linux-kernel - Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value

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Message-ID: <20140724202755.GU11241@linux.vnet.ibm.com>
Date:	Thu, 24 Jul 2014 13:27:55 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value

On Thu, Jul 24, 2014 at 03:59:33PM -0400, Pranith Kumar wrote:
> Adding peterz to CC as git blames him for wait_event code. :)
> 
> (original LKML link: https://lkml.org/lkml/2014/7/23/45)
> 
> On Thu, Jul 24, 2014 at 2:12 PM, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

[ . . . ]

> >> If we care about what wait_event_interruptible() returns, we can go
> >> back and wait for an actual wakeup much earlier without the additional
> >> overhead of calling rcu_gp_init().
> >
> > The key phrase here is "If we care".  Should we care?  If so, why?
> >
> > I suggest running some random benchmark and counting how many times
> > rcu_gp_init() is called and how many times rcu_gp_init() returns
> > because ->gp_flags is not set.  If rcu_gp_init() returns because
> > ->gp_flags is not set a significant fraction of the time, then this
> > -might- be worth worrying about.  (Extra credit: Under what conditions
> 
> In the grand scheme of things, I agree that minor optimizations may not seem
> to be worth much. But when the optimizationss are straight forward and 
> are _actually_ improving things, even by a small margin, I think they are
> worth considering.
> 
> Think of the billions of cycles we will save ;-)

If there are significant savings.  You have yet to demonstrate this.
In fact, you have yet to demonstrate that your change doesn't make
things worse.

							Thanx, Paul

> > -might- be worth worrying about.  (Extra credit: Under what conditions
> > would it be worth worrying about, and how would you go about checking
> > to see whether those conditions hold?)
> >
> 
> 
> -- 
> Pranith
> 

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