Age Owner Branch data TLA Line data Source code
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) 1996-2024, 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 : : #ifdef S_LOCK_TEST
65 : : /*
66 : : * These are needed by pgstat_report_wait_start in the standalone compile of
67 : : * s_lock_test.
68 : : */
69 : : static uint32 local_my_wait_event_info;
70 : : uint32 *my_wait_event_info = &local_my_wait_event_info;
71 : : #endif
72 : :
73 : : static int spins_per_delay = DEFAULT_SPINS_PER_DELAY;
74 : :
75 : :
76 : : /*
77 : : * s_lock_stuck() - complain about a stuck spinlock
78 : : */
79 : : static void
2923 andres@anarazel.de 80 :UBC 0 : s_lock_stuck(const char *file, int line, const char *func)
81 : : {
82 [ # # ]: 0 : if (!func)
83 : 0 : func = "(unknown)";
84 : : #if defined(S_LOCK_TEST)
85 : : fprintf(stderr,
86 : : "\nStuck spinlock detected at %s, %s:%d.\n",
87 : : func, file, line);
88 : : exit(1);
89 : : #else
90 [ # # ]: 0 : elog(PANIC, "stuck spinlock detected at %s, %s:%d",
91 : : func, file, line);
92 : : #endif
93 : : }
94 : :
95 : : /*
96 : : * s_lock(lock) - platform-independent portion of waiting for a spinlock.
97 : : */
98 : : int
2923 andres@anarazel.de 99 :CBC 19312 : s_lock(volatile slock_t *lock, const char *file, int line, const char *func)
100 : : {
101 : : SpinDelayStatus delayStatus;
102 : :
2922 103 : 19312 : init_spin_delay(&delayStatus, file, line, func);
104 : :
4612 rhaas@postgresql.org 105 [ + + + + ]: 121556 : while (TAS_SPIN(lock))
106 : : {
2926 andres@anarazel.de 107 : 102244 : perform_spin_delay(&delayStatus);
108 : : }
109 : :
110 : 19312 : finish_spin_delay(&delayStatus);
111 : :
112 : 19312 : return delayStatus.delays;
113 : : }
114 : :
115 : : #ifdef USE_DEFAULT_S_UNLOCK
116 : : void
117 : : s_unlock(volatile slock_t *lock)
118 : : {
119 : : #ifdef TAS_ACTIVE_WORD
120 : : /* HP's PA-RISC */
121 : : *TAS_ACTIVE_WORD(lock) = -1;
122 : : #else
123 : : *lock = 0;
124 : : #endif
125 : : }
126 : : #endif
127 : :
128 : : /*
129 : : * Wait while spinning on a contended spinlock.
130 : : */
131 : : void
132 : 1046096 : perform_spin_delay(SpinDelayStatus *status)
133 : : {
134 : : /* CPU-specific delay each time through the loop */
135 : 1046096 : SPIN_DELAY();
136 : :
137 : : /* Block the process every spins_per_delay tries */
138 [ + + ]: 1046096 : if (++(status->spins) >= spins_per_delay)
139 : : {
140 [ - + ]: 185 : if (++(status->delays) > NUM_DELAYS)
2923 andres@anarazel.de 141 :UBC 0 : s_lock_stuck(status->file, status->line, status->func);
142 : :
2489 tgl@sss.pgh.pa.us 143 [ + + ]:CBC 185 : if (status->cur_delay == 0) /* first time to delay? */
2926 andres@anarazel.de 144 : 158 : status->cur_delay = MIN_DELAY_USEC;
145 : :
146 : : /*
147 : : * Once we start sleeping, the overhead of reporting a wait event is
148 : : * justified. Actively spinning easily stands out in profilers, but
149 : : * sleeping with an exponential backoff is harder to spot...
150 : : *
151 : : * We might want to report something more granular at some point, but
152 : : * this is better than nothing.
153 : : */
510 154 : 185 : pgstat_report_wait_start(WAIT_EVENT_SPIN_DELAY);
2926 155 : 185 : pg_usleep(status->cur_delay);
510 156 : 185 : pgstat_report_wait_end();
157 : :
158 : : #if defined(S_LOCK_TEST)
159 : : fprintf(stdout, "*");
160 : : fflush(stdout);
161 : : #endif
162 : :
163 : : /* increase delay by a random fraction between 1X and 2X */
2926 164 : 370 : status->cur_delay += (int) (status->cur_delay *
868 tgl@sss.pgh.pa.us 165 : 185 : pg_prng_double(&pg_global_prng_state) + 0.5);
166 : : /* wrap back to minimum delay when max is exceeded */
2926 andres@anarazel.de 167 [ - + ]: 185 : if (status->cur_delay > MAX_DELAY_USEC)
2926 andres@anarazel.de 168 :UBC 0 : status->cur_delay = MIN_DELAY_USEC;
169 : :
2926 andres@anarazel.de 170 :CBC 185 : status->spins = 0;
171 : : }
172 : 1046096 : }
173 : :
174 : : /*
175 : : * After acquiring a spinlock, update estimates about how long to loop.
176 : : *
177 : : * If we were able to acquire the lock without delaying, it's a good
178 : : * indication we are in a multiprocessor. If we had to delay, it's a sign
179 : : * (but not a sure thing) that we are in a uniprocessor. Hence, we
180 : : * decrement spins_per_delay slowly when we had to delay, and increase it
181 : : * rapidly when we didn't. It's expected that spins_per_delay will
182 : : * converge to the minimum value on a uniprocessor and to the maximum
183 : : * value on a multiprocessor.
