sched_clock.c

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/*
 * sched_clock.c: support for extending counters to full 64-bit ns counter
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */
#include <linux/clocksource.h>
#include <linux/init.h>
#include <linux/jiffies.h>
#include <linux/kernel.h>
#include <linux/moduleparam.h>
#include <linux/sched.h>
#include <linux/syscore_ops.h>
#include <linux/timer.h>

#include <asm/sched_clock.h>

struct clock_data {
    u64 epoch_ns;
    u32 epoch_cyc;
    u32 epoch_cyc_copy;
    u32 mult;
    u32 shift;
    bool suspended;
    bool needs_suspend;
};

static void sched_clock_poll(unsigned long wrap_ticks);
static DEFINE_TIMER(sched_clock_timer, sched_clock_poll, 0, 0);
static int irqtime = -1;

core_param(irqtime, irqtime, int, 0400);

static struct clock_data cd = {
    .mult   = NSEC_PER_SEC / HZ,
};

static u32 __read_mostly sched_clock_mask = 0xffffffff;

static u32 notrace jiffy_sched_clock_read(void)
{
    return (u32)(jiffies - INITIAL_JIFFIES);
}

static u32 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;

static inline u64 cyc_to_ns(u64 cyc, u32 mult, u32 shift)
{
    return (cyc * mult) >> shift;
}

static unsigned long long cyc_to_sched_clock(u32 cyc, u32 mask)
{
    u64 epoch_ns;
    u32 epoch_cyc;

    if (cd.suspended)
        return cd.epoch_ns;

    /*
     * Load the epoch_cyc and epoch_ns atomically.  We do this by
     * ensuring that we always write epoch_cyc, epoch_ns and
     * epoch_cyc_copy in strict order, and read them in strict order.
     * If epoch_cyc and epoch_cyc_copy are not equal, then we're in
     * the middle of an update, and we should repeat the load.
     */
    do {
        epoch_cyc = cd.epoch_cyc;
        smp_rmb();
        epoch_ns = cd.epoch_ns;
        smp_rmb();
    } while (epoch_cyc != cd.epoch_cyc_copy);

    return epoch_ns + cyc_to_ns((cyc - epoch_cyc) & mask, cd.mult, cd.shift);
}

/*
 * Atomically update the sched_clock epoch.
 */
static void notrace update_sched_clock(void)
{
    unsigned long flags;
    u32 cyc;
    u64 ns;

    cyc = read_sched_clock();
    ns = cd.epoch_ns +
        cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
              cd.mult, cd.shift);
    /*
     * Write epoch_cyc and epoch_ns in a way that the update is
     * detectable in cyc_to_fixed_sched_clock().
     */
    raw_local_irq_save(flags);
    cd.epoch_cyc = cyc;
    smp_wmb();
    cd.epoch_ns = ns;
    smp_wmb();
    cd.epoch_cyc_copy = cyc;
    raw_local_irq_restore(flags);
}

static void sched_clock_poll(unsigned long wrap_ticks)
{
    mod_timer(&sched_clock_timer, round_jiffies(jiffies + wrap_ticks));
    update_sched_clock();
}

void __init setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate)
{
    unsigned long r, w;
    u64 res, wrap;
    char r_unit;

    BUG_ON(bits > 32);
    WARN_ON(!irqs_disabled());
    WARN_ON(read_sched_clock != jiffy_sched_clock_read);
    read_sched_clock = read;
    sched_clock_mask = (1 << bits) - 1;

    /* calculate the mult/shift to convert counter ticks to ns. */
    clocks_calc_mult_shift(&cd.mult, &cd.shift, rate, NSEC_PER_SEC, 0);

    r = rate;
    if (r >= 4000000) {
        r /= 1000000;
        r_unit = 'M';
    } else if (r >= 1000) {
        r /= 1000;
        r_unit = 'k';
    } else
        r_unit = ' ';

    /* calculate how many ns until we wrap */
    wrap = cyc_to_ns((1ULL << bits) - 1, cd.mult, cd.shift);
    do_div(wrap, NSEC_PER_MSEC);
    w = wrap;

    /* calculate the ns resolution of this counter */
    res = cyc_to_ns(1ULL, cd.mult, cd.shift);
    pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lums\n",
        bits, r, r_unit, res, w);

    /*
     * Start the timer to keep sched_clock() properly updated and
     * sets the initial epoch.
     */
    sched_clock_timer.data = msecs_to_jiffies(w - (w / 10));
    update_sched_clock();

    /*
     * Ensure that sched_clock() starts off at 0ns
     */
    cd.epoch_ns = 0;

    /* Enable IRQ time accounting if we have a fast enough sched_clock */
    if (irqtime > 0 || (irqtime == -1 && rate >= 1000000))
        enable_sched_clock_irqtime();

    pr_debug("Registered %pF as sched_clock source\n", read);
}

unsigned long long notrace sched_clock(void)
{
    u32 cyc = read_sched_clock();
    return cyc_to_sched_clock(cyc, sched_clock_mask);
}

void __init sched_clock_postinit(void)
{
    /*
     * If no sched_clock function has been provided at that point,
     * make it the final one one.
     */
    if (read_sched_clock == jiffy_sched_clock_read)
        setup_sched_clock(jiffy_sched_clock_read, 32, HZ);

    sched_clock_poll(sched_clock_timer.data);
}

static int sched_clock_suspend(void)
{
    sched_clock_poll(sched_clock_timer.data);
    cd.suspended = true;
    return 0;
}

static void sched_clock_resume(void)
{
    cd.epoch_cyc = read_sched_clock();
    cd.epoch_cyc_copy = cd.epoch_cyc;
    cd.suspended = false;
}

static struct syscore_ops sched_clock_ops = {
    .suspend = sched_clock_suspend,
    .resume = sched_clock_resume,
};

static int __init sched_clock_syscore_init(void)
{
    register_syscore_ops(&sched_clock_ops);
    return 0;
}
device_initcall(sched_clock_syscore_init);