Re: [PATCH 09/16] rcu: Remove redundant check for online cpu

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Pranith Kumar <bobby.prani@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <laijs@cn.fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 09/16] rcu: Remove redundant check for online cpu
Date: Wed, 23 Jul 2014 12:15:42 -0700	[thread overview]
Message-ID: <20140723191542.GA9398@linux.vnet.ibm.com> (raw)
In-Reply-To: <CAJhHMCBDZyHYCo01T421f+No0ZBWfjsUXijg+MT1A2U621izpA@mail.gmail.com>

On Wed, Jul 23, 2014 at 11:44:43AM -0400, Pranith Kumar wrote:
> On Wed, Jul 23, 2014 at 11:30 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Wed, Jul 23, 2014 at 11:11:45AM -0400, Pranith Kumar wrote:
> >> On Wed, Jul 23, 2014 at 10:23 AM, Paul E. McKenney
> >> <paulmck@linux.vnet.ibm.com> wrote:
> >> > On Wed, Jul 23, 2014 at 10:12:54AM -0400, Pranith Kumar wrote:
> >> >> On Wed, Jul 23, 2014 at 9:50 AM, Paul E. McKenney
> >> >> <paulmck@linux.vnet.ibm.com> wrote:
> >> >> > On Wed, Jul 23, 2014 at 08:59:06AM -0400, Pranith Kumar wrote:
> >> >> >> On Wed, Jul 23, 2014 at 8:21 AM, Paul E. McKenney
> >> >> >> <paulmck@linux.vnet.ibm.com> wrote:
> >> >> >> > On Wed, Jul 23, 2014 at 01:09:46AM -0400, Pranith Kumar wrote:
> >> >> >> >> There are two checks for an online CPU if two if() conditions. This commit
> >> >> >> >> simplies this by replacing it with only one check for the online CPU.
> >> >> >> >>
> >> >> >> >> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> >> >> >> >
> >> >> >> > I admit that it is very early in the morning my time, but I don't see
> >> >> >> > this change as preserving the semantics in all cases.  Please recheck
> >> >> >> > your changes to the second check.
> >> >> >> >
> >> >> >> >                                                         Thanx, Paul
> >> >> >>
> >> >> >> I guess you must be thrown off by the complementary checks, the first
> >> >> >> check is for cpu_online() and second is for cpu_is_offline(). :)
> >> >> >>
> >> >> >> Previously, if a cpu is offline, the first condition is false and the
> >> >> >> second condition is true, so we return from the second if() condition.
> >> >> >> The same semantics are being preserved.
> >> >> >
> >> >> > Fair enough!
> >> >> >
> >> >> > Nevertheless, I am not seeing this as a simplification.
> >> >>
> >> >> I am not sure what you mean here, do you mean that both the checks are
> >> >> actually required?
> >> >
> >> > I mean that the current compound tests each mean something.  Pulling out
> >> > the offline test adds lines of code and obscures that meaning.   This means
> >> > that it is easier (for me, anyway) to see why the current code is correct
> >> > than it is to see why your suggested change is correct.
> >> >
> >>
> >> That is a valid point. I did not mean to reduce readability of the
> >> code. Just trying to avoid the overhead of smp_processor_id().
> >>
> >> Not sure if you would prefer this, but how about the following?
> >
> > If you change the "awake" to something like "am_online", I could get
> > behind this one.
> 
> OK! I will submit that in the next series(with the zalloc check).

You caught me at a weak moment...  This change just adds an extra
line of code and doesn't really help anything.

So please leave this one out.

							Thanx, Paul