Message-ID: <54E772C3.30508@linux.intel.com>
Date: Fri, 20 Feb 2015 09:45:39 -0800
From: Arjan van de Ven <arjan@linux.intel.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, mingo@kernel.org,
        laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org,
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        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> <54E76FB7.4060005@linux.intel.com> <20150220174359.GW5029@twins.programming.kicks-ass.net>
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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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