  1 file changed, 8 insertions(+), 28 deletions(-)
> 
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 2d3f5c504939..6e8220ec8a62 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -93,6 +93,7 @@ struct tvec_base {
>  struct tvec_base boot_tvec_bases;
>  EXPORT_SYMBOL(boot_tvec_bases);
>  static DEFINE_PER_CPU(struct tvec_base *, tvec_bases) = &boot_tvec_bases;
> +static DEFINE_PER_CPU(struct tvec_base, __tvec_bases);
>  
>  /* Functions below help us manage 'deferrable' flag */
>  static inline unsigned int tbase_get_deferrable(struct tvec_base *base)
> @@ -1534,46 +1535,25 @@ EXPORT_SYMBOL(schedule_timeout_uninterruptible);
>  
>  static int init_timers_cpu(int cpu)
>  {
> -	int j;
> -	struct tvec_base *base;
> +	struct tvec_base *base = per_cpu(tvec_bases, cpu);
>  	static char tvec_base_done[NR_CPUS];
> +	int j;
>  
>  	if (!tvec_base_done[cpu]) {
>  		static char boot_done;
>  
> +		if (!boot_done) {
> +			boot_done = 1; /* skip the boot cpu */

So it would be a lot more descriptive to name this flag 
'boot_cpu_skipped'?

>  		} else {
> +			base = per_cpu_ptr(&__tvec_bases, cpu);
> +			per_cpu(tvec_bases, cpu) = base;
>  		}
> +
>  		spin_lock_init(&base->lock);
>  		tvec_base_done[cpu] = 1;
>  		base->cpu = cpu;
>  	}

Also, I'd put a description about the PER_CPU background into comments 
as well, because it's not obvious at first sight at all what the whole 
(boot_tvec_bases, tvec_bases, __tvec_bases) dance does.

Thanks,

	Ingo
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