/*
* 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/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 atomic_t start_count;
static 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 arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;
static cycles_t last_tsc;
static cycles_t max_warp;
static int nr_warps;
/*
* TSC-warp measurement loop running on both CPUs:
*/
static void check_tsc_warp(unsigned int timeout)
{
cycles_t start, now, prev, end;
int i;
rdtsc_barrier();
start = get_cycles();
rdtsc_barrier();
/*
* The measurement runs for 'timeout' msecs:
*/
end = start + (cycles_t) tsc_khz * timeout;
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.
*/
arch_spin_lock(&sync_lock);
prev = last_tsc;
rdtsc_barrier();
now = get_cycles();
rdtsc_barrier();
last_tsc = now;
arch_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)) {
arch_spin_lock(&sync_lock);
max_warp = max(max_warp, prev - now);
nr_warps++;
arch_spin_unlock(&sync_lock);
}
}
WARN(!(now-start),
"Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
now-start, end-start);
}
/*
* If the target CPU coming online doesn't have any of its core-siblings
* online, a timeout of 20msec will be used for the TSC-warp measurement
* loop. Otherwise a smaller timeout of 2msec will be used, as we have some
* information about this socket already (and this information grows as we
* have more and more logical-siblings in that socket).
*
* Ideally we should be able to skip the TSC sync check on the other
* core-siblings, if the first logical CPU in a socket passed the sync test.
* But as the TSC is per-logical CPU and can potentially be modified wrongly
* by the bios, TSC sync test for smaller duration should be able
* to catch such errors. Also this will catch the condition where all the
* cores in the socket doesn't get reset at the same time.
*/
static inline unsigned int loop_timeout(int cpu)
{
return (cpumask_weight(cpu_core_mask(cpu)) > 1) ? 2 : 20;
}
/*
* Source CPU calls into this - it waits for the freshly booted
* target CPU to arrive and then starts the measurement:
*/
void 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;
if (tsc_clocksource_reliable) {
if (cpu == (nr_cpu_ids-1) || system_state != SYSTEM_BOOTING)
pr_info(
"Skipped synchronization checks as TSC is reliable.\n");
return;
}
/*
* 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(loop_timeout(cpu));
while (atomic_read(&stop_count) != cpus-1)
cpu_relax();
if (nr_warps) {
pr_warning("TSC synchronization [CPU#%d -> CPU#%d]:\n",
smp_processor_id(), cpu);
pr_warning("Measured %Ld cycles TSC warp between CPUs, "
"turning off TSC clock.\n", max_warp);
mark_tsc_unstable("check_tsc_sync_source failed");
} else {
pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
smp_processor_id(), cpu);
}
/*
* 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 check_tsc_sync_target(void)
{
int cpus = 2;
if (unsynchronized_tsc() || tsc_clocksource_reliable)
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(loop_timeout(smp_processor_id()));
/*
* Ok, we are done:
*/
atomic_inc(&stop_count);
/*
* Wait for the source CPU to print stuff:
*/
while (atomic_read(&stop_count) != cpus)
cpu_relax();
}
