Date: Wed, 25 Jul 2012 05:40:06 GMT From: Jim Harris <jimharris@freebsd.org> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/167106: [eventtimers] eventtimer tsc smp_test unreliable Message-ID: <201207250540.q6P5e6ND048624@freefall.freebsd.org>
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The following reply was made to PR kern/167106; it has been noted by GNATS. From: Jim Harris <jimharris@freebsd.org> To: bug-followup@FreeBSD.org, claudius@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)
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