linux-kernel - Re: [tip/core/rcu, 05/21] rcu: Make rcu_gp_cleanup() more accurately predict need for new GP

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Message-Id: <20180511162443.GA26088@linux.vnet.ibm.com>
Date:   Fri, 11 May 2018 09:24:43 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel.opensrc@...il.com,
        torvalds@...ux-foundation.org, npiggin@...il.com
Subject: Re: [tip/core/rcu, 05/21] rcu: Make rcu_gp_cleanup() more accurately
 predict need for new GP

On Thu, May 10, 2018 at 10:37:54AM -0700, Joel Fernandes wrote:
> On Thu, May 10, 2018 at 06:15:46AM -0700, Paul E. McKenney wrote:
> [...] 
> > > Also in rcu_future_gp_cleanup, we call:
> > > 	trace_rcu_future_gp(rnp, rdp, c,
> > > 			    needmore ? TPS("CleanupMore") : TPS("Cleanup"));
> > > For this case, in the final trace event record, rnp->completed and c will be
> > > the same, since c is set to rnp->completed before calling
> > > trace_rcu_future_gp. I was thinking they should be different, do you expect
> > > them to be the same?
> > 
> > Hmmm...  That does look a bit inconsistent.  And it currently uses
> > rnp->gp_seq instead of rnp->gp_seq_needed despite having the same
> > "CleanupMore" name.
> > 
> > Looks like a review of the calls to trace_rcu_this_gp() is in order.
> 
> I see you changed trace_rcu_future_gp to use trace_rcu_this_gp in 15/21.. I
> am not sure if the concern is still valid then since you seem to be correctly
> getting the future GP in those cases, except for the naming which I suggest
> be changed from 'c' to 'future_gp' just for clarity / self-documenting code.

Indeed, "c" for "->completed" is completely outdated.  ;-)

Would you be willing to send a patch providing a better name?

								Thanx, Paul