calibrate: home in on correct lpj value more quickly
Binary chop with a jiffy-resync on each step to find an upper bound is
slow, so just race in a tight-ish loop to find an underestimate.
If done with lots of individual steps, sometimes several hundreds of
iterations would be required, which would impose a significant overhead,
and make the initial estimate very low. By taking slowly increasing steps
there will be less overhead.
E.g. an x86_64 2.67GHz could have fitted in 613 individual small delays,
but in reality should have been able to fit in a single delay 644 times
longer, so underestimated by 31 steps. To reach the equivalent of 644
small delays with the accelerating scheme now requires about 130
iterations, so has <1/4th of the overhead, and can therefore be expected
to underestimate by only 7 steps.
As now we have a better initial estimate we can binary chop over a smaller
range. With the loop overhead in the initial estimate kept low, and the
step sizes moderate, we won't have under-estimated by much, so chose as
tight a range as we can.
Signed-off-by: Phil Carmody <ext-phil.2.carmody@nokia.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Tested-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/init/calibrate.c b/init/calibrate.c
index b71643a..f9000df 100644
--- a/init/calibrate.c
+++ b/init/calibrate.c
@@ -110,8 +110,8 @@
/*
* This is the number of bits of precision for the loops_per_jiffy. Each
- * bit takes on average 1.5/HZ seconds. This (like the original) is a little
- * better than 1%
+ * time we refine our estimate after the first takes 1.5/HZ seconds, so try
+ * to start with a good estimate.
* For the boot cpu we can skip the delay calibration and assign it a value
* calculated based on the timer frequency.
* For the rest of the CPUs we cannot assume that the timer frequency is same as
@@ -121,38 +121,49 @@
static unsigned long __cpuinit calibrate_delay_converge(void)
{
- unsigned long lpj, ticks, loopbit;
- int lps_precision = LPS_PREC;
+ /* First stage - slowly accelerate to find initial bounds */
+ unsigned long lpj, ticks, loopadd, chop_limit;
+ int trials = 0, band = 0, trial_in_band = 0;
lpj = (1<<12);
- while ((lpj <<= 1) != 0) {
- /* wait for "start of" clock tick */
- ticks = jiffies;
- while (ticks == jiffies)
- /* nothing */;
- /* Go .. */
- ticks = jiffies;
- __delay(lpj);
- ticks = jiffies - ticks;
- if (ticks)
- break;
- }
+
+ /* wait for "start of" clock tick */
+ ticks = jiffies;
+ while (ticks == jiffies)
+ ; /* nothing */
+ /* Go .. */
+ ticks = jiffies;
+ do {
+ if (++trial_in_band == (1<<band)) {
+ ++band;
+ trial_in_band = 0;
+ }
+ __delay(lpj * band);
+ trials += band;
+ } while (ticks == jiffies);
+ /*
+ * We overshot, so retreat to a clear underestimate. Then estimate
+ * the largest likely undershoot. This defines our chop bounds.
+ */
+ trials -= band;
+ loopadd = lpj * band;
+ lpj *= trials;
+ chop_limit = lpj >> (LPS_PREC + 1);
/*
* Do a binary approximation to get lpj set to
- * equal one clock (up to lps_precision bits)
+ * equal one clock (up to LPS_PREC bits)
*/
- lpj >>= 1;
- loopbit = lpj;
- while (lps_precision-- && (loopbit >>= 1)) {
- lpj |= loopbit;
+ while (loopadd > chop_limit) {
+ lpj += loopadd;
ticks = jiffies;
while (ticks == jiffies)
- /* nothing */;
+ ; /* nothing */
ticks = jiffies;
__delay(lpj);
if (jiffies != ticks) /* longer than 1 tick */
- lpj &= ~loopbit;
+ lpj -= loopadd;
+ loopadd >>= 1;
}
return lpj;