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

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Date:	Fri, 20 Feb 2015 18:43:59 +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 09:32:39AM -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 proposed interface is very much exposed to everybody who wants
   it; this again is wide open to (ab)use.

   Once it exists people will start to use, and before you know it we'll
   always have that fast counter incremented and we're in IPI hell. Most
   likely because someone was lazy and it seemed like a quick fix for
   some stupid code.

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.
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