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s_lock.c
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1 /*-------------------------------------------------------------------------
2  *
3  * s_lock.c
4  * Hardware-dependent implementation of spinlocks.
5  *
6  * When waiting for a contended spinlock we loop tightly for awhile, then
7  * delay using pg_usleep() and try again. Preferably, "awhile" should be a
8  * small multiple of the maximum time we expect a spinlock to be held. 100
9  * iterations seems about right as an initial guess. However, on a
10  * uniprocessor the loop is a waste of cycles, while in a multi-CPU scenario
11  * it's usually better to spin a bit longer than to call the kernel, so we try
12  * to adapt the spin loop count depending on whether we seem to be in a
13  * uniprocessor or multiprocessor.
14  *
15  * Note: you might think MIN_SPINS_PER_DELAY should be just 1, but you'd
16  * be wrong; there are platforms where that can result in a "stuck
17  * spinlock" failure. This has been seen particularly on Alphas; it seems
18  * that the first TAS after returning from kernel space will always fail
19  * on that hardware.
20  *
21  * Once we do decide to block, we use randomly increasing pg_usleep()
22  * delays. The first delay is 1 msec, then the delay randomly increases to
23  * about one second, after which we reset to 1 msec and start again. The
24  * idea here is that in the presence of heavy contention we need to
25  * increase the delay, else the spinlock holder may never get to run and
26  * release the lock. (Consider situation where spinlock holder has been
27  * nice'd down in priority by the scheduler --- it will not get scheduled
28  * until all would-be acquirers are sleeping, so if we always use a 1-msec
29  * sleep, there is a real possibility of starvation.) But we can't just
30  * clamp the delay to an upper bound, else it would take a long time to
31  * make a reasonable number of tries.
32  *
33  * We time out and declare error after NUM_DELAYS delays (thus, exactly
34  * that many tries). With the given settings, this will usually take 2 or
35  * so minutes. It seems better to fix the total number of tries (and thus
36  * the probability of unintended failure) than to fix the total time
37  * spent.
38  *
39  * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
40  * Portions Copyright (c) 1994, Regents of the University of California
41  *
42  *
43  * IDENTIFICATION
44  * src/backend/storage/lmgr/s_lock.c
45  *
46  *-------------------------------------------------------------------------
47  */
48 #include "postgres.h"
49 
50 #include <time.h>
51 #include <unistd.h>
52 
53 #include "common/pg_prng.h"
54 #include "port/atomics.h"
55 #include "storage/s_lock.h"
56 #include "utils/wait_event.h"
57 
58 #define MIN_SPINS_PER_DELAY 10
59 #define MAX_SPINS_PER_DELAY 1000
60 #define NUM_DELAYS 1000
61 #define MIN_DELAY_USEC 1000L
62 #define MAX_DELAY_USEC 1000000L
63 
64 
66 
68 
69 
70 /*
71  * s_lock_stuck() - complain about a stuck spinlock
72  */
73 static void
74 s_lock_stuck(const char *file, int line, const char *func)
75 {
76  if (!func)
77  func = "(unknown)";
78 #if defined(S_LOCK_TEST)
79  fprintf(stderr,
80  "\nStuck spinlock detected at %s, %s:%d.\n",
81  func, file, line);
82  exit(1);
83 #else
84  elog(PANIC, "stuck spinlock detected at %s, %s:%d",
85  func, file, line);
86 #endif
87 }
88 
89 /*
90  * s_lock(lock) - platform-independent portion of waiting for a spinlock.
91  */
92 int
93 s_lock(volatile slock_t *lock, const char *file, int line, const char *func)
94 {
95  SpinDelayStatus delayStatus;
96 
97  init_spin_delay(&delayStatus, file, line, func);
98 
99  while (TAS_SPIN(lock))
100  {
101  perform_spin_delay(&delayStatus);
102  }
103 
104  finish_spin_delay(&delayStatus);
105 
106  return delayStatus.delays;
107 }
108 
109 #ifdef USE_DEFAULT_S_UNLOCK
110 void
111 s_unlock(volatile slock_t *lock)
112 {
113 #ifdef TAS_ACTIVE_WORD
114  /* HP's PA-RISC */
115  *TAS_ACTIVE_WORD(lock) = -1;
116 #else
117  *lock = 0;
118 #endif
119 }
120 #endif
121 
122 /*
123  * Wait while spinning on a contended spinlock.
