root/arch/x86/kernel/tsc_sync.c

DEFINITIONS

1. check_tsc_warp
2. check_tsc_sync_source
3. check_tsc_sync_target

/*
 * check TSC synchronization.
 *
 * Copyright (C) 2006, Red Hat, Inc., Ingo Molnar
 *
 * We check whether all boot CPUs have their TSC's synchronized,
 * print a warning if not and turn off the TSC clock-source.
 *
 * The warp-check is point-to-point between two CPUs, the CPU
 * initiating the bootup is the 'source CPU', the freshly booting
 * CPU is the 'target CPU'.
 *
 * Only two CPUs may participate - they can enter in any order.
 * ( The serial nature of the boot logic and the CPU hotplug lock
 *   protects against more than 2 CPUs entering this code. )
 */
#include <linux/spinlock.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/smp.h>
#include <linux/nmi.h>
#include <asm/tsc.h>

/*
 * Entry/exit counters that make sure that both CPUs
 * run the measurement code at once:
 */
static __cpuinitdata atomic_t start_count;
static __cpuinitdata atomic_t stop_count;

/*
 * We use a raw spinlock in this exceptional case, because
 * we want to have the fastest, inlined, non-debug version
 * of a critical section, to be able to prove TSC time-warps:
 */
static __cpuinitdata raw_spinlock_t sync_lock = __RAW_SPIN_LOCK_UNLOCKED;
static __cpuinitdata cycles_t last_tsc;
static __cpuinitdata cycles_t max_warp;
static __cpuinitdata int nr_warps;

/*
 * TSC-warp measurement loop running on both CPUs:
 */
static __cpuinit void check_tsc_warp(void)
{
        cycles_t start, now, prev, end;
        int i;

        start = get_cycles();
        /*
         * The measurement runs for 20 msecs:
         */
        end = start + tsc_khz * 20ULL;
        now = start;

        for (i = 0; ; i++) {
                /*
                 * We take the global lock, measure TSC, save the
                 * previous TSC that was measured (possibly on
                 * another CPU) and update the previous TSC timestamp.
                 */
                __raw_spin_lock(&sync_lock);
                prev = last_tsc;
                now = get_cycles();
                last_tsc = now;
                __raw_spin_unlock(&sync_lock);

                /*
                 * Be nice every now and then (and also check whether
                 * measurement is done [we also insert a 10 million
                 * loops safety exit, so we dont lock up in case the
                 * TSC readout is totally broken]):
                 */
                if (unlikely(!(i & 7))) {
                        if (now > end || i > 10000000)
                                break;
                        cpu_relax();
                        touch_nmi_watchdog();
                }
                /*
                 * Outside the critical section we can now see whether
                 * we saw a time-warp of the TSC going backwards:
                 */
                if (unlikely(prev > now)) {
                        __raw_spin_lock(&sync_lock);
                        max_warp = max(max_warp, prev - now);
                        nr_warps++;
                        __raw_spin_unlock(&sync_lock);
                }
        }
        WARN(!(now-start),
                "Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
                        now-start, end-start);
}

/*
 * Source CPU calls into this - it waits for the freshly booted
 * target CPU to arrive and then starts the measurement:
 */
void __cpuinit check_tsc_sync_source(int cpu)
{
        int cpus = 2;

        /*
         * No need to check if we already know that the TSC is not
         * synchronized:
         */
        if (unsynchronized_tsc())
                return;

        printk(KERN_INFO "checking TSC synchronization [CPU#%d -> CPU#%d]:",
                          smp_processor_id(), cpu);

        /*
         * Reset it - in case this is a second bootup:
         */
        atomic_set(&stop_count, 0);

        /*
         * Wait for the target to arrive:
         */
        while (atomic_read(&start_count) != cpus-1)
                cpu_relax();
        /*
         * Trigger the target to continue into the measurement too:
         */
        atomic_inc(&start_count);

        check_tsc_warp();

        while (atomic_read(&stop_count) != cpus-1)
                cpu_relax();

        if (nr_warps) {
                printk("\n");
                printk(KERN_WARNING "Measured %Ld cycles TSC warp between CPUs,"
                                    " turning off TSC clock.\n", max_warp);
                mark_tsc_unstable("check_tsc_sync_source failed");
        } else {
                printk(" passed.\n");
        }

        /*
         * Reset it - just in case we boot another CPU later:
         */
        atomic_set(&start_count, 0);
        nr_warps = 0;
        max_warp = 0;
        last_tsc = 0;

        /*
         * Let the target continue with the bootup:
         */
        atomic_inc(&stop_count);
}

/*
 * Freshly booted CPUs call into this:
 */
void __cpuinit check_tsc_sync_target(void)
{
        int cpus = 2;

        if (unsynchronized_tsc())
                return;

        /*
         * Register this CPU's participation and wait for the
         * source CPU to start the measurement:
         */
        atomic_inc(&start_count);
        while (atomic_read(&start_count) != cpus)
                cpu_relax();

        check_tsc_warp();

        /*
         * Ok, we are done:
         */
        atomic_inc(&stop_count);

        /*
         * Wait for the source CPU to print stuff:
         */
        while (atomic_read(&stop_count) != cpus)
                cpu_relax();
}
#undef NR_LOOPS