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Date: Thu, 19 Mar 2015 13:58:33 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, rusty@rustcorp.com.au, mathieu.desnoyers@efficios.com,
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        Andrea Arcangeli <aarcange@redhat.com>,
        David Woodhouse <David.Woodhouse@intel.com>,
        Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH 6/8] rbtree: Implement generic latch_tree
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On Thu, 19 Mar 2015 08:25:02 +0100 Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Mar 18, 2015 at 10:14:46PM -0700, Andrew Morton wrote:
> > On Wed, 18 Mar 2015 14:36:32 +0100 Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > >  include/linux/rbtree_latch.h |  223 +++++++++++++++++++++++++++++++++++++++++++
> > 
> > Did it really need to all be inlined?
> 
> Without that you get actual function calls to the less() and comp()
> operators. This way GCC can inline the lot even though its function
> pointers.
> 
> The typical RB tree user open-codes all this every single time.

Is it a good tradeoff?

> > How much of this code is unneeded on uniprocessor?
> 
> None, UP has NMIs too.

OK.  This code is basically required to support perf/ftrace and
modules, yes?  Presumably small and space-constrained systems aren't
using either, so they don't take the hit.

However CONFIG_MODULES systems which aren't using perf/ftrace _do_ take
a hit.  How many systems are we talking here?  All non-x86?