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Date: Mon, 01 Jun 2015 10:57:46 +0300
From: Adrian Hunter <adrian.hunter@...el.com>
To: Thomas Gleixner <tglx@...utronix.de>
CC: linux-kernel@...r.kernel.org,
Andy Lutomirski <luto@...capital.net>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andi Kleen <ak@...ux.intel.com>, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC] x86, tsc: Allow for high latency in quick_pit_calibrate()
On 21/05/15 10:55, Adrian Hunter wrote:
> If it takes longer than 12us to read the PIT counter lsb/msb,
> then the error margin will never fall below 500ppm within 50ms,
> and Fast TSC calibration will always fail.
Hi
This does happen, so it would be nice to have this fix.
Are there any comments?
Regards
Adrian
>
> This patch detects when that will happen and switches to using
> a slightly different algorithm that takes advantage of the PIT's
> latch comand.
>
> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> ---
> arch/x86/kernel/tsc.c | 133 +++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 132 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 5054497..0035682 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -553,6 +553,40 @@ static inline int pit_expect_msb(unsigned char val, u64 *tscp, unsigned long *de
> }
>
> /*
> + * High latency version of pit_expect_msb(). Instead of using the read latency
> + * as the error margin, latch the counter and use that latency. The counter
> + * latches at the current value which means there is an addition error margin
> + * of 1 timer tick which is not accounted for here.
> + */
> +static inline int pit_expect_msb_hl(unsigned char val, u64 *tscp,
> + unsigned long *deltap, u16 *cnt)
> +{
> + u64 tsc0, tsc1;
> + int count;
> + u8 lsb, msb;
> +
> + for (count = 0; count < 50000; count++) {
> + tsc0 = get_cycles();
> + /* Latch counter 2 */
> + outb(0x80, 0x43);
> + tsc1 = get_cycles();
> + lsb = inb(0x42);
> + msb = inb(0x42);
> + if (msb != val)
> + break;
> + }
> + *deltap = tsc1 - tsc0;
> + *tscp = tsc0;
> + *cnt = lsb | ((u16)msb << 8);
> +
> + /*
> + * We require _some_ success, but the quality control
> + * will be based on the error terms on the TSC values.
> + */
> + return count > 5;
> +}
> +
> +/*
> * 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
> @@ -561,6 +595,94 @@ static inline int pit_expect_msb(unsigned char val, u64 *tscp, unsigned long *de
> #define MAX_QUICK_PIT_MS 50
> #define MAX_QUICK_PIT_ITERATIONS (MAX_QUICK_PIT_MS * PIT_TICK_RATE / 1000 / 256)
>
> +/*
> + * High latency version of quick_pit_calibrate() that works even if there is a
> + * high latency reading the PIT lsb/msb. This is needed if it takes longer than
> + * 12us to read the lsb/msb because then the error margin will never fall below
> + * 500ppm within 50ms.
> + */
> +static unsigned long quick_pit_calibrate_hl(void)
> +{
> + int i;
> + u64 tsc, delta;
> + unsigned long d1, d2;
> + u16 cnt0, cnt1, dc;
> +
> + /* Re-start at 0xffff */
> + outb(0xff, 0x42);
> + outb(0xff, 0x42);
> +
> + /*
> + * The PIT starts counting at the next edge, so we
> + * need to delay for a microsecond. The easiest way
> + * to do that is to just read back the 16-bit counter
> + * once from the PIT.
> + */
> + pit_verify_msb(0);
> +
> + /*
> + * Iterate until the error is less than 500 ppm. The error margin due to
> + * the time to latch the counter is d1 + d2. The counter latches the
> + * current count which introduces a one tick error at the start and end.
> + * So the total error is d1 + d2 + 2 timer ticks. A timer tick is
> + * approximately the TSC delta divided by the timer delta. So the error
> + * margin is too high while:
> + * d1 + d2 + 2 * (delta / dc) >= delta >> 11
> + * => d1 + d2 >= (delta >> 11) - 2 * (delta / dc)
> + * => d1 + d2 >= (delta - 4096 * (delta / dc)) / 2048
> + * => (d1 + d2) * dc >= (dc * delta - 4096 * delta) / 2048
> + * => (d1 + d2) * dc >= (dc - 4096) * delta >> 11
> + */
> + if (pit_expect_msb_hl(0xff, &tsc, &d1, &cnt0)) {
> + for (i = 1; i <= MAX_QUICK_PIT_ITERATIONS; i++) {
> + if (!pit_expect_msb_hl(0xff - i, &delta, &d2, &cnt1))
> + break;
> +
> + dc = cnt0 - cnt1;
> + if (dc < 4096)
> + continue;
> +
> + delta -= tsc;
> +
> + if ((d1 + d2) * (u64)dc >= (dc - 4096) * delta >> 11)
> + continue;
> +
> + /*
> + * Check the PIT one more time to verify that
> + * all TSC reads were stable wrt the PIT.
> + *
> + * This also guarantees serialization of the
> + * last cycle read ('d2') in pit_expect_msb.
> + */
> + if (!pit_verify_msb(0xfe - i))
> + break;
> + goto success;
> + }
> + }
> + pr_info("Fast TSC calibration (with latch) failed\n");
> + return 0;
> +
> +success:
> + /*
> + * Ok, if we get here, then we've seen the
> + * MSB of the PIT decrement 'i' times, and the
> + * error has shrunk to less than 500 ppm.
> + *
> + * As a result, we can depend on there not being
> + * any odd delays anywhere, and the TSC reads are
> + * reliable (within the error).
> + *
> + * kHz = ticks / time-in-seconds / 1000;
> + * kHz = ticks / (PIT count / PIT_TICK_RATE) / 1000
> + * kHz = delta / (dc / PIT_TICK_RATE) / 1000
> + * kHz = (delta * PIT_TICK_RATE) / (dc * 1000)
> + */
> + delta *= PIT_TICK_RATE;
> + do_div(delta, dc * 1000);
> + pr_info("Fast TSC calibration (with latch) using PIT\n");
> + return delta;
> +}
> +
> static unsigned long quick_pit_calibrate(void)
> {
> int i;
> @@ -598,10 +720,19 @@ static unsigned long quick_pit_calibrate(void)
> if (!pit_expect_msb(0xff-i, &delta, &d2))
> break;
>
> + delta -= tsc;
> +
> + /*
> + * Extrapolate the error and switch to high-latency
> + * algorithm if the error will never be below 500 ppm.
> + */
> + if (i == 1 &&
> + d1 + d2 >= (delta * MAX_QUICK_PIT_ITERATIONS) >> 11)
> + return quick_pit_calibrate_hl();
> +
> /*
> * Iterate until the error is less than 500 ppm
> */
> - delta -= tsc;
> if (d1+d2 >= delta >> 11)
> continue;
>
>
--
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