1 /*
2 * 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/smp.h>
20 #include <linux/nmi.h>
21 #include <asm/tsc.h>
22
23 /*
24 * Entry/exit counters that make sure that both CPUs
25 * run the measurement code at once:
26 */
27 static atomic_t start_count;
28 static atomic_t stop_count;
29
30 /*
31 * We use a raw spinlock in this exceptional case, because
32 * we want to have the fastest, inlined, non-debug version
33 * of a critical section, to be able to prove TSC time-warps:
34 */
35 static arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;
36
37 static cycles_t last_tsc;
38 static cycles_t max_warp;
39 static int nr_warps;
40
41 /*
42 * TSC-warp measurement loop running on both CPUs:
43 */
check_tsc_warp(unsigned int timeout)44 static void check_tsc_warp(unsigned int timeout)
45 {
46 cycles_t start, now, prev, end;
47 int i;
48
49 rdtsc_barrier();
50 start = get_cycles();
51 rdtsc_barrier();
52 /*
53 * The measurement runs for 'timeout' msecs:
54 */
55 end = start + (cycles_t) tsc_khz * timeout;
56 now = start;
57
58 for (i = 0; ; i++) {
59 /*
60 * We take the global lock, measure TSC, save the
61 * previous TSC that was measured (possibly on
62 * another CPU) and update the previous TSC timestamp.
63 */
64 arch_spin_lock(&sync_lock);
65 prev = last_tsc;
66 rdtsc_barrier();
67 now = get_cycles();
68 rdtsc_barrier();
69 last_tsc = now;
70 arch_spin_unlock(&sync_lock);
71
72 /*
73 * Be nice every now and then (and also check whether
74 * measurement is done [we also insert a 10 million
75 * loops safety exit, so we dont lock up in case the
76 * TSC readout is totally broken]):
77 */
78 if (unlikely(!(i & 7))) {
79 if (now > end || i > 10000000)
80 break;
81 cpu_relax();
82 touch_nmi_watchdog();
83 }
84 /*
85 * Outside the critical section we can now see whether
86 * we saw a time-warp of the TSC going backwards:
87 */
88 if (unlikely(prev > now)) {
89 arch_spin_lock(&sync_lock);
90 max_warp = max(max_warp, prev - now);
91 nr_warps++;
92 arch_spin_unlock(&sync_lock);
93 }
94 }
95 WARN(!(now-start),
96 "Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
97 now-start, end-start);
98 }
99
100 /*
101 * If the target CPU coming online doesn't have any of its core-siblings
102 * online, a timeout of 20msec will be used for the TSC-warp measurement
103 * loop. Otherwise a smaller timeout of 2msec will be used, as we have some
104 * information about this socket already (and this information grows as we
105 * have more and more logical-siblings in that socket).
106 *
107 * Ideally we should be able to skip the TSC sync check on the other
108 * core-siblings, if the first logical CPU in a socket passed the sync test.
109 * But as the TSC is per-logical CPU and can potentially be modified wrongly
110 * by the bios, TSC sync test for smaller duration should be able
111 * to catch such errors. Also this will catch the condition where all the
112 * cores in the socket doesn't get reset at the same time.
113 */
loop_timeout(int cpu)114 static inline unsigned int loop_timeout(int cpu)
115 {
116 return (cpumask_weight(cpu_core_mask(cpu)) > 1) ? 2 : 20;
117 }
118
119 /*
120 * Source CPU calls into this - it waits for the freshly booted
121 * target CPU to arrive and then starts the measurement:
122 */
check_tsc_sync_source(int cpu)123 void check_tsc_sync_source(int cpu)
124 {
125 int cpus = 2;
126
127 /*
128 * No need to check if we already know that the TSC is not
129 * synchronized:
130 */
131 if (unsynchronized_tsc())
132 return;
133
134 if (tsc_clocksource_reliable) {
135 if (cpu == (nr_cpu_ids-1) || system_state != SYSTEM_BOOTING)
136 pr_info(
137 "Skipped synchronization checks as TSC is reliable.\n");
138 return;
139 }
140
141 /*
142 * Reset it - in case this is a second bootup:
143 */
144 atomic_set(&stop_count, 0);
145
146 /*
147 * Wait for the target to arrive:
148 */
149 while (atomic_read(&start_count) != cpus-1)
150 cpu_relax();
151 /*
152 * Trigger the target to continue into the measurement too:
153 */
154 atomic_inc(&start_count);
155
156 check_tsc_warp(loop_timeout(cpu));
157
158 while (atomic_read(&stop_count) != cpus-1)
159 cpu_relax();
160
161 if (nr_warps) {
162 pr_warning("TSC synchronization [CPU#%d -> CPU#%d]:\n",
163 smp_processor_id(), cpu);
164 pr_warning("Measured %Ld cycles TSC warp between CPUs, "
165 "turning off TSC clock.\n", max_warp);
166 mark_tsc_unstable("check_tsc_sync_source failed");
167 } else {
168 pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
169 smp_processor_id(), cpu);
170 }
171
172 /*
173 * Reset it - just in case we boot another CPU later:
174 */
175 atomic_set(&start_count, 0);
176 nr_warps = 0;
177 max_warp = 0;
178 last_tsc = 0;
179
180 /*
181 * Let the target continue with the bootup:
182 */
183 atomic_inc(&stop_count);
184 }
185
186 /*
187 * Freshly booted CPUs call into this:
188 */
check_tsc_sync_target(void)189 void check_tsc_sync_target(void)
190 {
191 int cpus = 2;
192
193 if (unsynchronized_tsc() || tsc_clocksource_reliable)
194 return;
195
196 /*
197 * Register this CPU's participation and wait for the
198 * source CPU to start the measurement:
199 */
200 atomic_inc(&start_count);
201 while (atomic_read(&start_count) != cpus)
202 cpu_relax();
203
204 check_tsc_warp(loop_timeout(smp_processor_id()));
205
206 /*
207 * Ok, we are done:
208 */
209 atomic_inc(&stop_count);
210
211 /*
212 * Wait for the source CPU to print stuff:
213 */
214 while (atomic_read(&stop_count) != cpus)
215 cpu_relax();
216 }
217