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

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Message-ID: <87sinth2oz.fsf@sejong.aot.lge.com>
Date:	Thu, 29 May 2014 09:02:36 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Jiri Olsa <jolsa@...hat.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, 27 May 2014 09:37:38 +0200, Jiri Olsa wrote:
> 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

Yes, I know.  But my question is why do this at close()?  Isn't it
sufficient to check the file limit at open() and close previous one if
necessary?

>
>> 
>> > +
>> > +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.

I'd rather suggest dropping the open/close idiom for this case since
it's confusing.  What about get/put or get_fd/put_fd?

Thanks,
Namhyung
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