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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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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