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

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Message-ID: <20150221160852.GI23367@worktop.ger.corp.intel.com>
Date:	Sat, 21 Feb 2015 17:08:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	paulmck@...ux.vnet.ibm.com, 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 09:45:39AM -0800, Arjan van de Ven wrote:
> On 2/20/2015 9:43 AM, Peter Zijlstra wrote:
> >On Fri, Feb 20, 2015 at 09:32:39AM -0800, Arjan van de Ven wrote:
> >>there's a few others as well that I'm chasing down...
> >>.. but the flip side, prior to running ring 3 code, why NOT do fast expedites?
> >
> >So my objections are twofold:
> >
> >  - I object to fast expedites in principle; they spray IPIs across the
> >    system, so ideally we'd not have them at all, therefore also not at
> >    boot.
> >
> >    Because as soon as the option exists, people will use it for other
> >    things too.
> 
> the option exists today in sysfs and kernel parameter...

Yeah, Paul and me have been having this argument for a while now ;-)

> >And esp. in bootup code you can special case a lot of stuff; there's
> >limited concurrency esp. because userspace it not there yet. So we might
> >not actually need those sync calls.
> 
> yeah I am going down that angle as well absolutely.
> but there are cases that may well be legit (or are 5 function calls deep into common code)

Good ;-)
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