svn commit: r246116 - head/sys/x86/x86

[Konstantin Belousov]

Author: kib
Date: Wed Jan 30 12:43:10 2013
New Revision: 246116
URL: http://svnweb.freebsd.org/changeset/base/246116

Log:
  Reduce default shift used to calculate the max frequency for the TSC
  timecounter to 1, and correspondingly increase the precision of the
  gettimeofday(2) and related functions in the default configuration.
  
  The motivation for the TSC-low timecounter, as described in the
  r222866, seems to provide a workaround for the non-serializing
  behaviour of the RDTSC on some Intel hardware.  Tests demonstrate that
  even with the pre-shift of 8, the cross-core non-monotonicity of the
  RDTSC is still observed reliably, e.g. on the Nehalems.  The r238755
  and r238973 implemented the proper fix for the issue.
  
  The pre-shift of 1 is applied to keep TSC not overflowing for the
  frequency of hardclock down to 2 sec/intr.  The pre-shift is made a
  tunable to allow the easy debugging of the issues users could see with
  the shift being too low.
  
  Reviewed by:	bde
  MFC after:	2 weeks

Modified:
  head/sys/x86/x86/tsc.c

Modified: head/sys/x86/x86/tsc.c
==============================================================================
--- head/sys/x86/x86/tsc.c	Wed Jan 30 10:59:42 2013	(r246115)
+++ head/sys/x86/x86/tsc.c	Wed Jan 30 12:43:10 2013	(r246116)
@@ -65,6 +65,12 @@ static int	smp_tsc;
 SYSCTL_INT(_kern_timecounter, OID_AUTO, smp_tsc, CTLFLAG_RDTUN, &smp_tsc, 0,
     "Indicates whether the TSC is safe to use in SMP mode");
 TUNABLE_INT("kern.timecounter.smp_tsc", &smp_tsc);
+
+static int	smp_tsc_shift = 1;
+SYSCTL_INT(_kern_timecounter, OID_AUTO, smp_tsc_shift, CTLFLAG_RDTUN,
+    &smp_tsc_shift, 0,
+    "Shift to pre-apply for the maximum TSC frequency in SMP mode");
+TUNABLE_INT("kern.timecounter.smp_tsc_shift", &smp_tsc_shift);
 #endif
 
 static int	tsc_disabled;
@@ -506,7 +512,7 @@ init_TSC_tc(void)
 			tsc_timecounter.tc_quality = -100;
 		} else {
 			tsc_timecounter.tc_quality = test_smp_tsc();
-			max_freq >>= 8;
+			max_freq >>= smp_tsc_shift;
 		}
 	} else
 #endif