linux-kernel - Re: [PATCH] perf record: mmap output file

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Message-ID: <20131015072540.GE24584@gmail.com>
Date:	Tue, 15 Oct 2013 09:25:40 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	David Ahern <dsahern@...il.com>, acme@...stprotocols.net,
	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf record: mmap output file - v2


* Namhyung Kim <namhyung@...nel.org> wrote:

> > 3)
> >
> > The rec->bytes_at_mmap_start field feels a bit weird. If I read the code 
> > correctly, in every 'perf record' invocation, rec->bytes_written starts at 
> > 0 - i.e. we don't have repeat invocations of cmd_record().
> 
> rec->bytes_written is updated when it writes to the output file for 
> synthesizing COMM/MMAP events (this mmap output is not used at that 
> time).

Btw., while looking into it, I think advance_output() needlessly 
obfuscates as well:

static void advance_output(struct perf_record *rec, size_t size)
{
        rec->bytes_written += size;
}

that code should just be written open coded.

So I think all this needs a few good rounds of cleanups, before we can 
complicate it with a new feature. (the cleanups can be on top of the 
feature, if they go in at the same time.)

> > That means that this:
> >
> > 		rec->bytes_at_mmap_start = st.st_size - rec->bytes_written;
> >
> > is really:
> >
> > 		rec->bytes_at_mmap_start = st.st_size;
> >
> > furthermore, since we don't allow appends anymore, st.st_size ought to be 
> > zero as well.
> >
> > Which means that ->bytes_at_mmap_start is always zero - and could be 
> > eliminated altogether.
> 
> No, st_size is bigger than rec->bytes_written due to the 
> perf_file_header which is written without updating rec->bytes_written.

Since all this is code that executes once during __cmd_record(), is this 
all about the header writeout?

That is what confused me about the stat() call and that's messy really: we 
are the ones who write the file header, we have a very good idea about how 
many bytes we wrote to the file! It should be entirely unnecessary to lose 
that information and then execute a system call to recover that 
information...

Thanks,

	Ingo
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