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

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Message-ID: <20181002095543.GA55344@gmail.com>
Date:   Tue, 2 Oct 2018 11:55:43 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Waiman Long <longman@...hat.com>, 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


* Peter Zijlstra <peterz@...radead.org> wrote:

> 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?

Just a spur of the moment statistics to satisfy curiousity, and it's useful to see how 'busy' a 
particular class is, right?

> Can't we simply ditch it?

We certainly could. Do we have roughly equivalent metrics to arrive at this number via other 
methods?

Thanks,

	Ingo