svn commit: r340450 - head/sys/sys

Warner Losh imp at FreeBSD.org
Thu Nov 15 16:02:14 UTC 2018


Author: imp
Date: Thu Nov 15 16:02:13 2018
New Revision: 340450
URL: https://svnweb.freebsd.org/changeset/base/340450

Log:
  When converting ns,us,ms to sbt, return the ceil() of the result
  rather than the floor(). Returning the floor means that
  sbttoX(Xtosbt(y)) != y for almost all values of y.  In practice, this
  results in a difference of at most 1 in the lsb of the sbintime_t.
  This difference is meaningless for all current users of these
  functions, but is important for the newly introduced sysctl conversion
  routines which implicitly rely on the transformation being idempotent.
  
  Sponsored by: Netflix, Inc

Modified:
  head/sys/sys/time.h

Modified: head/sys/sys/time.h
==============================================================================
--- head/sys/sys/time.h	Thu Nov 15 08:43:17 2018	(r340449)
+++ head/sys/sys/time.h	Thu Nov 15 16:02:13 2018	(r340450)
@@ -161,6 +161,10 @@ sbttobt(sbintime_t _sbt)
  * Decimal<->sbt conversions.  Multiplying or dividing by SBT_1NS results in
  * large roundoff errors which sbttons() and nstosbt() avoid.  Millisecond and
  * microsecond functions are also provided for completeness.
+ *
+ * These functions return the smallest sbt larger or equal to the number of
+ * seconds requested so that sbttoX(Xtosbt(y)) == y. The 1 << 32 - 1 term added
+ * transforms the >> 32 from floor() to ceil().
  */
 static __inline int64_t
 sbttons(sbintime_t _sbt)
@@ -173,7 +177,7 @@ static __inline sbintime_t
 nstosbt(int64_t _ns)
 {
 
-	return ((_ns * (((uint64_t)1 << 63) / 500000000)) >> 32);
+	return ((_ns * (((uint64_t)1 << 63) / 500000000) + (1ull << 32) - 1) >> 32);
 }
 
 static __inline int64_t
@@ -187,7 +191,7 @@ static __inline sbintime_t
 ustosbt(int64_t _us)
 {
 
-	return ((_us * (((uint64_t)1 << 63) / 500000)) >> 32);
+	return ((_us * (((uint64_t)1 << 63) / 500000) + (1ull << 32) - 1) >> 32);
 }
 
 static __inline int64_t
@@ -201,7 +205,7 @@ static __inline sbintime_t
 mstosbt(int64_t _ms)
 {
 
-	return ((_ms * (((uint64_t)1 << 63) / 500)) >> 32);
+	return ((_ms * (((uint64_t)1 << 63) / 500) + (1ull << 32) - 1) >> 32);
 }
 
 /*-


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