linux-kernel - Re: [PATCH 18/18] rcu: Use pr_fmt to prefix "rcu: " to logging output

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Message-Id: <20180514222456.GO26088@linux.vnet.ibm.com>
Date:   Mon, 14 May 2018 15:24:56 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 18/18] rcu: Use pr_fmt to prefix "rcu: " to logging output

On Mon, May 14, 2018 at 02:41:59PM -0700, Joe Perches wrote:
> On Mon, 2018-05-14 at 13:29 -0700, Paul E. McKenney wrote:
> > On Thu, May 10, 2018 at 08:45:44AM -0700, Joe Perches wrote:
> > > Use a consistent logging prefix for all rcu related output.
> > > 
> > > Signed-off-by: Joe Perches <joe@...ches.com>
> > 
> > I took parts of this (thank you!) but have concerns about other parts.
> []
> > > diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
> > > index 076a50fb22ad..ebdd77b45470 100644
> > > --- a/kernel/rcu/rcuperf.c
> > > +++ b/kernel/rcu/rcuperf.c
> > > @@ -19,6 +19,9 @@
> > >   *
> > >   * Authors: Paul E. McKenney <paulmck@...ibm.com>
> > >   */
> > > +
> > > +#define pr_fmt(fmt) "rcu: " fmt
> > 
> > This is going to get us messages of the form "rcu: rcu-perf:", not?
> > (And other odd combinations, depending on the flavor of RCU under test.)
> > If so, this does not seem to be an improvement.
> 
> That depends on the existing embedded content of the format.
> This will prefix just "rcu: " to pr_<level> output.

OK, so to make this work, I need to create special-purpose pr_fmt()
definitions for torture, rcutorture, locktorture, and rcuperf.  Most
of the rest don't care.

Or am I missing something basic here?

> > > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> []
> > > @@ -908,7 +909,7 @@ rcu_torture_writer(void *arg)
> > >  	VERBOSE_TOROUT_STRING("rcu_torture_writer task started");
> > >  	if (!can_expedite)
> > >  		pr_alert("%s" TORTURE_FLAG
> > > -			 " GP expediting controlled from boot/sysfs for %s.\n",
> > > +			 " GP expediting controlled from boot/sysfs for %s\n",
> > >  			 torture_type, cur_ops->name);
> 
> As there is _no_ pr_fmt #defined in this file,
> output will/could be prefixed with KBUILD_MODNAME ": "
> (in this case "rcutorture: ") if/when the generic
> pr_fmt conversion patch is applied.

Not a fan, NACK.

> > >  	} else if (gp_sync && !cur_ops->sync) {
> > > -		pr_alert("%s: gp_sync without primitives.\n", __func__);
> > > +		pr_alert("%s: gp_sync without primitives\n", __func__);
> > 
> > I used a CDC Cyber 73 in the 1970s.
> 
> Fancy.  I used a CDC 6400 and an IBM 1620, but
> those were pretty old when I started.

;-)

> > It had tiny memory by today's
> > standards, but even it had periods in its error messages.  We can easily
> > afford them today, especially given that rcutorture is not included in
> > small-memory Linux configurations.
> 
> OK, but I like consistency too.
> 
> ~90 percent of linux logging does not use terminating periods.
> For instance, on my laptop:
> 
> $ uptime -p
> up 1 week, 1 day, 13 hours, 37 minute
> $ dmesg | wc -l
> 4240
> $ dmesg | grep -P "\w\.$"| wc -l
> 381

Why is this a problem worth fixing?  From where I sit, it is not.

Even assuming that this is somehow worth solving, why is it buried
in an unrelated patch?

> > > @@ -1711,11 +1712,11 @@ static void rcu_test_debug_objects(void)
> > > 
> > >  	/* Wait for them all to get done so we can safely return. */
> > >  	rcu_barrier();
> > > -	pr_alert("%s: WARN: Duplicate call_rcu() test complete.\n", KBUILD_MODNAME);
> > > +	pr_alert("WARN: Duplicate call_rcu() test complete\n");
> > 
> > I would like to keep these, as they mark the region of console output where
> > splats are expected.
> 
> The prefixes are still there...
> 
> > >  	if (cur_ops->fqs == NULL && fqs_duration != 0) {
> > > -		pr_alert("rcu-torture: ->fqs NULL and non-zero fqs_duration, fqs disabled.\n");
> > > +		pr_alert("->fqs NULL and non-zero fqs_duration, fqs disabled\n");
> > 
> > This I would like to keep.  Easier to find.
> 
> One thing that you could use to validate the
> output string format is after compilation:
> 
> $ strings kernel/rcu/rcutorture.o | grep -P "^[0-6]\w+:"
> 
> With your change, you will see duplicated prefixes.

Except that right now there are not duplicated prefixes.  Those
apparently only show up after some earlier patch in/before your set is
applied, correct?  Plus with your change, I apparently get quite a few
semi-duplicated prefixes, which I would rather avoid, as it would add
pointless thrash to my rcutorture scripting.

Is there some C-preprocessor macro indicating whether or not your changes
have been applied?

							Thanx, Paul