[RFC][PATCH] spin loop arch primitives for busy waiting

[Peter Zijlstra]

On Thu, Apr 06, 2017 at 08:16:19AM -0700, Linus Torvalds wrote:

> In theory x86 could use monitor/mwait for it too, in practice I think
> it tends to still be too high latency (because it was originally just
> designed for the idle loop). mwait got extended to actually be useful,
> but I'm not sure what the latency is for the modern one.

I've been meaning to test mwait-c0 for this, but never got around to it.

Something like the below, which is ugly (because I couldn't be bothered
to resolve the header recursion and thus duplicates the monitor/mwait
functions) and broken (because it hard assumes the hardware can do
monitor/mwait).

But it builds and boots, no clue if its better or worse. Changing mwait
eax to 0 would give us C1 and might also be worth a try I suppose.

---
 arch/x86/include/asm/barrier.h | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index bfb28ca..faab9cd 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -80,6 +80,36 @@ do {									\
 #define __smp_mb__before_atomic()	barrier()
 #define __smp_mb__after_atomic()	barrier()
 
+static inline void ___monitor(const void *eax, unsigned long ecx,
+			     unsigned long edx)
+{
+	/* "monitor %eax, %ecx, %edx;" */
+	asm volatile(".byte 0x0f, 0x01, 0xc8;"
+		     :: "a" (eax), "c" (ecx), "d"(edx));
+}
+
+static inline void ___mwait(unsigned long eax, unsigned long ecx)
+{
+	/* "mwait %eax, %ecx;" */
+	asm volatile(".byte 0x0f, 0x01, 0xc9;"
+		     :: "a" (eax), "c" (ecx));
+}
+
+#define smp_cond_load_acquire(ptr, cond_expr)				\
+({									\
+	typeof(ptr) __PTR = (ptr);					\
+	typeof(*ptr) VAL;						\
+	for (;;) {							\
+		___monitor(__PTR, 0, 0);				\
+		VAL = READ_ONCE(*__PTR);				\
+		if (cond_expr)						\
+			break;						\
+		___mwait(0xf0 /* C0 */, 0x01 /* INT */);		\
+	}								\
+	smp_acquire__after_ctrl_dep();					\
+	VAL;								\
+})
+
 #include <asm-generic/barrier.h>
 
 #endif /* _ASM_X86_BARRIER_H */