kernel/locking/qspinlock_stat.h

   1 /*
   2  * This program is free software; you can redistribute it and/or modify
   3  * it under the terms of the GNU General Public License as published by
   4  * the Free Software Foundation; either version 2 of the License, or
   5  * (at your option) any later version.
   6  *
   7  * This program is distributed in the hope that it will be useful,
   8  * but WITHOUT ANY WARRANTY; without even the implied warranty of
   9  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  10  * GNU General Public License for more details.
  11  *
  12  * Authors: Waiman Long <waiman.long@hpe.com>
  13  */
  14
  15 /*
  16  * When queued spinlock statistical counters are enabled, the following
  17  * debugfs files will be created for reporting the counter values:
  18  *
  19  * <debugfs>/qlockstat/
  20  *   pv_hash_hops       - average # of hops per hashing operation
  21  *   pv_kick_unlock     - # of vCPU kicks issued at unlock time
  22  *   pv_kick_wake       - # of vCPU kicks used for computing pv_latency_wake
  23  *   pv_latency_kick    - average latency (ns) of vCPU kick operation
  24  *   pv_latency_wake    - average latency (ns) from vCPU kick to wakeup
  25  *   pv_lock_stealing   - # of lock stealing operations
  26  *   pv_spurious_wakeup - # of spurious wakeups in non-head vCPUs
  27  *   pv_wait_again      - # of wait's after a queue head vCPU kick
  28  *   pv_wait_early      - # of early vCPU wait's
  29  *   pv_wait_head       - # of vCPU wait's at the queue head
  30  *   pv_wait_node       - # of vCPU wait's at a non-head queue node
  31  *   lock_pending       - # of locking operations via pending code
  32  *   lock_slowpath      - # of locking operations via MCS lock queue
  33  *
  34  * Writing to the "reset_counters" file will reset all the above counter
  35  * values.
  36  *
  37  * These statistical counters are implemented as per-cpu variables which are
  38  * summed and computed whenever the corresponding debugfs files are read. This
  39  * minimizes added overhead making the counters usable even in a production
  40  * environment.
  41  *
  42  * There may be slight difference between pv_kick_wake and pv_kick_unlock.
  43  */
  44 enum qlock_stats {
  45         qstat_pv_hash_hops,
  46         qstat_pv_kick_unlock,
  47         qstat_pv_kick_wake,
  48         qstat_pv_latency_kick,
  49         qstat_pv_latency_wake,
  50         qstat_pv_lock_stealing,
  51         qstat_pv_spurious_wakeup,
  52         qstat_pv_wait_again,
  53         qstat_pv_wait_early,
  54         qstat_pv_wait_head,
  55         qstat_pv_wait_node,
  56         qstat_lock_pending,
  57         qstat_lock_slowpath,
  58         qstat_num,      /* Total number of statistical counters */
  59         qstat_reset_cnts = qstat_num,
  60 };
  61
  62 #ifdef CONFIG_QUEUED_LOCK_STAT
  63 /*
  64  * Collect pvqspinlock statistics
  65  */
  66 #include <linux/debugfs.h>
  67 #include <linux/sched.h>
  68 #include <linux/sched/clock.h>
  69 #include <linux/fs.h>
  70
  71 static const char * const qstat_names[qstat_num + 1] = {
  72         [qstat_pv_hash_hops]       = "pv_hash_hops",
  73         [qstat_pv_kick_unlock]     = "pv_kick_unlock",
  74         [qstat_pv_kick_wake]       = "pv_kick_wake",
  75         [qstat_pv_spurious_wakeup] = "pv_spurious_wakeup",
  76         [qstat_pv_latency_kick]    = "pv_latency_kick",
  77         [qstat_pv_latency_wake]    = "pv_latency_wake",
  78         [qstat_pv_lock_stealing]   = "pv_lock_stealing",
  79         [qstat_pv_wait_again]      = "pv_wait_again",
  80         [qstat_pv_wait_early]      = "pv_wait_early",
  81         [qstat_pv_wait_head]       = "pv_wait_head",
  82         [qstat_pv_wait_node]       = "pv_wait_node",
  83         [qstat_lock_pending]       = "lock_pending",
  84         [qstat_lock_slowpath]      = "lock_slowpath",
  85         [qstat_reset_cnts]         = "reset_counters",
  86 };
  87
  88 /*
  89  * Per-cpu counters
  90  */
  91 static DEFINE_PER_CPU(unsigned long, qstats[qstat_num]);
  92 static DEFINE_PER_CPU(u64, pv_kick_time);
  93
  94 /*
  95  * Function to read and return the qlock statistical counter values
  96  *
  97  * The following counters are handled specially:
  98  * 1. qstat_pv_latency_kick
  99  *    Average kick latency (ns) = pv_latency_kick/pv_kick_unlock
 100  * 2. qstat_pv_latency_wake
 101  *    Average wake latency (ns) = pv_latency_wake/pv_kick_wake
 102  * 3. qstat_pv_hash_hops
 103  *    Average hops/hash = pv_hash_hops/pv_kick_unlock
 104  */
 105 static ssize_t qstat_read(struct file *file, char __user *user_buf,
 106                           size_t count, loff_t *ppos)
 107 {
 108         char buf[64];
 109         int cpu, counter, len;
 110         u64 stat = 0, kicks = 0;
 111
 112         /*
 113          * Get the counter ID stored in file->f_inode->i_private
 114          */
 115         counter = (long)file_inode(file)->i_private;
 116
 117         if (counter >= qstat_num)
 118                 return -EBADF;
 119
 120         for_each_possible_cpu(cpu) {
 121                 stat += per_cpu(qstats[counter], cpu);
 122                 /*
 123                  * Need to sum additional counter for some of them
 124                  */
 125                 switch (counter) {
 126
 127                 case qstat_pv_latency_kick:
 128                 case qstat_pv_hash_hops:
 129                         kicks += per_cpu(qstats[qstat_pv_kick_unlock], cpu);
 130                         break;
 131
 132                 case qstat_pv_latency_wake:
 133                         kicks += per_cpu(qstats[qstat_pv_kick_wake], cpu);
 134                         break;
 135                 }
 136         }
 137
 138         if (counter == qstat_pv_hash_hops) {
 139                 u64 frac = 0;
 140
 141                 if (kicks) {
 142                         frac = 100ULL * do_div(stat, kicks);
 143                         frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
 144                 }
 145
 146                 /*
 147                  * Return a X.XX decimal number
 148                  */
 149                 len = snprintf(buf, sizeof(buf) - 1, "%llu.%02llu\n", stat, frac);
 150         } else {
 151                 /*
 152                  * Round to the nearest ns
 153                  */
 154                 if ((counter == qstat_pv_latency_kick) ||
 155                     (counter == qstat_pv_latency_wake)) {
 156                         if (kicks)
 157                                 stat = DIV_ROUND_CLOSEST_ULL(stat, kicks);
 158                 }
 159                 len = snprintf(buf, sizeof(buf) - 1, "%llu\n", stat);
 160         }
 161
 162         return simple_read_from_buffer(user_buf, count, ppos, buf, len);
 163 }
 164
 165 /*
 166  * Function to handle write request
 167  *
 168  * When counter = reset_cnts, reset all the counter values.
