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Add rmb() to tsc_read_##x to enforce serialization of rdtsc captures.

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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
This commit is contained in:
Jim Harris 2012-07-24 22:10:11 +00:00
parent b18b53bad0
commit 3f6e7b9b11
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=238755
1 changed files with 2 additions and 0 deletions
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2
sys/x86/x86/tsc.c
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@ -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)