Commit-ID: f125be1469303f7b9324447f251d74a0da24952f
Gitweb: http://git.kernel.org/tip/f125be1469303f7b9324447f251d74a0da24952f
Author: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
AuthorDate: Mon, 18 Jan 2010 11:25:45 -0600
Committer: Robert Richter <robert.richter@xxxxxxx>
CommitDate: Fri, 26 Feb 2010 15:14:02 +0100
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@xxxxxxx>
Signed-off-by: Robert Richter <robert.richter@xxxxxxx>
---
arch/x86/oprofile/op_model_amd.c | 23 +++++++++++++++++++++++
1 files changed, 23 insertions(+), 0 deletions(-)
diff --git a/arch/x86/oprofile/op_model_amd.c b/arch/x86/oprofile/op_model_amd.c
index 6557683..97c84eb 100644
--- a/arch/x86/oprofile/op_model_amd.c
+++ b/arch/x86/oprofile/op_model_amd.c
@@ -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)
--
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