[PATCH v2 2/7] asm-generic: barrier: add wait_policy handlers

[Ankur Arora]

smp_cond_load_relaxed_timewait() waits on a conditional variable
while either spinning or via some architectural primitive, while
also watching the clock.

The generic code presents the simple case where the waiting is done
exclusively via a cpu_relax() spin-wait loop. To keep the pipeline
as idle as possible, we want to do the time-check only intermittently.
How often the time-check is done -- which also determines how much we
overshoot the timeout by, is configured via the __smp_cond_timewait_coarse()
and __smp_cond_timewait_fine() wait policies.

Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Will Deacon <will at kernel.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Kumar Kartikeya Dwivedi <memxor at gmail.com>
Cc: Alexei Starovoitov <ast at kernel.org>
Cc: linux-arch at vger.kernel.org
Signed-off-by: Ankur Arora <ankur.a.arora at oracle.com>
---
 include/asm-generic/barrier.h | 66 +++++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index a7be98e906f4..76124683be4b 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -15,6 +15,7 @@
 
 #include <linux/compiler.h>
 #include <linux/kcsan-checks.h>
+#include <linux/minmax.h>
 #include <asm/rwonce.h>
 
 #ifndef nop
@@ -273,6 +274,64 @@ do {									\
 })
 #endif
 
+#ifndef SMP_TIMEWAIT_SPIN_BASE
+#define SMP_TIMEWAIT_SPIN_BASE		16
+#endif
+
+static inline u64 ___cond_spinwait(u64 now, u64 prev, u64 end,
+				   u32 *spin, bool *wait, u64 slack)
+{
+	if (now >= end)
+		return 0;
+
+	*wait = false;
+
+	/*
+	 * Scale the spin-count up or down so we evaluate the time-expr every
+	 * slack unit of time or so.
+	 */
+	if ((now - prev) < slack)
+		*spin <<= 1;
+	else
+		/*
+		 * Ensure the spin-count is at least SMP_TIMEWAIT_SPIN_BASE
+		 * when scaling down to guard against artificially low values
+		 * due to interrupts etc. Clamping down also handles the case
+		 * of the first iteration (*spin == 0).
+		 */
+		*spin = max((*spin >> 1) + (*spin >> 2), SMP_TIMEWAIT_SPIN_BASE);
+
+	return now;
+}
+
+#ifndef SMP_TIMEWAIT_SLACK_FINE_US
+#define SMP_TIMEWAIT_SLACK_FINE_US	2UL
+#endif
+
+#ifndef SMP_TIMEWAIT_SLACK_COARSE_US
+#define SMP_TIMEWAIT_SLACK_COARSE_US	5UL
+#endif
+
+/*
+ * wait_policy: to minimize how often we do the (typically) expensive
+ * time-check, expect a slack duration which would vary based on
+ * architecture.
+ *
+ * For the generic variant, the fine and coarse variants have a slack
+ * duration of SMP_TIMEWAIT_SLACK_FINE_US and SMP_TIMEWAIT_SLACK_COARSE_US.
+ */
+#ifndef __smp_cond_timewait_fine
+#define __smp_cond_timewait_fine(now, prev, end, spin, wait)	\
+	___cond_spinwait(now, prev, end, spin, wait,		\
+			    SMP_TIMEWAIT_SLACK_FINE_US)
+#endif
+
+#ifndef __smp_cond_timewait_coarse
+#define __smp_cond_timewait_coarse(now, prev, end, spin, wait)	\
+	___cond_spinwait(now, prev, end, spin, wait,		\
+			    SMP_TIMEWAIT_SLACK_COARSE_US)
+#endif
+
 /*
  * Non-spin primitive that allows waiting for stores to an address,
  * with support for a timeout. This works in conjunction with an
@@ -320,11 +379,18 @@ do {									\
  * @time_expr: monotonic expression that evaluates to the current time
  * @time_end: compared against time_expr
  *
+ * The default policies (__smp_cond_timewait_coarse, __smp_cond_timewait_fine)
+ * assume that time_expr and time_end evaluate to time in us (both with a user
+ * specified precision.)
+ * With a user specified policy, any units and precision can be used.
+ *
  * Equivalent to using READ_ONCE() on the condition variable.
  */
 #define smp_cond_load_relaxed_timewait(ptr, cond_expr, wait_policy,	\
 					 time_expr, time_end) ({	\
 	__unqual_scalar_typeof(*ptr) _val;;				\
+	BUILD_BUG_ON_MSG(!__same_type(typeof(time_expr), u64),		\
+			 "incompatible time units");			\
 	_val = __smp_cond_load_relaxed_timewait(ptr, cond_expr,		\
 					      wait_policy, time_expr,	\
 					      time_end);		\
-- 
2.43.5