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Old 03-27-2012, 02:34 PM
Tim Gardner
Default x86, tsc: Fix SMI induced variation in quick_pit_calibrate()

From: Linus Torvalds <torvalds@linux-foundation.org>

BugLink: http://bugs.launchpad.net/bugs/965586

pit_expect_msb() returns success wrongly in the below SMI scenario:

a. pit_verify_msb() has not yet seen the MSB transition.

b. we are close to the MSB transition though and got a SMI immediately after
returning from pit_verify_msb() which didn't see the MSB transition. PIT MSB
transition has happened somewhere during SMI execution.

c. returned from SMI and we noted down the 'tsc', saw the pit MSB change now and
exited the loop to calculate 'deltatsc'. Instead of noting the TSC at the MSB
transition, we are way off because of the SMI. And as the SMI happened
between the pit_verify_msb() and before the 'tsc' is recorded in the
for loop, 'delattsc' (d1/d2 in quick_pit_calibrate()) will be small and
quick_pit_calibrate() will not notice this error.

Depending on whether SMI disturbance happens while computing d1 or d2, we will
see the TSC calibrated value smaller or bigger than the expected value. As a
result, in a cluster we were seeing a variation of approximately +/- 20MHz in
the calibrated values, resulting in NTP failures.

[ As far as the SMI source is concerned, this is a periodic SMI that gets
disabled after ACPI is enabled by the OS. But the TSC calibration happens
before the ACPI is enabled. ]

To address this, change pit_expect_msb() so that

- the 'tsc' is the TSC in between the two reads that read the MSB
change from the PIT (same as before)

- the 'delta' is the difference in TSC from *before* the MSB changed
to *after* the MSB changed.

Now the delta is twice as big as before (it covers four PIT accesses,
roughly 4us) and quick_pit_calibrate() will loop a bit longer to get
the calibrated value with in the 500ppm precision. As the delta (d1/d2)
covers four PIT accesses, actual calibrated result might be closer to
250ppm precision.

As the loop now takes longer to stabilize, double MAX_QUICK_PIT_MS to 50.

SMI disturbance will showup as much larger delta's and the loop will take
longer than usual for the result to be with in the accepted precision. Or will
fallback to slow PIT calibration if it takes more than 50msec.

Also while we are at this, remove the calibration correction that aims to
get the result to the middle of the error bars. We really don't know which
direction to correct into, so remove it.

Reported-and-tested-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Link: http://lkml.kernel.org/r/1326843337.5291.4.camel@sbsiddha-mobl2
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
(cherry picked from commit 68f30fbee19cc67849b9fa8e153ede70758afe81)

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
arch/x86/kernel/tsc.c | 14 ++++++--------
1 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 6cc6922..fc60bd9 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -291,14 +291,15 @@ static inline int pit_verify_msb(unsigned char val)
static inline int pit_expect_msb(unsigned char val, u64 *tscp, unsigned long *deltap)
int count;
- u64 tsc = 0;
+ u64 tsc = 0, prev_tsc = 0;

for (count = 0; count < 50000; count++) {
if (!pit_verify_msb(val))
+ prev_tsc = tsc;
tsc = get_cycles();
- *deltap = get_cycles() - tsc;
+ *deltap = get_cycles() - prev_tsc;
*tscp = tsc;

@@ -312,9 +313,9 @@ static inline int pit_expect_msb(unsigned char val, u64 *tscp, unsigned long *de
* How many MSB values do we want to see? We aim for
* a maximum error rate of 500ppm (in practice the
* real error is much smaller), but refuse to spend
- * more than 25ms on it.
+ * more than 50ms on it.
-#define MAX_QUICK_PIT_MS 25
+#define MAX_QUICK_PIT_MS 50

static unsigned long quick_pit_calibrate(void)
@@ -384,15 +385,12 @@ success:
* As a result, we can depend on there not being
* any odd delays anywhere, and the TSC reads are
- * reliable (within the error). We also adjust the
- * delta to the middle of the error bars, just
- * because it looks nicer.
+ * reliable (within the error).
* kHz = ticks / time-in-seconds / 1000;
* kHz = (t2 - t1) / (I * 256 / PIT_TICK_RATE) / 1000
* kHz = ((t2 - t1) * PIT_TICK_RATE) / (I * 256 * 1000)
- delta += (long)(d2 - d1)/2;
delta *= PIT_TICK_RATE;
do_div(delta, i*256*1000);
printk("Fast TSC calibration using PIT

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