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

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Message-ID: <20150220091107.GN21418@twins.programming.kicks-ass.net>
Date:	Fri, 20 Feb 2015 10:11:07 +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 Thu, Feb 19, 2015 at 09:08:50PM -0800, Paul E. McKenney wrote:
> Hello!
> 
> This series, possibly for v3.21, contains changes that allow in-kernel
> code to specify that all subsequent synchronous grace-period primitives
> (synchronize_rcu() and friends) be expedited.  New rcu_expedite_gp()
> and rcu_unexpedite_gp() primitives enable and disable expediting,
> and these may be nested.  Note that the rcu_expedited boot/sysfs
> variable, if non-zero, causes expediting to happen regardless of calls
> to rcu_expedite_gp().
> 
> Because one of the use cases for these primitives is to expedite
> grace periods during the in-kernel portion of boot, a new Kconfig
> parameter named CONFIG_RCU_EXPEDITE_BOOT causes the kernel to act
> as if rcu_expedite_gp() was called very early in boot.  At the end
> of boot (presumably just before init is spawned), a call to
> rcu_end_inkernel_boot() will provide the matching rcu_unexpedite_gp()
> if required.

So I though we wanted to get rid / limit the expedited stuff because its
IPI happy, and here its spreading.

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?
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