 169  * Since the counter updates aren't atomic, the resetting is done twice
 170  * to make sure that the counters are very likely to be all cleared.
 171  */
 172 static ssize_t qstat_write(struct file *file, const char __user *user_buf,
 173                            size_t count, loff_t *ppos)
 174 {
 175         int cpu;
 176
 177         /*
 178          * Get the counter ID stored in file->f_inode->i_private
 179          */
 180         if ((long)file_inode(file)->i_private != qstat_reset_cnts)
 181                 return count;
 182
 183         for_each_possible_cpu(cpu) {
 184                 int i;
 185                 unsigned long *ptr = per_cpu_ptr(qstats, cpu);
 186
 187                 for (i = 0 ; i < qstat_num; i++)
 188                         WRITE_ONCE(ptr[i], 0);
 189         }
 190         return count;
 191 }
 192
 193 /*
 194  * Debugfs data structures
 195  */
 196 static const struct file_operations fops_qstat = {
 197         .read = qstat_read,
 198         .write = qstat_write,
 199         .llseek = default_llseek,
 200 };
 201
 202 /*
 203  * Initialize debugfs for the qspinlock statistical counters
 204  */
 205 static int __init init_qspinlock_stat(void)
 206 {
 207         struct dentry *d_qstat = debugfs_create_dir("qlockstat", NULL);
 208         int i;
 209
 210         if (!d_qstat)
 211                 goto out;
 212
 213         /*
 214          * Create the debugfs files
 215          *
 216          * As reading from and writing to the stat files can be slow, only
 217          * root is allowed to do the read/write to limit impact to system
 218          * performance.
 219          */
 220         for (i = 0; i < qstat_num; i++)
 221                 if (!debugfs_create_file(qstat_names[i], 0400, d_qstat,
 222                                          (void *)(long)i, &fops_qstat))
 223                         goto fail_undo;
 224
 225         if (!debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
 226                                  (void *)(long)qstat_reset_cnts, &fops_qstat))
 227                 goto fail_undo;
 228
 229         return 0;
 230 fail_undo:
 231         debugfs_remove_recursive(d_qstat);
 232 out:
 233         pr_warn("Could not create 'qlockstat' debugfs entries\n");
 234         return -ENOMEM;
 235 }
 236 fs_initcall(init_qspinlock_stat);
 237
 238 /*
 239  * Increment the PV qspinlock statistical counters
 240  */
 241 static inline void qstat_inc(enum qlock_stats stat, bool cond)
 242 {
 243         if (cond)
 244                 this_cpu_inc(qstats[stat]);
 245 }
 246
 247 /*
 248  * PV hash hop count
 249  */
 250 static inline void qstat_hop(int hopcnt)
 251 {
 252         this_cpu_add(qstats[qstat_pv_hash_hops], hopcnt);
 253 }
 254
 255 /*
 256  * Replacement function for pv_kick()
 257  */
 258 static inline void __pv_kick(int cpu)
 259 {
 260         u64 start = sched_clock();
 261
 262         per_cpu(pv_kick_time, cpu) = start;
 263         pv_kick(cpu);
 264         this_cpu_add(qstats[qstat_pv_latency_kick], sched_clock() - start);
 265 }
 266
 267 /*
 268  * Replacement function for pv_wait()
 269  */
 270 static inline void __pv_wait(u8 *ptr, u8 val)
 271 {
 272         u64 *pkick_time = this_cpu_ptr(&pv_kick_time);
 273
 274         *pkick_time = 0;
 275         pv_wait(ptr, val);
 276         if (*pkick_time) {
 277                 this_cpu_add(qstats[qstat_pv_latency_wake],
 278                              sched_clock() - *pkick_time);
 279                 qstat_inc(qstat_pv_kick_wake, true);
 280         }
 281 }
 282
 283 #define pv_kick(c)      __pv_kick(c)
 284 #define pv_wait(p, v)   __pv_wait(p, v)
 285
 286 #else /* CONFIG_QUEUED_LOCK_STAT */
 287
 288 static inline void qstat_inc(enum qlock_stats stat, bool cond)  { }
 289 static inline void qstat_hop(int hopcnt)                        { }
 290
 291 #endif /* CONFIG_QUEUED_LOCK_STAT */