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

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Date:	Fri, 20 Feb 2015 09:32:39 -0800
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	paulmck@...ux.vnet.ibm.com, Peter Zijlstra <peterz@...radead.org>
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

>>>> 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.
>
> I must defer to the people who took the exact measurements.
>
> But yes, once I have that info, I should add it to the commit log.

so the two most obvious cases are

Registering sysrq keys ... even when the old key code had no handler
(have a patch pending for this)

registering idle handlers
(this is more tricky, it's very obvious abuse but the fix is less clear)

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?

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