linux-kernel - Re: [PATCH 13/14] tools lib fd array: Allow associating an integer cookie with each entry

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Message-ID: <20140911154922.GK10158@kernel.org>
Date:	Thu, 11 Sep 2014 12:49:22 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Adrian Hunter <adrian.hunter@...el.com>,
	Borislav Petkov <bp@...e.de>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Jean Pihet <jean.pihet@...aro.org>,
	Jiri Olsa <jolsa@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 13/14] tools lib fd array: Allow associating an integer
 cookie with each entry

Em Thu, Sep 11, 2014 at 05:35:27PM +0200, Jiri Olsa escreveu:
> On Thu, Sep 11, 2014 at 12:23:30PM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Thu, Sep 11, 2014 at 04:59:45PM +0200, Jiri Olsa escreveu:
> > > On Thu, Sep 11, 2014 at 10:29:11AM -0300, Arnaldo Carvalho de Melo wrote:
> > > > But then I had no use whatsoever for the ->ptr one at this point, so I
> > > > just nuked it, to keep _just_ what is used _right now_, and added the
> > > > comment to the changelog :-)

> > > if we are treeting tools/lib/api as 'external' lib, I think we should
> > > use 'void *' for priv and let the user retype it to whatever he wants

> > > but I dont care/insist here too much.. it just seems strange to me ;-)
> > 
> > Ok, so perhaps it should be:
> > 
> > 	struct fdarray {
> > 		...
> > 		union {
> > 			int idx;
> > 		} *priv;
> > 		...
> > }
> > 
> > And then perf's evlist will use:
> > 
> > 	fda->priv[fd].idx;
> > 
> > 	Then, when we get the need for a pointer, we can go there and
> > add it to the union.
> > 
> > 	This will make tools/lib/api/fd/ a little bit more friendly to
> > use outside the kernel sources, since then a new version will be source
> > code compatible (but not necessarily binary compatible).
> 
> you lost me ;-)
> 
> could you please make an example of the !compatibility you mentioned?

If I leave it as is now, when I add that void * pointer to each entry in
fdarray->priv[], I will have to change fdarray's users, i.e. right now I
will have to go and change evlist.h, i.e. every place that has:

	fda->priv[fd];

	will have to be changed to:

	fda->priv[fd].idx;

If I instead change it to:

 	struct fdarray {
 		...
 		union {
 			int idx;
 		} *priv;
 		...
	}

Then I will not have to change anything when I add that void * pointer
to the union.
 
> > 	If we commit to that we can't just go on and change all its
> > users as we can now do for things like 'struct sk_buff', 'struct sock'
> > and any other kernel data structures.
> > 	There are costs in making it 'external' in the sense you're
> > implying. I want to make it a bit less 'external' (in your sense), at
> > least while we initially implement new stuff like this one.
 
> and the costs?
 
> also I did not mean that union above, what I meant was:
 
> struct fdarray {
> 	...
> 	void *priv;
> 	...
> }

Yeah, that is what I am trying to avoid. This will make it use
sizeof(void *) for each entry, and I have no use for that right now. All
I need is sizeof(int) for each entry.

Remember, this is an array, so its not just a matter of changing:

	int *priv;

to:

	void *priv;

But since it is an array, it will use, in this case, double the space,
on 64-bit, for no gain _right now_.

- Arnaldo
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