184 : : *
185 : : * Note: spins_per_delay is local within our current process. We want to
186 : : * average these observations across multiple backends, since it's
187 : : * relatively rare for this function to even get entered, and so a single
188 : : * backend might not live long enough to converge on a good value. That
189 : : * is handled by the two routines below.
190 : : */
191 : : void
192 : 27182021 : finish_spin_delay(SpinDelayStatus *status)
193 : : {
194 [ + + ]: 27182021 : if (status->cur_delay == 0)
195 : : {
196 : : /* we never had to delay */
6760 tgl@sss.pgh.pa.us 197 [ + + ]: 27181863 : if (spins_per_delay < MAX_SPINS_PER_DELAY)
198 : 54935 : spins_per_delay = Min(spins_per_delay + 100, MAX_SPINS_PER_DELAY);
199 : : }
200 : : else
201 : : {
202 [ + - ]: 158 : if (spins_per_delay > MIN_SPINS_PER_DELAY)
203 : 158 : spins_per_delay = Max(spins_per_delay - 1, MIN_SPINS_PER_DELAY);
204 : : }
9477 scrappy@hub.org 205 : 27182021 : }
206 : :
207 : : /*
208 : : * Set local copy of spins_per_delay during backend startup.
209 : : *
210 : : * NB: this has to be pretty fast as it is called while holding a spinlock
211 : : */
212 : : void
6760 tgl@sss.pgh.pa.us 213 : 19580 : set_spins_per_delay(int shared_spins_per_delay)
214 : : {
215 : 19580 : spins_per_delay = shared_spins_per_delay;
216 : 19580 : }
217 : :
218 : : /*
219 : : * Update shared estimate of spins_per_delay during backend exit.
220 : : *
221 : : * NB: this has to be pretty fast as it is called while holding a spinlock
222 : : */
223 : : int
224 : 18048 : update_spins_per_delay(int shared_spins_per_delay)
225 : : {
226 : : /*
227 : : * We use an exponential moving average with a relatively slow adaption
228 : : * rate, so that noise in any one backend's result won't affect the shared
229 : : * value too much. As long as both inputs are within the allowed range,
230 : : * the result must be too, so we need not worry about clamping the result.
231 : : *
232 : : * We deliberately truncate rather than rounding; this is so that single
233 : : * adjustments inside a backend can affect the shared estimate (see the
234 : : * asymmetric adjustment rules above).
235 : : */
236 : 18048 : return (shared_spins_per_delay * 15 + spins_per_delay) / 16;
237 : : }
238 : :
239 : :
240 : : /*****************************************************************************/
241 : : #if defined(S_LOCK_TEST)
242 : :
243 : : /*
244 : : * test program for verifying a port's spinlock support.
245 : : */
246 : :
247 : : struct test_lock_struct
248 : : {
249 : : char pad1;
250 : : slock_t lock;
251 : : char pad2;
252 : : };
253 : :
254 : : volatile struct test_lock_struct test_lock;
255 : :
256 : : int
257 : : main()
258 : : {
259 : : pg_prng_seed(&pg_global_prng_state, (uint64) time(NULL));
260 : :
261 : : test_lock.pad1 = test_lock.pad2 = 0x44;
262 : :
263 : : S_INIT_LOCK(&test_lock.lock);
264 : :
265 : : if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
266 : : {
267 : : printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
268 : : return 1;
269 : : }
270 : :
271 : : if (!S_LOCK_FREE(&test_lock.lock))
272 : : {
273 : : printf("S_LOCK_TEST: failed, lock not initialized\n");
274 : : return 1;
275 : : }
276 : :
277 : : S_LOCK(&test_lock.lock);
278 : :
279 : : if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
280 : : {
281 : : printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
282 : : return 1;
283 : : }
284 : :
285 : : if (S_LOCK_FREE(&test_lock.lock))
286 : : {
287 : : printf("S_LOCK_TEST: failed, lock not locked\n");
288 : : return 1;
289 : : }
290 : :
291 : : S_UNLOCK(&test_lock.lock);
292 : :
293 : : if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
294 : : {
295 : : printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
296 : : return 1;
297 : : }
298 : :
299 : : if (!S_LOCK_FREE(&test_lock.lock))
300 : : {
301 : : printf("S_LOCK_TEST: failed, lock not unlocked\n");
302 : : return 1;
303 : : }
304 : :
305 : : S_LOCK(&test_lock.lock);
306 : :
307 : : if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
308 : : {
309 : : printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
310 : : return 1;
311 : : }
312 : :
313 : : if (S_LOCK_FREE(&test_lock.lock))
314 : : {
315 : : printf("S_LOCK_TEST: failed, lock not re-locked\n");
316 : : return 1;
317 : : }
318 : :
319 : : printf("S_LOCK_TEST: this will print %d stars and then\n", NUM_DELAYS);
320 : : printf(" exit with a 'stuck spinlock' message\n");
321 : : printf(" if S_LOCK() and TAS() are working.\n");
322 : : fflush(stdout);
323 : :
324 : : s_lock(&test_lock.lock, __FILE__, __LINE__, __func__);
325 : :
326 : : printf("S_LOCK_TEST: failed, lock not locked\n");
327 : : return 1;
328 : : }
329 : :
330 : : #endif /* S_LOCK_TEST */
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