linux-kernel - Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace periods

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Message-ID: <20150220165409.GU5029@twins.programming.kicks-ass.net>
Date:	Fri, 20 Feb 2015 17:54:09 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace
 periods

On Fri, Feb 20, 2015 at 08:37:37AM -0800, Paul E. McKenney wrote:
> On Fri, Feb 20, 2015 at 10:11:07AM +0100, Peter Zijlstra wrote:
> > So I though we wanted to get rid / limit the expedited stuff because its
> > IPI happy, and here its spreading.
> 
> Well, at least it no longer IPIs idle CPUs.  ;-)
> 
> And this is during boot, when a few extra IPIs should not be a big deal.

Well the one application now is during boot; but you expose the
interface for all to use, and therefore someone will.

> > Does it really make a machine boot much faster? Why are people using
> > synchronous gp primitives if they care about speed? Should we not fix
> > that instead?
> 
> The report I heard was that it provided 10-15% faster boot times.

That's not insignificant; got more details? I think we should really
look at why people are using the sync primitives.
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