1 /*
2 * Copyright (c) 2005 Jeffrey M. Hsu. All rights reserved.
3 *
4 * This code is derived from software contributed to The DragonFly Project
5 * by Jeffrey M. Hsu. and Matthew Dillon
6 *
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
9 * are met:
10 * 1. Redistributions of source code must retain the above copyright
11 * notice, this list of conditions and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
13 * notice, this list of conditions and the following disclaimer in the
14 * documentation and/or other materials provided with the distribution.
15 * 3. Neither the name of The DragonFly Project nor the names of its
16 * contributors may be used to endorse or promote products derived
17 * from this software without specific, prior written permission.
18 *
19 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
20 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
21 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
22 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
23 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
24 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
25 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
26 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
27 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
28 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
29 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
30 * SUCH DAMAGE.
31 */
32
33 /*
34 * The implementation is designed to avoid looping when compatible operations
35 * are executed.
36 *
37 * To acquire a spinlock we first increment counta. Then we check if counta
38 * meets our requirements. For an exclusive spinlock it must be 1, of a
39 * shared spinlock it must either be 1 or the SHARED_SPINLOCK bit must be set.
40 *
41 * Shared spinlock failure case: Decrement the count, loop until we can
42 * transition from 0 to SHARED_SPINLOCK|1, or until we find SHARED_SPINLOCK
43 * is set and increment the count.
44 *
45 * Exclusive spinlock failure case: While maintaining the count, clear the
46 * SHARED_SPINLOCK flag unconditionally. Then use an atomic add to transfer
47 * the count from the low bits to the high bits of counta. Then loop until
48 * all low bits are 0. Once the low bits drop to 0 we can transfer the
49 * count back with an atomic_cmpset_int(), atomically, and return.
50 */
51 #include <sys/param.h>
52 #include <sys/systm.h>
53 #include <sys/types.h>
54 #include <sys/kernel.h>
55 #include <sys/sysctl.h>
56 #ifdef INVARIANTS
57 #include <sys/proc.h>
58 #endif
59 #include <sys/priv.h>
60 #include <machine/atomic.h>
61 #include <machine/cpu.h>
62 #include <machine/cpufunc.h>
63 #include <machine/specialreg.h>
64 #include <machine/clock.h>
65 #include <sys/spinlock.h>
66 #include <sys/spinlock2.h>
67 #include <sys/ktr.h>
68
69 #ifdef _KERNEL_VIRTUAL
70 #include <pthread.h>
71 #endif
72
73 struct spinlock pmap_spin = SPINLOCK_INITIALIZER(pmap_spin);
74
75 struct indefinite_info {
76 sysclock_t base;
77 int secs;
78 const char *ident;
79 };
80
81 /*
82 * Kernal Trace
83 */
84 #if !defined(KTR_SPIN_CONTENTION)
85 #define KTR_SPIN_CONTENTION KTR_ALL
86 #endif
87 #define SPIN_STRING "spin=%p type=%c"
88 #define SPIN_ARG_SIZE (sizeof(void *) + sizeof(int))
89
90 KTR_INFO_MASTER(spin);
91 #if 0
92 KTR_INFO(KTR_SPIN_CONTENTION, spin, beg, 0, SPIN_STRING, SPIN_ARG_SIZE);
93 KTR_INFO(KTR_SPIN_CONTENTION, spin, end, 1, SPIN_STRING, SPIN_ARG_SIZE);
94 #endif
95
96 #define logspin(name, spin, type) \
97 KTR_LOG(spin_ ## name, spin, type)
98
99 #ifdef INVARIANTS
100 static int spin_lock_test_mode;
101 #endif
102
103 #ifdef DEBUG_LOCKS_LATENCY
104
105 static long spinlocks_add_latency;
106 SYSCTL_LONG(_debug, OID_AUTO, spinlocks_add_latency, CTLFLAG_RW,
107 &spinlocks_add_latency, 0,
108 "Add spinlock latency");
109
110 #endif
111
112
113 /*
114 * We need a fairly large pool to avoid contention on large SMP systems,
115 * particularly multi-chip systems.
