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Linux/arch/x86/kernel/tsc_sync.c

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  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  */
 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  */
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  */
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  */
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 

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