Message-ID: <54E76FB7.4060005@linux.intel.com>
Date: Fri, 20 Feb 2015 09:32:39 -0800
From: Arjan van de Ven <arjan@linux.intel.com>
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To: paulmck@linux.vnet.ibm.com, Peter Zijlstra <peterz@infradead.org>
CC: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com,
        oleg@redhat.com, bobby.prani@gmail.com
Subject: Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace
 periods
References: <20150220050850.GA32639@linux.vnet.ibm.com> <20150220091107.GN21418@twins.programming.kicks-ass.net> <20150220163737.GL5745@linux.vnet.ibm.com> <20150220165409.GU5029@twins.programming.kicks-ass.net> <20150220171442.GM5745@linux.vnet.ibm.com>
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>>>> 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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