124  */
125 void
127 {
128  /* CPU-specific delay each time through the loop */
129  SPIN_DELAY();
130 
131  /* Block the process every spins_per_delay tries */
132  if (++(status->spins) >= spins_per_delay)
133  {
134  if (++(status->delays) > NUM_DELAYS)
135  s_lock_stuck(status->file, status->line, status->func);
136 
137  if (status->cur_delay == 0) /* first time to delay? */
138  status->cur_delay = MIN_DELAY_USEC;
139 
140  /*
141  * Once we start sleeping, the overhead of reporting a wait event is
142  * justified. Actively spinning easily stands out in profilers, but
143  * sleeping with an exponential backoff is harder to spot...
144  *
145  * We might want to report something more granular at some point, but
146  * this is better than nothing.
147  */
149  pg_usleep(status->cur_delay);
151 
152 #if defined(S_LOCK_TEST)
153  fprintf(stdout, "*");
154  fflush(stdout);
155 #endif
156 
157  /* increase delay by a random fraction between 1X and 2X */
158  status->cur_delay += (int) (status->cur_delay *
160  /* wrap back to minimum delay when max is exceeded */
161  if (status->cur_delay > MAX_DELAY_USEC)
162  status->cur_delay = MIN_DELAY_USEC;
163 
164  status->spins = 0;
165  }
166 }
167 
168 /*
169  * After acquiring a spinlock, update estimates about how long to loop.
170  *
171  * If we were able to acquire the lock without delaying, it's a good
172  * indication we are in a multiprocessor. If we had to delay, it's a sign
173  * (but not a sure thing) that we are in a uniprocessor. Hence, we
174  * decrement spins_per_delay slowly when we had to delay, and increase it
175  * rapidly when we didn't. It's expected that spins_per_delay will
176  * converge to the minimum value on a uniprocessor and to the maximum
177  * value on a multiprocessor.
178  *
179  * Note: spins_per_delay is local within our current process. We want to
180  * average these observations across multiple backends, since it's
181  * relatively rare for this function to even get entered, and so a single
182  * backend might not live long enough to converge on a good value. That
183  * is handled by the two routines below.
184  */
185 void
187 {
188  if (status->cur_delay == 0)
189  {
190  /* we never had to delay */
193  }
194  else
195  {
198  }
199 }
200 
201 /*
202  * Set local copy of spins_per_delay during backend startup.
203  *
204  * NB: this has to be pretty fast as it is called while holding a spinlock
205  */
206 void
207 set_spins_per_delay(int shared_spins_per_delay)
208 {
209  spins_per_delay = shared_spins_per_delay;
210 }
211 
212 /*
213  * Update shared estimate of spins_per_delay during backend exit.
214  *
215  * NB: this has to be pretty fast as it is called while holding a spinlock
216  */
217 int
218 update_spins_per_delay(int shared_spins_per_delay)
219 {
220  /*
221  * We use an exponential moving average with a relatively slow adaption
222  * rate, so that noise in any one backend's result won't affect the shared
223  * value too much. As long as both inputs are within the allowed range,
224  * the result must be too, so we need not worry about clamping the result.
225  *
226  * We deliberately truncate rather than rounding; this is so that single
227  * adjustments inside a backend can affect the shared estimate (see the
228  * asymmetric adjustment rules above).
229  */
230  return (shared_spins_per_delay * 15 + spins_per_delay) / 16;
231 }
232 
233 
234 /*****************************************************************************/
235 #if defined(S_LOCK_TEST)
236 
237 /*
238  * test program for verifying a port's spinlock support.
