xref: /openbmc/linux/arch/x86/xen/spinlock.c (revision 8b036556)
1 /*
2  * Split spinlock implementation out into its own file, so it can be
3  * compiled in a FTRACE-compatible way.
4  */
5 #include <linux/kernel_stat.h>
6 #include <linux/spinlock.h>
7 #include <linux/debugfs.h>
8 #include <linux/log2.h>
9 #include <linux/gfp.h>
10 #include <linux/slab.h>
11 
12 #include <asm/paravirt.h>
13 
14 #include <xen/interface/xen.h>
15 #include <xen/events.h>
16 
17 #include "xen-ops.h"
18 #include "debugfs.h"
19 
20 enum xen_contention_stat {
21 	TAKEN_SLOW,
22 	TAKEN_SLOW_PICKUP,
23 	TAKEN_SLOW_SPURIOUS,
24 	RELEASED_SLOW,
25 	RELEASED_SLOW_KICKED,
26 	NR_CONTENTION_STATS
27 };
28 
29 
30 #ifdef CONFIG_XEN_DEBUG_FS
31 #define HISTO_BUCKETS	30
32 static struct xen_spinlock_stats
33 {
34 	u32 contention_stats[NR_CONTENTION_STATS];
35 	u32 histo_spin_blocked[HISTO_BUCKETS+1];
36 	u64 time_blocked;
37 } spinlock_stats;
38 
39 static u8 zero_stats;
40 
41 static inline void check_zero(void)
42 {
43 	u8 ret;
44 	u8 old = READ_ONCE(zero_stats);
45 	if (unlikely(old)) {
46 		ret = cmpxchg(&zero_stats, old, 0);
47 		/* This ensures only one fellow resets the stat */
48 		if (ret == old)
49 			memset(&spinlock_stats, 0, sizeof(spinlock_stats));
50 	}
51 }
52 
53 static inline void add_stats(enum xen_contention_stat var, u32 val)
54 {
55 	check_zero();
56 	spinlock_stats.contention_stats[var] += val;
57 }
58 
59 static inline u64 spin_time_start(void)
60 {
61 	return xen_clocksource_read();
62 }
63 
64 static void __spin_time_accum(u64 delta, u32 *array)
65 {
66 	unsigned index = ilog2(delta);
67 
68 	check_zero();
69 
70 	if (index < HISTO_BUCKETS)
71 		array[index]++;
72 	else
73 		array[HISTO_BUCKETS]++;
74 }
75 
76 static inline void spin_time_accum_blocked(u64 start)
77 {
78 	u32 delta = xen_clocksource_read() - start;
79 
80 	__spin_time_accum(delta, spinlock_stats.histo_spin_blocked);
81 	spinlock_stats.time_blocked += delta;
82 }
83 #else  /* !CONFIG_XEN_DEBUG_FS */
84 static inline void add_stats(enum xen_contention_stat var, u32 val)
85 {
86 }
87 
88 static inline u64 spin_time_start(void)
89 {
90 	return 0;
91 }
92 
93 static inline void spin_time_accum_blocked(u64 start)
94 {
95 }
96 #endif  /* CONFIG_XEN_DEBUG_FS */
97 
98 struct xen_lock_waiting {
99 	struct arch_spinlock *lock;
100 	__ticket_t want;
101 };
102 
103 static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
104 static DEFINE_PER_CPU(char *, irq_name);
105 static DEFINE_PER_CPU(struct xen_lock_waiting, lock_waiting);
106 static cpumask_t waiting_cpus;
107 
108 static bool xen_pvspin = true;
109 __visible void xen_lock_spinning(struct arch_spinlock *lock, __ticket_t want)
110 {
111 	int irq = __this_cpu_read(lock_kicker_irq);
112 	struct xen_lock_waiting *w = this_cpu_ptr(&lock_waiting);
113 	int cpu = smp_processor_id();
114 	u64 start;
115 	__ticket_t head;
116 	unsigned long flags;
117 
118 	/* If kicker interrupts not initialized yet, just spin */
119 	if (irq == -1)
120 		return;
121 
122 	start = spin_time_start();
123 
124 	/*
125 	 * Make sure an interrupt handler can't upset things in a
126 	 * partially setup state.
