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calibrate: home in on correct lpj value more quickly

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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>
This commit is contained in:
Phil Carmody 2011-03-22 16:34:13 -07:00 committed by Linus Torvalds
parent 71c696b1d0
commit 191e56880a
1 changed files with 34 additions and 23 deletions
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57
init/calibrate.c
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@ -110,8 +110,8 @@ static unsigned long __cpuinit calibrate_delay_direct(void) {return 0;}
/*
* 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_direct(void) {return 0;}
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;