linux-kernel - Re: [PATCH 4/5] locking/lockdep: Make class->ops a percpu counter

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Message-ID: <20181002093907.GV3439@hirez.programming.kicks-ass.net>
Date:   Tue, 2 Oct 2018 11:39:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] locking/lockdep: Make class->ops a percpu counter

On Fri, Sep 28, 2018 at 01:53:20PM -0400, Waiman Long wrote:
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index ca002c0..7a0ed1d 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -139,6 +139,7 @@ static inline int debug_locks_off_graph_unlock(void)
>   */
>  unsigned long nr_lock_classes;
>  static struct lock_class lock_classes[MAX_LOCKDEP_KEYS];
> +static DEFINE_PER_CPU(unsigned long [MAX_LOCKDEP_KEYS], lock_class_ops);

> @@ -1387,11 +1391,15 @@ static inline int usage_match(struct lock_list *entry, void *bit)
>  
>  static void print_lock_class_header(struct lock_class *class, int depth)
>  {
> -	int bit;
> +	int bit, cpu;
> +	unsigned long ops = 0UL;
> +
> +	for_each_possible_cpu(cpu)
> +		ops += *per_cpu(class->pops, cpu);
>  
>  	printk("%*s->", depth, "");
>  	print_lock_name(class);
> -	printk(KERN_CONT " ops: %lu", class->ops);
> +	printk(KERN_CONT " ops: %lu", ops);
>  	printk(KERN_CONT " {\n");
>  
>  	for (bit = 0; bit < LOCK_USAGE_STATES; bit++) {

That is an aweful lot of storage for a stupid number. Some archs
(sparc64) are bzImage size constrained and this will hurt them.

Ingo, do you happen to remember what that number was good for?
Can't we simply ditch it?