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

From: Nicholas Piggin

Sent: 16 September 2016 09:58
Implementing busy wait loops with cpu_relax() in callers poses
some difficulties for powerpc.
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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 exiting
the loop.  Dependong on the CPU design, 'HMT_low() ; HMT_medium();' as
cpu_relax() does may have HMT_medium take effect before HMT_low made
any (or much) difference.
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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).
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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 want
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.

	David