linux-kernel - Re: [PATCH tip/core/rcu 17/86] rcu: fix boost-tracing bug and update tracing documentation

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Date:	Mon, 2 May 2011 01:33:01 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	patches@...aro.org, "Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 17/86] rcu: fix boost-tracing bug and
 update tracing documentation

On Sun, May 01, 2011 at 08:43:10AM -0700, Josh Triplett wrote:
> The bugfix here should really go in a separate commit from the updated
> documentation.  Also:
> 
> On Sun, May 01, 2011 at 06:20:57AM -0700, Paul E. McKenney wrote:
> > --- a/Documentation/RCU/trace.txt
> > +++ b/Documentation/RCU/trace.txt
> > @@ -10,34 +10,37 @@ for rcutree and next for rcutiny.
> >  
> >  CONFIG_TREE_RCU and CONFIG_TREE_PREEMPT_RCU debugfs Files and Formats
> >  
> > -These implementations of RCU provides five debugfs files under the
> > +These implementations of RCU provides seven debugfs files under the
> 
> s/seven/several/, so this doesn't continue to need updates as you add
> more tracing?  It doesn't seem entirely accurate even now, given that
> the set of files available now depends on CONFIG_RCU_BOOST.

Good point, will fix.

> >  top-level directory RCU: rcu/rcudata (which displays fields in struct
> 
> RCU or rcu?

Indeed, "rcu".  Will fix.

> >  rcu_data), rcu/rcudata.csv (which is a .csv spreadsheet version of
> >  rcu/rcudata), rcu/rcugp (which displays grace-period counters),
> > -rcu/rcuhier (which displays the struct rcu_node hierarchy), and
> > +rcu/rcuhier (which displays the struct rcu_node hierarchy),
> >  rcu/rcu_pending (which displays counts of the reasons that the
> > -rcu_pending() function decided that there was core RCU work to do).
> > +rcu_pending() function decided that there was core RCU work to do),
> > +rcu/rcutorture (which displays rcutorture test progress), and, if the
> > +kernel is built with CONFIG_RCU_BOOST, rcu/rcuboost (which displays RCU
> > +boosting statistics).
> 
> I think this would work better as a list:
> 
> rcu/rcudata:
>     Displays fields in struct rcu_data.
> rcu/rcudata.csv:
>     Comma-separated values spreadsheet version of rcudata.
> rcu/rcugp:
>     Displays grace-period counters.
> rcu/rcuhier
>     Displays the struct rcu_node hierarchy.
> rcu/rcu_pending:
>     Displays counts of the reasons rcu_pending() decided that RCU had
>     work to do.
> rcu/rcutorture:
>     Displays rcutorture test progress.
> rcu/rcuboost
>     Displays RCU boosting statistics. Only present if
>     CONFIG_RCU_BOOST=y.
> 
> Feel free to put the descriptions on the same lines as the filenames if
> you prefer; however, I thought this style worked better for the
> descriptions longer than one line.

Makes sense, will do a list.

> Also, does rcu/rcutorture appear with rcutorture not loaded?  Hopefully
> not, since rcutorture shouldn't take up any additional resources when
> not loaded.

Not much choice, unfortunately.  If I put the info into rcutorture, then
I lose it when rcutorture is unloaded.  This is problematic in automated
tests that repeatedly load and unload rcutorture.

But the amount of data is small, and I set things up so that TINY_RCU
can omit this.  TREE_RCU is big enough that it doesn't really matter.

> > --- a/kernel/rcutree_plugin.h
> > +++ b/kernel/rcutree_plugin.h
> > @@ -1086,7 +1086,7 @@ static void rcu_initiate_boost_trace(struct rcu_node *rnp)
> >  	else if (rnp->gp_tasks != NULL && rnp->qsmask != 0)
> >  		rnp->n_balk_notblocked++;
> >  	else if (rnp->gp_tasks != NULL &&
> > -		 ULONG_CMP_GE(jiffies, rnp->boost_time))
> > +		 ULONG_CMP_LT(jiffies, rnp->boost_time))
> >  		rnp->n_balk_notyet++;
> >  	else
> >  		rnp->n_balk_nos++;
> 
> This bugfix should go in a separate commit.

Excellent point, will fix.

							Thanx, Paul
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