| title: | Re PATCH 1 1 mm unify pmd free implemen |
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On Mon, 28 Jul 2008, James Bottomley wrote:
Sorry ... should have been clearer. My main concern is the cost of
barrier() which is just a memory clobber ... we have to use barriers to
place the probe points correctly in the code.
Oh, "barrier()" itself has _much_ less cost.
It still has all the "needs to flush any global/address-taken-of variables
to memory" property and can thus cause reloads, but thats kind of the
point of it, after all. So in that sense "barrier()" is free: the only
cost of a barrier is the cost of what you actually need to get done. Its
not really "free", but its also not any more costly than what your
objective was.
In contrast, the "objective" in an empty function call is seldom the
serialization, so in that case the serialization is all just unnecessary
overhead.
Also, barrier() avoids the big hit of turning a leaf function into a
non-leaf one. It also avoids all the fixed registers and the register
clobbers (although for tracing purposes you may end up setting up fixed
regs, of course).
The leaf - non-leaf thing is actually often the major thing. Yes, the
compiler will often inline functions that are simple enough to be leaf
functions with no stack frame, so we dont have _that_ many of them, but
when it hits, its often the most noticeable part of an unnecessary
function call. And "barrier()" should never trigger that problem.
Linus
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