[PATCH] x86: rewrite SMP TSC sync code

make the TSC synchronization code more robust, and unify it between x86_64 and
i386.

The biggest change is the removal of the 'fix up TSCs' code on x86_64 and
i386, in some rare cases it was /causing/ time-warps on SMP systems.

The new code only checks for TSC asynchronity - and if it can prove a
time-warp (if it can observe the TSC going backwards when going from one CPU
to another within a critical section), then the TSC clock-source is turned
off.

The TSC synchronization-checking code also got moved into a separate file.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 187 insertions, 0 deletions

diff --git a/arch/x86_64/kernel/tsc_sync.c b/arch/x86_64/kernel/tsc_sync.c
new file mode 100644
index 0000000..014f0db
--- /dev/null
+++ b/arch/x86_64/kernel/tsc_sync.c
@@ -0,0 +1,187 @@
+/*
+ * arch/x86_64/kernel/tsc_sync.c: 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_sync();
+	/*
+	 * The measurement runs for 20 msecs:
+	 */
+	end = start + cpu_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_sync();
+		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 100 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 > 100000000)
+				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);
+		}
+
+	}
+}
+
+/*
+ * 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();
+
+	/*
+	 * Reset it - just in case we boot another CPU later:
+	 */
+	atomic_set(&start_count, 0);
+
+	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();
+		nr_warps = 0;
+		max_warp = 0;
+		last_tsc = 0;
+	} else {
+		printk(" passed.\n");
+	}
+
+	/*
+	 * 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
+