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oprofile/x86: implement lsfr pseudo-random number generator for IBS 

This patch implements a linear feedback shift register (LFSR) for
pseudo-random number generation for IBS.

For IBS measurements it would be good to minimize memory traffic in
the interrupt handler since every access pollutes the data
caches. Computing a maximal period LFSR just needs shifts and ORs.

The LFSR method is good enough to randomize the ops at low
overhead. 16 pseudo-random bits are enough for the implementation and
it doesn't matter that the pattern repeats with a fairly short
cycle. It only needs to break up (hard) periodic sampling behavior.

The logic was designed by Paul Drongowski.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>
This commit is contained in:
Suravee Suthikulpanit 2010-01-18 11:25:45 -06:00 committed by Robert Richter
parent 64683da664
commit f125be1469
1 changed files with 23 additions and 0 deletions
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23
arch/x86/oprofile/op_model_amd.c
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@ -218,6 +218,29 @@ static void op_amd_setup_ctrs(struct op_x86_model_spec const *model,
}
}
/*
* 16-bit Linear Feedback Shift Register (LFSR)
*
* 16 14 13 11
* Feedback polynomial = X + X + X + X + 1
*/
static unsigned int lfsr_random(void)
{
static unsigned int lfsr_value = 0xF00D;
unsigned int bit;
/* Compute next bit to shift in */
bit = ((lfsr_value >> 0) ^
(lfsr_value >> 2) ^
(lfsr_value >> 3) ^
(lfsr_value >> 5)) & 0x0001;
/* Advance to next register value */
lfsr_value = (lfsr_value >> 1) | (bit << 15);
return lfsr_value;
}
static inline void
op_amd_handle_ibs(struct pt_regs * const regs,
struct op_msrs const * const msrs)