linux-kernel - Re: [PATCH 2/3] DEBUGFS: Add per cpu counters

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Message-ID: <20111209030025.GA17149@localhost>
Date:	Fri, 9 Dec 2011 11:00:25 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	"greg@...ah.com" <greg@...ah.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 2/3] DEBUGFS: Add per cpu counters

On Wed, Dec 07, 2011 at 01:17:51AM +0800, Andi Kleen wrote:
> On Tue, Dec 06, 2011 at 07:44:38PM +0800, Wu Fengguang wrote:
> > > +#include <linux/seq_file.h>
> > > +#include <linux/percpu.h>
> > > +#include <linux/kernel.h>
> > > +
> > > +/* OPEN: implement module support */
> > 
> > Yeah, I think the module support would benefit my case as well.
> > 
> > To support module users, init_counters() will be exported with the
> > __start___debugfs/__stop___debugfs hard coding removed. Then I'll be
> > call it from the readahead initilization code:
> 
> No, the module loader should take care of this: look for the magic
> section, register it. This is already done for other magic
> sections and not too difficult, i just didn't write the code
> so far.
>
> Then the DEFINE_* macros would work seamlessly in modules too.
 
OK.

> Do you really need it for the readahead code? I thought that
> was builtin.

Yeah because in the current form I'm initializing an array of counters
at runtime in the readahead call site. 

> > DEFINE_PER_CPU(unsigned long[RA_PATTERN_MAX][RA_ACCOUNT_MAX], ra_counter);
> > struct debugfs_counter ra_pcpu_counter[RA_PATTERN_MAX][RA_ACCOUNT_MAX];
> > 
> > // init ra_pcpu_counter in a loop
> > for each pattern
> >         init_counters(ra_pcpu_counter[pattern], ra_pcpu_counter[pattern+1]);
> 
> Ok so you need arrays.
> 
> The idea is to not call some init function, but just put it into section
> and let the init code walk it.
> 
> Should probably have a macro that handles arrays nicely.

That would be nice!

> > > +static int show_debugfs_counter(struct seq_file *m, void *arg)
> > > +{
> > > +	int n;
> > > +	n = dump_counters(m, __start___debugfs, __stop___debugfs, m->private);
> > 
> > That hard coded __start___debugfs/__stop___debugfs is OK for POC, and
> > will need to be improved to work with multiple users in kernel.
> 
> For Modules it obviously has to walk a list. It's a straight forward extension.
> 
> I don't think it should allow everyone to register their own lists,
> that doesn't make sense because they could as well create the debug 
> files themselves.

OK. It just looks a bit inefficient when (this becomes so popular that)
the list grows large.

Thanks,
Fengguang
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