116 */
117 /*#define SPINLOCK_NUM_POOL 8101*/
118 #define SPINLOCK_NUM_POOL 8192
119 #define SPINLOCK_NUM_POOL_MASK (SPINLOCK_NUM_POOL - 1)
120
121 static __cachealign struct {
122 struct spinlock spin;
123 char filler[32 - sizeof(struct spinlock)];
124 } pool_spinlocks[SPINLOCK_NUM_POOL];
125
126 static int spin_indefinite_check(struct spinlock *spin,
127 struct indefinite_info *info);
128
129 /*
130 * We contested due to another exclusive lock holder. We lose.
131 *
132 * We have to unwind the attempt and may acquire the spinlock
133 * anyway while doing so. countb was incremented on our behalf.
134 */
135 int
136 spin_trylock_contested(struct spinlock *spin)
137 {
138 globaldata_t gd = mycpu;
139
140 /*atomic_add_int(&spin->counta, -1);*/
141 --gd->gd_spinlocks;
142 --gd->gd_curthread->td_critcount;
143 return (FALSE);
144 }
145
146 /*
147 * The spin_lock() inline was unable to acquire the lock.
148 *
149 * atomic_swap_int() is the absolute fastest spinlock instruction, at
150 * least on multi-socket systems. All instructions seem to be about
151 * the same on single-socket multi-core systems. However, atomic_swap_int()
152 * does not result in an even distribution of successful acquisitions.
153 *
154 * UNFORTUNATELY we cannot really use atomic_swap_int() when also implementing
155 * shared spin locks, so as we do a better job removing contention we've
156 * moved to atomic_cmpset_int() to be able handle multiple states.
157 *
158 * Another problem we have is that (at least on the 48-core opteron we test
159 * with) having all 48 cores contesting the same spin lock reduces
160 * performance to around 600,000 ops/sec, verses millions when fewer cores
161 * are going after the same lock.
162 *
163 * Backoff algorithms can create even worse starvation problems, and don't
164 * really improve performance when a lot of cores are contending.
165 *
166 * Our solution is to allow the data cache to lazy-update by reading it
167 * non-atomically and only attempting to acquire the lock if the lazy read
168 * looks good. This effectively limits cache bus bandwidth. A cpu_pause()
169 * (for intel/amd anyhow) is not strictly needed as cache bus resource use
170 * is governed by the lazy update.
171 *
172 * WARNING!!!! Performance matters here, by a huge margin.
173 *
174 * 48-core test with pre-read / -j 48 no-modules kernel compile
175 * with fanned-out inactive and active queues came in at 55 seconds.
176 *
177 * 48-core test with pre-read / -j 48 no-modules kernel compile
178 * came in at 75 seconds. Without pre-read it came in at 170 seconds.
179 *
180 * 4-core test with pre-read / -j 48 no-modules kernel compile
181 * came in at 83 seconds. Without pre-read it came in at 83 seconds
182 * as well (no difference).
183 */
184 void
185 _spin_lock_contested(struct spinlock *spin, const char *ident)
186 {
187 struct indefinite_info info = { 0, 0, ident };
188 int i;
189
190 /*
191 * Transfer our count to the high bits, then loop until we can
192 * acquire the low counter (== 1). No new shared lock can be
193 * acquired while we hold the EXCLWAIT bits.
194 *
195 * Force any existing shared locks to exclusive. The shared unlock
196 * understands that this may occur.
197 */
198 atomic_add_int(&spin->counta, SPINLOCK_EXCLWAIT - 1);
199 atomic_clear_int(&spin->counta, SPINLOCK_SHARED);
200
201 #ifdef DEBUG_LOCKS_LATENCY
202 long j;
203 for (j = spinlocks_add_latency; j > 0; --j)
204 cpu_ccfence();
205 #endif
206 #if defined(INVARIANTS)
207 if (spin_lock_test_mode > 10 &&
208 spin->countb > spin_lock_test_mode &&
209 (spin_lock_test_mode & 0xFF) == mycpu->gd_cpuid) {
210 spin->countb = 0;
211 print_backtrace(-1);
212 }
213 ++spin->countb;
214 #endif
215 i = 0;
216
217 /*logspin(beg, spin, 'w');*/
218 for (;;) {
219 /*
220 * If the low bits are zero, try to acquire the exclusive lock
221 * by transfering our high bit counter to the low bits.