239  */
240 
241 struct test_lock_struct
242 {
243  char pad1;
244  slock_t lock;
245  char pad2;
246 };
247 
248 volatile struct test_lock_struct test_lock;
249 
250 int
251 main()
252 {
253  pg_prng_seed(&pg_global_prng_state, (uint64) time(NULL));
254 
255  test_lock.pad1 = test_lock.pad2 = 0x44;
256 
257  S_INIT_LOCK(&test_lock.lock);
258 
259  if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
260  {
261  printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
262  return 1;
263  }
264 
265  if (!S_LOCK_FREE(&test_lock.lock))
266  {
267  printf("S_LOCK_TEST: failed, lock not initialized\n");
268  return 1;
269  }
270 
271  S_LOCK(&test_lock.lock);
272 
273  if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
274  {
275  printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
276  return 1;
277  }
278 
279  if (S_LOCK_FREE(&test_lock.lock))
280  {
281  printf("S_LOCK_TEST: failed, lock not locked\n");
282  return 1;
283  }
284 
285  S_UNLOCK(&test_lock.lock);
286 
287  if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
288  {
289  printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
290  return 1;
291  }
292 
293  if (!S_LOCK_FREE(&test_lock.lock))
294  {
295  printf("S_LOCK_TEST: failed, lock not unlocked\n");
296  return 1;
297  }
298 
299  S_LOCK(&test_lock.lock);
300 
301  if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
302  {
303  printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
304  return 1;
305  }
306 
307  if (S_LOCK_FREE(&test_lock.lock))
308  {
309  printf("S_LOCK_TEST: failed, lock not re-locked\n");
310  return 1;
311  }
312 
313  printf("S_LOCK_TEST: this will print %d stars and then\n", NUM_DELAYS);
314  printf(" exit with a 'stuck spinlock' message\n");
315  printf(" if S_LOCK() and TAS() are working.\n");
316  fflush(stdout);
317 
318  s_lock(&test_lock.lock, __FILE__, __LINE__, __func__);
319 
320  printf("S_LOCK_TEST: failed, lock not locked\n");
321  return 1;
322 }
323 
324 #endif /* S_LOCK_TEST */
#define Min(x, y)
Definition: c.h:937
#define Max(x, y)
Definition: c.h:931
#define PANIC
Definition: elog.h:38
static void const char fflush(stdout)
exit(1)
int main(int argc, char **argv)
Definition: oid2name.c:583
double pg_prng_double(pg_prng_state *state)
Definition: pg_prng.c:226
void pg_prng_seed(pg_prng_state *state, uint64 seed)
Definition: pg_prng.c:83
pg_prng_state pg_global_prng_state
Definition: pg_prng.c:28
static void static void status(const char *fmt,...) pg_attribute_printf(1
Definition: pg_regress.c:225
#define fprintf
Definition: port.h:242
#define printf(...)
Definition: port.h:244
void set_spins_per_delay(int shared_spins_per_delay)
Definition: s_lock.c:207
void perform_spin_delay(SpinDelayStatus *status)
Definition: s_lock.c:126
#define MIN_DELAY_USEC
Definition: s_lock.c:61
void finish_spin_delay(SpinDelayStatus *status)
Definition: s_lock.c:186
int s_lock(volatile slock_t *lock, const char *file, int line, const char *func)
Definition: s_lock.c:93
#define NUM_DELAYS
Definition: s_lock.c:60
#define MAX_SPINS_PER_DELAY
Definition: s_lock.c:59
#define MIN_SPINS_PER_DELAY
Definition: s_lock.c:58
int update_spins_per_delay(int shared_spins_per_delay)
Definition: s_lock.c:218
#define MAX_DELAY_USEC
Definition: s_lock.c:62
static int spins_per_delay
Definition: s_lock.c:67
static void s_lock_stuck(const char *file, int line, const char *func)
Definition: s_lock.c:74
slock_t dummy_spinlock
Definition: s_lock.c:65
#define S_UNLOCK(lock)
Definition: s_lock.h:762
#define TAS_SPIN(lock)
Definition: s_lock.h:821
#define DEFAULT_SPINS_PER_DELAY
Definition: s_lock.h:832
#define S_LOCK_FREE(lock)
Definition: s_lock.h:761
static void init_spin_delay(SpinDelayStatus *status, const char *file, int line, const char *func)
Definition: s_lock.h:852
#define S_INIT_LOCK(lock)
Definition: s_lock.h:763
#define S_LOCK(lock)
Definition: s_lock.h:775
int slock_t
Definition: s_lock.h:754
#define SPIN_DELAY()
Definition: s_lock.h:810
void pg_usleep(long microsec)
Definition: signal.c:53
@ WAIT_EVENT_SPIN_DELAY
Definition: wait_event.h:148
static void pgstat_report_wait_start(uint32 wait_event_info)
Definition: wait_event.h:268
static void pgstat_report_wait_end(void)
Definition: wait_event.h:284