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

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Message-ID: <54E772C3.30508@linux.intel.com>
Date:	Fri, 20 Feb 2015 09:45:39 -0800
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 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...
>
> 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)

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