127 	 */
128 	local_irq_save(flags);
129 	/*
130 	 * We don't really care if we're overwriting some other
131 	 * (lock,want) pair, as that would mean that we're currently
132 	 * in an interrupt context, and the outer context had
133 	 * interrupts enabled.  That has already kicked the VCPU out
134 	 * of xen_poll_irq(), so it will just return spuriously and
135 	 * retry with newly setup (lock,want).
136 	 *
137 	 * The ordering protocol on this is that the "lock" pointer
138 	 * may only be set non-NULL if the "want" ticket is correct.
139 	 * If we're updating "want", we must first clear "lock".
140 	 */
141 	w->lock = NULL;
142 	smp_wmb();
143 	w->want = want;
144 	smp_wmb();
145 	w->lock = lock;
146 
147 	/* This uses set_bit, which atomic and therefore a barrier */
148 	cpumask_set_cpu(cpu, &waiting_cpus);
149 	add_stats(TAKEN_SLOW, 1);
150 
151 	/* clear pending */
152 	xen_clear_irq_pending(irq);
153 
154 	/* Only check lock once pending cleared */
155 	barrier();
156 
157 	/*
158 	 * Mark entry to slowpath before doing the pickup test to make
159 	 * sure we don't deadlock with an unlocker.
160 	 */
161 	__ticket_enter_slowpath(lock);
162 
163 	/* make sure enter_slowpath, which is atomic does not cross the read */
164 	smp_mb__after_atomic();
165 
166 	/*
167 	 * check again make sure it didn't become free while
168 	 * we weren't looking
169 	 */
170 	head = READ_ONCE(lock->tickets.head);
171 	if (__tickets_equal(head, want)) {
172 		add_stats(TAKEN_SLOW_PICKUP, 1);
173 		goto out;
174 	}
175 
176 	/* Allow interrupts while blocked */
177 	local_irq_restore(flags);
178 
179 	/*
180 	 * If an interrupt happens here, it will leave the wakeup irq
181 	 * pending, which will cause xen_poll_irq() to return
182 	 * immediately.
183 	 */
184 
185 	/* Block until irq becomes pending (or perhaps a spurious wakeup) */
186 	xen_poll_irq(irq);
187 	add_stats(TAKEN_SLOW_SPURIOUS, !xen_test_irq_pending(irq));
188 
189 	local_irq_save(flags);
190 
191 	kstat_incr_irq_this_cpu(irq);
192 out:
193 	cpumask_clear_cpu(cpu, &waiting_cpus);
194 	w->lock = NULL;
195 
196 	local_irq_restore(flags);
197 
198 	spin_time_accum_blocked(start);
199 }
200 PV_CALLEE_SAVE_REGS_THUNK(xen_lock_spinning);
201 
202 static void xen_unlock_kick(struct arch_spinlock *lock, __ticket_t next)
203 {
204 	int cpu;
205 
206 	add_stats(RELEASED_SLOW, 1);
207 
208 	for_each_cpu(cpu, &waiting_cpus) {
209 		const struct xen_lock_waiting *w = &per_cpu(lock_waiting, cpu);
210 
211 		/* Make sure we read lock before want */
212 		if (READ_ONCE(w->lock) == lock &&
213 		    READ_ONCE(w->want) == next) {
214 			add_stats(RELEASED_SLOW_KICKED, 1);
215 			xen_send_IPI_one(cpu, XEN_SPIN_UNLOCK_VECTOR);
216 			break;
217 		}
218 	}
219 }
220 
221 static irqreturn_t dummy_handler(int irq, void *dev_id)
222 {
223 	BUG();
224 	return IRQ_HANDLED;
225 }
226 
227 void xen_init_lock_cpu(int cpu)
228 {
229 	int irq;
230 	char *name;
231 
232 	if (!xen_pvspin)
233 		return;
234 
235 	WARN(per_cpu(lock_kicker_irq, cpu) >= 0, "spinlock on CPU%d exists on IRQ%d!\n",
236 	     cpu, per_cpu(lock_kicker_irq, cpu));
237 
238 	name = kasprintf(GFP_KERNEL, "spinlock%d", cpu);
239 	irq = bind_ipi_to_irqhandler(XEN_SPIN_UNLOCK_VECTOR,
240 				     cpu,
241 				     dummy_handler,
242 				     IRQF_PERCPU|IRQF_NOBALANCING,
243 				     name,
244 				     NULL);
245 
246 	if (irq >= 0) {
247 		disable_irq(irq); /* make sure it's never delivered */
248 		per_cpu(lock_kicker_irq, cpu) = irq;
249 		per_cpu(irq_name, cpu) = name;
250 	}
251 
252 	printk("cpu %d spinlock event irq %d\n", cpu, irq);
253 }
254 
255 void xen_uninit_lock_cpu(int cpu)
256 {
257 	if (!xen_pvspin)
258 		return;
259 
260 	unbind_from_irqhandler(per_cpu(lock_kicker_irq, cpu), NULL);
261 	per_cpu(lock_kicker_irq, cpu) = -1;
262 	kfree(per_cpu(irq_name, cpu));
263 	per_cpu(irq_name, cpu) = NULL;
264 }
265 
266 
267 /*
268  * Our init of PV spinlocks is split in two init functions due to us
269  * using paravirt patching and jump labels patching and having to do
270  * all of this before SMP code is invoked.
