RE: [PATCH][RFC] Implement arch primitives for busywait loops

From: Nicholas Piggin

Sent: 16 September 2016 12:52
On Fri, 16 Sep 2016 11:30:58 +0000
David Laight [off-list ref] wrote:
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From: Nicholas Piggin

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Sent: 16 September 2016 09:58
Implementing busy wait loops with cpu_relax() in callers poses
some difficulties for powerpc.

First, we want to put our SMT thread into a low priority mode for the
duration of the loop, but then return to normal priority after exitin=

g

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the loop.  Dependong on the CPU design, 'HMT_low() ; HMT_medium();' a=

s

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cpu_relax() does may have HMT_medium take effect before HMT_low made
any (or much) difference.

Second, it can be beneficial for some implementations to spin on the
exit condition with a statically predicted-not-taken branch (i.e.,
always predict the loop will exit).

This is a quick RFC with a couple of users converted to see what
people think. I don't use a C branch with hints, because we don't wan=

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the compiler moving the loop body out of line, which makes it a bit
messy unfortunately. If there's a better way to do it, I'm all ears.

I think it will still all go wrong if the conditional isn't trivial.
In particular if the condition contains || or && it is likely to
have a branch - which could invert the loop.

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I don't know that it will.
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Yes, if we have exit condition that requires more branches in order to
be computed then we lose our nice property of never taking a branch
miss on loop exit. But we still avoid *this* branch miss, and still
prevent multiple iterations of the wait loop being speculatively
executed concurrently when there's no work to be done.
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And C doesn't know about the loop, so it can't do any transformation
except to compute the final condition.
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Or have I missed something?

Try putting the code inside a conditional or at the bottom of a loop.
gcc can replicate code to remove a branch.

So:
	for (;;) {
		a;
		if (b)
			c;
		d;
	}

can become:
   x1:
	a;
	if (b) to x2;
	d;
	goto x1;

    x2:
	c;
	d;
	goto x1;

Which won't work.

	David