222 *
223 * NOTE: Reading spin->counta prior to the swap is extremely
224 * important on multi-chip/many-core boxes. On 48-core
225 * this one change improves fully concurrent all-cores
226 * compiles by 100% or better.
227 *
228 * I can't emphasize enough how important the pre-read
229 * is in preventing hw cache bus armageddon on
230 * multi-chip systems. And on single-chip/multi-core
231 * systems it just doesn't hurt.
232 */
233 uint32_t ovalue = spin->counta;
234 cpu_ccfence();
235 if ((ovalue & (SPINLOCK_EXCLWAIT - 1)) == 0 &&
236 atomic_cmpset_int(&spin->counta, ovalue,
237 (ovalue - SPINLOCK_EXCLWAIT) | 1)) {
238 break;
239 }
240 if ((++i & 0x7F) == 0x7F) {
241 mycpu->gd_cnt.v_lock_name[0] = 'X';
242 strncpy(mycpu->gd_cnt.v_lock_name + 1,
243 ident,
244 sizeof(mycpu->gd_cnt.v_lock_name) - 2);
245 ++mycpu->gd_cnt.v_lock_colls;
246 #if defined(INVARIANTS)
247 ++spin->countb;
248 #endif
249 if (spin_indefinite_check(spin, &info))
250 break;
251 }
252 #ifdef _KERNEL_VIRTUAL
253 pthread_yield();
254 #endif
255 }
256 /*logspin(end, spin, 'w');*/
257 }
258
259 /*
260 * Shared spinlock attempt was contested.
261 *
262 * The caller has not modified counta.
263 */
264 void
265 _spin_lock_shared_contested(struct spinlock *spin, const char *ident)
266 {
267 struct indefinite_info info = { 0, 0, ident };
268 int i;
269
270 #ifdef DEBUG_LOCKS_LATENCY
271 long j;
272 for (j = spinlocks_add_latency; j > 0; --j)
273 cpu_ccfence();
274 #endif
275 #if defined(INVARIANTS)
276 if (spin_lock_test_mode > 10 &&
277 spin->countb > spin_lock_test_mode &&
278 (spin_lock_test_mode & 0xFF) == mycpu->gd_cpuid) {
279 spin->countb = 0;
280 print_backtrace(-1);
281 }
282 ++spin->countb;
283 #endif
284 i = 0;
285
286 /*logspin(beg, spin, 'w');*/
287 for (;;) {
288 /*
289 * Loop until we can acquire the shared spinlock. Note that
290 * the low bits can be zero while the high EXCLWAIT bits are
291 * non-zero. In this situation exclusive requesters have
292 * priority (otherwise shared users on multiple cpus can hog
293 * the spinlnock).
294 *
295 * NOTE: Reading spin->counta prior to the swap is extremely
296 * important on multi-chip/many-core boxes. On 48-core
297 * this one change improves fully concurrent all-cores
298 * compiles by 100% or better.
299 *
300 * I can't emphasize enough how important the pre-read
301 * is in preventing hw cache bus armageddon on
302 * multi-chip systems. And on single-chip/multi-core
303 * systems it just doesn't hurt.
