linux-kernel - Re: [Bcache v14 16/16] bcache: Debug and tracing code

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Date:	Tue, 12 Jun 2012 10:35:43 -0700
From:	Joe Perches <joe@...ches.com>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	g@...gle.com, linux-bcache@...r.kernel.org,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com,
	tejun@...gle.com, agk@...hat.com, dan.j.williams@...el.com
Subject: Re: [Bcache v14 16/16] bcache: Debug and tracing code

On Tue, 2012-06-12 at 10:24 -0700, Kent Overstreet wrote:
> On Tue, Jun 12, 2012 at 09:50:58AM -0700, Joe Perches wrote:
> > On Tue, 2012-06-12 at 08:39 -0700, Kent Overstreet wrote:
> > 
> > > diff --git a/drivers/md/bcache/debug.c b/drivers/md/bcache/debug.c
> > 
> > > +static void dump_bset(struct btree *b, struct bset *i)
> > > +{
> > > +	for (struct bkey *k = i->start; k < end(i); k = bkey_next(k)) {
> > > +		printk(KERN_ERR "block %zu key %zu/%i: %s", index(i, b),
> > > +		       (uint64_t *) k - i->d, i->keys, pkey(k));
> > 
> > Add #define pr_fmt and use pr_<level> not printk everywhere.
> 
> I've got the pr_fmt, but I don't want to use it here because it's
> dumping a btree node (100s of lines) so the bcache: would be redundant,
> but more importantly I don't want lines getting truncated.
> 
> > Doesn't this throw a gcc warning for argument mismatch?
> 
> No, why?

"(uint64_t *)k - i->d" is what type again?
What is a %zu?

Isn't that a mismatch?

> > > +static int debug_seq_show(struct seq_file *f, void *data)
> > > +{
> > > +	static const char *tabs = "\t\t\t\t\t";
> > 
> > Seems a _very_ odd use.
> 
> It is a strange hack.
> 
> The idea is that we want to indent more as we recurse; we could build up
> a new string of tabs each time we recurse that's got one more tab than
> our parent's, but that'd be a pain in the ass and it'd use more stack
> space (though that should be fine here), so instead it's just
> decrementing the pointer to the tab string to produce a string with one
> more tab.

It's a nice idea, but that's not what's happening
as I believe you reference tabs only once as &tabs[4]

> I'm not opposed to taking it out if you know cleaner way that isn't
> ridiculously verbose. But this code needs to be rewritten to not use
> single_open() (which I tihnk is going to be a pain in the ass) so it's
> not really at the top of my list.
> 
> > 
> > > +	uint64_t last = 0, sectors = 0;
> > > +	struct cache_set *c = f->private;
> > > +
> > > +	struct btree_op op;
> > > +	bch_btree_op_init_stack(&op);
> > > +
> > > +	btree_root(dump, c, &op, f, &tabs[4], &last, &sectors);
> > > +
> > 
> > Why not just:
> > 
> > 	btree_root(dump, c, &op, "\t", &last, &sectors);


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