kern/167106: [eventtimers] eventtimer tsc smp_test unreliable

[Jim Harris]

The following reply was made to PR kern/167106; it has been noted by GNATS.

From: Jim Harris <jimharris at freebsd.org>
To: bug-followup at FreeBSD.org, claudius at ambtec.de
Cc:  
Subject: Re: kern/167106: [eventtimers] eventtimer tsc smp_test unreliable
Date: Tue, 24 Jul 2012 22:30:31 -0700

 I am almost certain kern/167106 is fixed by r238755, committed today.
 kib@ has already approved this to be MFC'd to stable/9 for upcoming
 9.1 release.  MFC will happen on Friday (27th July) barring any
 unforeseen issues with the patch.
 
 Full commit message below.
 
 Regards,
 
 -Jim
 
 
 Author: jimharris
 Date: Tue Jul 24 22:10:11 2012
 New Revision: 238755
 URL: http://svn.freebsd.org/changeset/base/238755
 
 Log:
   Add rmb() to tsc_read_##x to enforce serialization of rdtsc captures.
 
   Intel Architecture Manual specifies that rdtsc instruction is not serialized,
   so without this change, TSC synchronization test would periodically fail,
   resulting in use of HPET timecounter instead of TSC-low.  This caused
   severe performance degradation (40-50%) when running high IO/s
 workloads due to
   HPET MMIO reads and GEOM stat collection.
 
   Tests on Xeon E5-2600 (Sandy Bridge) 8C systems were seeing TSC
 synchronization
   fail approximately 20% of the time.
 
   Sponsored by: Intel
   Reviewed by: kib
   MFC after: 3 days
 
 Modified:
   head/sys/x86/x86/tsc.c
 
 Modified: head/sys/x86/x86/tsc.c
 ==============================================================================
 --- head/sys/x86/x86/tsc.c      Tue Jul 24 20:15:41 2012        (r238754)
 +++ head/sys/x86/x86/tsc.c      Tue Jul 24 22:10:11 2012        (r238755)
 @@ -328,6 +328,7 @@ init_TSC(void)
 
  #ifdef SMP
 
 +/* rmb is required here because rdtsc is not a serializing instruction. */
  #define        TSC_READ(x)                     \
  static void                            \
  tsc_read_##x(void *arg)                        \
 @@ -335,6 +336,7 @@ tsc_read_##x(void *arg)                     \
         uint32_t *tsc = arg;            \
         u_int cpu = PCPU_GET(cpuid);    \
                                         \
 +       rmb();                          \
         tsc[cpu * 3 + x] = rdtsc32();   \
  }
  TSC_READ(0)