304 */
305 uint32_t ovalue = spin->counta;
306
307 cpu_ccfence();
308 if (ovalue == 0) {
309 if (atomic_cmpset_int(&spin->counta, 0,
310 SPINLOCK_SHARED | 1))
311 break;
312 } else if (ovalue & SPINLOCK_SHARED) {
313 if (atomic_cmpset_int(&spin->counta, ovalue,
314 ovalue + 1))
315 break;
316 }
317 if ((++i & 0x7F) == 0x7F) {
318 mycpu->gd_cnt.v_lock_name[0] = 'S';
319 strncpy(mycpu->gd_cnt.v_lock_name + 1,
320 ident,
321 sizeof(mycpu->gd_cnt.v_lock_name) - 2);
322 ++mycpu->gd_cnt.v_lock_colls;
323 #if defined(INVARIANTS)
324 ++spin->countb;
325 #endif
326 if (spin_indefinite_check(spin, &info))
327 break;
328 }
329 #ifdef _KERNEL_VIRTUAL
330 pthread_yield();
331 #endif
332 }
333 /*logspin(end, spin, 'w');*/
334 }
335
336 /*
337 * Pool functions (SHARED SPINLOCKS NOT SUPPORTED)
338 */
339 static __inline int
340 _spin_pool_hash(void *ptr)
341 {
342 int i;
343
344 i = ((int)(uintptr_t) ptr >> 5) ^ ((int)(uintptr_t)ptr >> 12);
345 i &= SPINLOCK_NUM_POOL_MASK;
346 return (i);
347 }
348
349 void
350 _spin_pool_lock(void *chan, const char *ident)
351 {
352 struct spinlock *sp;
353
354 sp = &pool_spinlocks[_spin_pool_hash(chan)].spin;
355 _spin_lock(sp, ident);
356 }
357
358 void
359 _spin_pool_unlock(void *chan)
360 {
361 struct spinlock *sp;
362
363 sp = &pool_spinlocks[_spin_pool_hash(chan)].spin;
364 spin_unlock(sp);
365 }
366
367
368 static
369 int
370 spin_indefinite_check(struct spinlock *spin, struct indefinite_info *info)
371 {
372 sysclock_t count;
373
374 cpu_spinlock_contested();
375
376 count = sys_cputimer->count();
377 if (info->secs == 0) {
378 info->base = count;
379 ++info->secs;
380 } else if (count - info->base > sys_cputimer->freq) {
381 kprintf("spin_lock: %s(%p), indefinite wait (%d secs)!\n",
382 info->ident, spin, info->secs);
383 info->base = count;
384 ++info->secs;
385 if (panicstr)
386 return (TRUE);
387 #if defined(INVARIANTS)
388 if (spin_lock_test_mode) {
389 print_backtrace(-1);
390 return (TRUE);
391 }
392 #endif
393 #if defined(INVARIANTS)
394 if (info->secs == 11)
395 print_backtrace(-1);
396 #endif
397 if (info->secs == 60)
398 panic("spin_lock: %s(%p), indefinite wait!",
399 info->ident, spin);
400 }
401 return (FALSE);
402 }
403
404 /*
405 * If INVARIANTS is enabled various spinlock timing tests can be run
406 * by setting debug.spin_lock_test:
407 *
408 * 1 Test the indefinite wait code
409 * 2 Time the best-case exclusive lock overhead (spin_test_count)
410 * 3 Time the best-case shared lock overhead (spin_test_count)
411 */
412
413 #ifdef INVARIANTS
414
415 static int spin_test_count = 10000000;
416 SYSCTL_INT(_debug, OID_AUTO, spin_test_count, CTLFLAG_RW, &spin_test_count, 0,
417 "Number of iterations to use for spinlock wait code test");
418
419 static int
420 sysctl_spin_lock_test(SYSCTL_HANDLER_ARGS)
421 {
422 struct spinlock spin;
423 int error;
424 int value = 0;
425 int i;
426
427 if ((error = priv_check(curthread, PRIV_ROOT)) != 0)
428 return (error);
429 if ((error = SYSCTL_IN(req, &value, sizeof(value))) != 0)
430 return (error);
431
432 /*
433 * Indefinite wait test
434 */
435 if (value == 1) {
436 spin_init(&spin);
437 spin_lock(&spin); /* force an indefinite wait */
438 spin_lock_test_mode = 1;
439 spin_lock(&spin);
440 spin_unlock(&spin); /* Clean up the spinlock count */
441 spin_unlock(&spin);
442 spin_lock_test_mode = 0;
443 }
444
445 /*
446 * Time best-case exclusive spinlocks
447 */
448 if (value == 2) {
449 globaldata_t gd = mycpu;
450
451 spin_init(&spin);
452 for (i = spin_test_count; i > 0; --i) {
453 _spin_lock_quick(gd, &spin, "test");
454 spin_unlock_quick(gd, &spin);
455 }
456 }
457
458 return (0);
459 }
460
461 SYSCTL_PROC(_debug, KERN_PROC_ALL, spin_lock_test, CTLFLAG_RW|CTLTYPE_INT,
462 0, 0, sysctl_spin_lock_test, "I", "Test spinlock wait code");
463
464 #endif /* INVARIANTS */
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