271  *
272  * The paravirt patching needs to be done _before_ the alternative asm code
273  * is started, otherwise we would not patch the core kernel code.
274  */
275 void __init xen_init_spinlocks(void)
276 {
277 
278 	if (!xen_pvspin) {
279 		printk(KERN_DEBUG "xen: PV spinlocks disabled\n");
280 		return;
281 	}
282 	printk(KERN_DEBUG "xen: PV spinlocks enabled\n");
283 	pv_lock_ops.lock_spinning = PV_CALLEE_SAVE(xen_lock_spinning);
284 	pv_lock_ops.unlock_kick = xen_unlock_kick;
285 }
286 
287 /*
288  * While the jump_label init code needs to happend _after_ the jump labels are
289  * enabled and before SMP is started. Hence we use pre-SMP initcall level
290  * init. We cannot do it in xen_init_spinlocks as that is done before
291  * jump labels are activated.
292  */
293 static __init int xen_init_spinlocks_jump(void)
294 {
295 	if (!xen_pvspin)
296 		return 0;
297 
298 	if (!xen_domain())
299 		return 0;
300 
301 	static_key_slow_inc(&paravirt_ticketlocks_enabled);
302 	return 0;
303 }
304 early_initcall(xen_init_spinlocks_jump);
305 
306 static __init int xen_parse_nopvspin(char *arg)
307 {
308 	xen_pvspin = false;
309 	return 0;
310 }
311 early_param("xen_nopvspin", xen_parse_nopvspin);
312 
313 #ifdef CONFIG_XEN_DEBUG_FS
314 
315 static struct dentry *d_spin_debug;
316 
317 static int __init xen_spinlock_debugfs(void)
318 {
319 	struct dentry *d_xen = xen_init_debugfs();
320 
321 	if (d_xen == NULL)
322 		return -ENOMEM;
323 
324 	if (!xen_pvspin)
325 		return 0;
326 
327 	d_spin_debug = debugfs_create_dir("spinlocks", d_xen);
328 
329 	debugfs_create_u8("zero_stats", 0644, d_spin_debug, &zero_stats);
330 
331 	debugfs_create_u32("taken_slow", 0444, d_spin_debug,
332 			   &spinlock_stats.contention_stats[TAKEN_SLOW]);
333 	debugfs_create_u32("taken_slow_pickup", 0444, d_spin_debug,
334 			   &spinlock_stats.contention_stats[TAKEN_SLOW_PICKUP]);
335 	debugfs_create_u32("taken_slow_spurious", 0444, d_spin_debug,
336 			   &spinlock_stats.contention_stats[TAKEN_SLOW_SPURIOUS]);
337 
338 	debugfs_create_u32("released_slow", 0444, d_spin_debug,
339 			   &spinlock_stats.contention_stats[RELEASED_SLOW]);
340 	debugfs_create_u32("released_slow_kicked", 0444, d_spin_debug,
341 			   &spinlock_stats.contention_stats[RELEASED_SLOW_KICKED]);
342 
343 	debugfs_create_u64("time_blocked", 0444, d_spin_debug,
344 			   &spinlock_stats.time_blocked);
345 
346 	debugfs_create_u32_array("histo_blocked", 0444, d_spin_debug,
347 				spinlock_stats.histo_spin_blocked, HISTO_BUCKETS + 1);
348 
349 	return 0;
350 }
351 fs_initcall(xen_spinlock_debugfs);
352 
353 #endif	/* CONFIG_XEN_DEBUG_FS */
354