Date: Sat, 21 Feb 2015 17:08:52 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Arjan van de Ven <arjan@linux.intel.com>
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,
        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
Message-ID: <20150221160852.GI23367@worktop.ger.corp.intel.com>
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 <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>
 <54E772C3.30508@linux.intel.com>
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On Fri, Feb 20, 2015 at 09:45:39AM -0800, Arjan van de Ven wrote:
> 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...

Yeah, Paul and me have been having this argument for a while now ;-)

> >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)

Good ;-)
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