1 /* 2 * arch/x86_64/kernel/tsc_sync.c: check TSC synchronization. 3 * 4 * Copyright (C) 2006, Red Hat, Inc., Ingo Molnar 5 * 6 * We check whether all boot CPUs have their TSC's synchronized, 7 * print a warning if not and turn off the TSC clock-source. 8 * 9 * The warp-check is point-to-point between two CPUs, the CPU 10 * initiating the bootup is the 'source CPU', the freshly booting 11 * CPU is the 'target CPU'. 12 * 13 * Only two CPUs may participate - they can enter in any order. 14 * ( The serial nature of the boot logic and the CPU hotplug lock 15 * protects against more than 2 CPUs entering this code. ) 16 */ 17 #include <linux/spinlock.h> 18 #include <linux/kernel.h> 19 #include <linux/init.h> 20 #include <linux/smp.h> 21 #include <linux/nmi.h> 22 #include <asm/tsc.h> 23 24 /* 25 * Entry/exit counters that make sure that both CPUs 26 * run the measurement code at once: 27 */ 28 static __cpuinitdata atomic_t start_count; 29 static __cpuinitdata atomic_t stop_count; 30 31 /* 32 * We use a raw spinlock in this exceptional case, because 33 * we want to have the fastest, inlined, non-debug version 34 * of a critical section, to be able to prove TSC time-warps: 35 */ 36 static __cpuinitdata raw_spinlock_t sync_lock = __RAW_SPIN_LOCK_UNLOCKED; 37 static __cpuinitdata cycles_t last_tsc; 38 static __cpuinitdata cycles_t max_warp; 39 static __cpuinitdata int nr_warps; 40 41 /* 42 * TSC-warp measurement loop running on both CPUs: 43 */ 44 static __cpuinit void check_tsc_warp(void) 45 { 46 cycles_t start, now, prev, end; 47 int i; 48 49 start = get_cycles_sync(); 50 /* 51 * The measurement runs for 20 msecs: 52 */ 53 end = start + tsc_khz * 20ULL; 54 now = start; 55 56 for (i = 0; ; i++) { 57 /* 58 * We take the global lock, measure TSC, save the 59 * previous TSC that was measured (possibly on 60 * another CPU) and update the previous TSC timestamp. 61 */ 62 __raw_spin_lock(&sync_lock); 63 prev = last_tsc; 64 now = get_cycles_sync(); 65 last_tsc = now; 66 __raw_spin_unlock(&sync_lock); 67 68 /* 69 * Be nice every now and then (and also check whether 70 * measurement is done [we also insert a 100 million 71 * loops safety exit, so we dont lock up in case the 72 * TSC readout is totally broken]): 73 */ 74 if (unlikely(!(i & 7))) { 75 if (now > end || i > 100000000) 76 break; 77 cpu_relax(); 78 touch_nmi_watchdog(); 79 } 80 /* 81 * Outside the critical section we can now see whether 82 * we saw a time-warp of the TSC going backwards: 83 */ 84 if (unlikely(prev > now)) { 85 __raw_spin_lock(&sync_lock); 86 max_warp = max(max_warp, prev - now); 87 nr_warps++; 88 __raw_spin_unlock(&sync_lock); 89 } 90 91 } 92 } 93 94 /* 95 * Source CPU calls into this - it waits for the freshly booted 96 * target CPU to arrive and then starts the measurement: 97 */ 98 void __cpuinit check_tsc_sync_source(int cpu) 99 { 100 int cpus = 2; 101 102 /* 103 * No need to check if we already know that the TSC is not 104 * synchronized: 105 */ 106 if (unsynchronized_tsc()) 107 return; 108 109 printk(KERN_INFO "checking TSC synchronization [CPU#%d -> CPU#%d]:", 110 smp_processor_id(), cpu); 111 112 /* 113 * Reset it - in case this is a second bootup: 114 */ 115 atomic_set(&stop_count, 0); 116 117 /* 118 * Wait for the target to arrive: 119 */ 120 while (atomic_read(&start_count) != cpus-1) 121 cpu_relax(); 122 /* 123 * Trigger the target to continue into the measurement too: 124 */ 125 atomic_inc(&start_count); 126 127 check_tsc_warp(); 128 129 while (atomic_read(&stop_count) != cpus-1) 130 cpu_relax(); 131 132 /* 133 * Reset it - just in case we boot another CPU later: 134 */ 135 atomic_set(&start_count, 0); 136 137 if (nr_warps) { 138 printk("\n"); 139 printk(KERN_WARNING "Measured %Ld cycles TSC warp between CPUs," 140 " turning off TSC clock.\n", max_warp); 141 mark_tsc_unstable("check_tsc_sync_source failed"); 142 nr_warps = 0; 143 max_warp = 0; 144 last_tsc = 0; 145 } else { 146 printk(" passed.\n"); 147 } 148 149 /* 150 * Let the target continue with the bootup: 151 */ 152 atomic_inc(&stop_count); 153 } 154 155 /* 156 * Freshly booted CPUs call into this: 157 */ 158 void __cpuinit check_tsc_sync_target(void) 159 { 160 int cpus = 2; 161 162 if (unsynchronized_tsc()) 163 return; 164 165 /* 166 * Register this CPU's participation and wait for the 167 * source CPU to start the measurement: 168 */ 169 atomic_inc(&start_count); 170 while (atomic_read(&start_count) != cpus) 171 cpu_relax(); 172 173 check_tsc_warp(); 174 175 /* 176 * Ok, we are done: 177 */ 178 atomic_inc(&stop_count); 179 180 /* 181 * Wait for the source CPU to print stuff: 182 */ 183 while (atomic_read(&stop_count) != cpus) 184 cpu_relax(); 185 } 186 #undef NR_LOOPS 187 188