linux-kernel - Re: [PATCH 06/14] perf tools: Cache dso data file descriptor

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Date:	Tue, 27 May 2014 09:37:38 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Namhyung Kim <namhyung@...il.com>
Cc:	Jiri Olsa <jolsa@...nel.org>, linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	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>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 06/14] perf tools: Cache dso data file descriptor

On Tue, May 27, 2014 at 10:05:28AM +0900, Namhyung Kim wrote:
> Hi Jiri,
> 
> On Thu, 15 May 2014 19:23:27 +0200, Jiri Olsa wrote:
> 
> [SNIP]
> > +static void data_close(void)
> > +{
> > +	bool cache_fd = may_cache_fd();
> > +
> > +	if (!cache_fd)
> > +		close_first_dso();
> > +}
> 
> Why do you do this at close()?  As long as there's no attempt to open a
> new file, we can keep existing fd, no?

so the way it works now is:

 - we keep up to the 'RLIMIT_NOFILE / 2' of open dso objects
 - if we try to open dso and it fails, because we are out of
   file descriptors, we close dso objects and try to reopen
   (check do_open function)
 - when we close the dso object we check if number of opened
   dso objects is below 'RLIMIT_NOFILE / 2'.. if it is, we keep
   the dso opened, if not we close first dso in the list

util/dso.h tries to describe that

> 
> > +
> > +void dso__data_close(struct dso *dso)
> > +{
> > +	if (dso->data.fd >= 0)
> > +		data_close();
> > +}
> 
> Hmm.. it's confusing dso__data_close(dso) closes an other dso rather
> than the given dso.  And this dso__data_close() is not paired with any
> _open() also these close calls make me confusing which one to use. ;-p

thats due to the caching.. as explained above

About the pairing.. originally the interface was only dso__data_fd
that opened and returned fd, which the caller needed to close.

I added dso__data_close so we could keep track of file descriptors.

I could add dso__data_open I guess, but it is dso__data_fd which is
needed for elf interface anyway.

thanks,
jirka
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