linux-kernel - Re: [PATCH] perf tools: Fix tracing_path

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Message-ID: <20181016152322.GC3849@kernel.org>
Date:   Tue, 16 Oct 2018 12:23:22 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Michael Petlan <mpetlan@...hat.com>
Subject: Re: [PATCH] perf tools: Fix tracing_path_mount proper path

Em Tue, Oct 16, 2018 at 12:18:13PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Tue, Oct 16, 2018 at 01:48:18PM +0200, Jiri Olsa escreveu:
> > If there's no tracefs (RHEL7) support the tracing_path_mount
> > returns debugfs path which results in following fail:
> > 
> >   # perf probe sys_write
> >   kprobe_events file does not exist - please rebuild kernel with CONFIG_KPROBE_EVENTS.
> >   Error: Failed to add events.
> > 
> > In tracing_path_debugfs_mount function we need to return the
> > 'tracing' path instead of just the mount to make it work:
> > 
> >   # perf probe sys_write
> >   Added new event:
> >     probe:sys_write      (on sys_write)
> > 
> >   You can now use it in all perf tools, such as:
> > 
> >           perf record -e probe:sys_write -aR sleep 1
> > 
> > Adding the 'return tracing_path;' also to tracing_path_tracefs_mount
> > function just for consistency with tracing_path_debugfs_mount.
> > 
> > Upstream keeps working, because it has the tracefs support.
> > 
> > Link: http://lkml.kernel.org/n/tip-yiwkzexq9fk1ey1xg3gnjlw4@git.kernel.org
> > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > ---
> >  tools/lib/api/fs/tracing_path.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/tools/lib/api/fs/tracing_path.c b/tools/lib/api/fs/tracing_path.c
> > index 120037496f77..5afb11b30fca 100644
> > --- a/tools/lib/api/fs/tracing_path.c
> > +++ b/tools/lib/api/fs/tracing_path.c
> > @@ -36,7 +36,7 @@ static const char *tracing_path_tracefs_mount(void)
> >  
> >  	__tracing_path_set("", mnt);
> >  
> > -	return mnt;
> > +	return tracing_path;
> >  }
> >  
> >  static const char *tracing_path_debugfs_mount(void)
> > @@ -49,7 +49,7 @@ static const char *tracing_path_debugfs_mount(void)
> >  
> >  	__tracing_path_set("tracing/", mnt);
> >  
> > -	return mnt;
> > +	return tracing_path;
> 
> Humm, colour me confused, by the name of the function I expected
> tracing_path_debugfs_mount() to return the debugfs mount.
> 
> tracing_path_tracefs_mount() would return the tracefs mount point, so,
> there, returning tracing_path would be ok.
> 
> Is there some other way to figure out that tracefs isn't available and
> thus we should use debugfs_mount + "/tracing/" for the "tracefs" mount?
> 
> This may well be just the naming confusion... lemme read this a bit
> more...

Indeed, naming confusion, these are static things, used only by:

const char *tracing_path_mount(void)
{
        const char *mnt;

        mnt = tracing_path_tracefs_mount();
        if (mnt)
                return mnt;

        mnt = tracing_path_debugfs_mount();

        return mnt;
}

This one could as well just return tracing_path and the return be used
just to figure if it managed to read the mount point for tracefs or
debugfs...

Something like:

static int tracing_path_tracefs_mount(void)
{
	const char *mnt = tracefs__mount();
	if (!mnt)
		return -1;

	__tracing_path_set("", mnt);
	return 0;
}

static const char *tracing_path_debugfs_mount(void)
{
	const char *mnt = debugfs__mount();
	if (!mnt)
		return -1;

	__tracing_path_set("tracing/", mnt);
	return 0;
}

const char *tracing_path_mount(void)
{
	if (tracing_path_tracefs_mount() < 0 &&
	    tracing_path_debugfs_mount() < 0)
		return NULL;

	return tracing_path;
}

But anyway, your patch fixes things, applying...

- Arnaldo