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_lock_idx1,
  59         qstat_lock_idx2,
  60         qstat_lock_idx3,
  61         qstat_num,      /* Total number of statistical counters */
  62         qstat_reset_cnts = qstat_num,
  63 };
  64
  65 #ifdef CONFIG_QUEUED_LOCK_STAT
  66 /*
  67  * Collect pvqspinlock statistics
  68  */
  69 #include <linux/debugfs.h>
  70 #include <linux/sched.h>
  71 #include <linux/sched/clock.h>
  72 #include <linux/fs.h>
  73
  74 static const char * const qstat_names[qstat_num + 1] = {
  75         [qstat_pv_hash_hops]       = "pv_hash_hops",
  76         [qstat_pv_kick_unlock]     = "pv_kick_unlock",
  77         [qstat_pv_kick_wake]       = "pv_kick_wake",
  78         [qstat_pv_spurious_wakeup] = "pv_spurious_wakeup",
  79         [qstat_pv_latency_kick]    = "pv_latency_kick",
  80         [qstat_pv_latency_wake]    = "pv_latency_wake",
  81         [qstat_pv_lock_stealing]   = "pv_lock_stealing",
  82         [qstat_pv_wait_again]      = "pv_wait_again",
  83         [qstat_pv_wait_early]      = "pv_wait_early",
  84         [qstat_pv_wait_head]       = "pv_wait_head",
  85         [qstat_pv_wait_node]       = "pv_wait_node",
  86         [qstat_lock_pending]       = "lock_pending",
  87         [qstat_lock_slowpath]      = "lock_slowpath",
  88         [qstat_lock_idx1]          = "lock_index1",
  89         [qstat_lock_idx2]          = "lock_index2",
  90         [qstat_lock_idx3]          = "lock_index3",
  91         [qstat_reset_cnts]         = "reset_counters",
  92 };
  93
  94 /*
  95  * Per-cpu counters
  96  */
  97 static DEFINE_PER_CPU(unsigned long, qstats[qstat_num]);
  98 static DEFINE_PER_CPU(u64, pv_kick_time);
  99
 100 /*
 101  * Function to read and return the qlock statistical counter values
 102  *
 103  * The following counters are handled specially:
 104  * 1. qstat_pv_latency_kick
 105  *    Average kick latency (ns) = pv_latency_kick/pv_kick_unlock
 106  * 2. qstat_pv_latency_wake
 107  *    Average wake latency (ns) = pv_latency_wake/pv_kick_wake
 108  * 3. qstat_pv_hash_hops
 109  *    Average hops/hash = pv_hash_hops/pv_kick_unlock
 110  */
 111 static ssize_t qstat_read(struct file *file, char __user *user_buf,
 112                           size_t count, loff_t *ppos)
 113 {
 114         char buf[64];
 115         int cpu, counter, len;
 116         u64 stat = 0, kicks = 0;
 117
 118         /*
 119          * Get the counter ID stored in file->f_inode->i_private
 120          */
 121         counter = (long)file_inode(file)->i_private;
 122
 123         if (counter >= qstat_num)
 124                 return -EBADF;
 125
 126         for_each_possible_cpu(cpu) {
 127                 stat += per_cpu(qstats[counter], cpu);
 128                 /*
 129                  * Need to sum additional counter for some of them
 130                  */
 131                 switch (counter) {
 132
 133                 case qstat_pv_latency_kick:
 134                 case qstat_pv_hash_hops:
 135                         kicks += per_cpu(qstats[qstat_pv_kick_unlock], cpu);
 136                         break;
 137
 138                 case qstat_pv_latency_wake:
 139                         kicks += per_cpu(qstats[qstat_pv_kick_wake], cpu);
 140                         break;
 141                 }
 142         }
 143
 144         if (counter == qstat_pv_hash_hops) {
 145                 u64 frac = 0;
 146
 147                 if (kicks) {
 148                         frac = 100ULL * do_div(stat, kicks);
 149                         frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
 150                 }
 151
 152                 /*
 153                  * Return a X.XX decimal number
 154                  */
 155                 len = snprintf(buf, sizeof(buf) - 1, "%llu.%02llu\n", stat, frac);
 156         } else {
 157                 /*
 158                  * Round to the nearest ns
 159                  */
 160                 if ((counter == qstat_pv_latency_kick) ||
 161                     (counter == qstat_pv_latency_wake)) {
 162                         if (kicks)
 163                                 stat = DIV_ROUND_CLOSEST_ULL(stat, kicks);
 164                 }
 165                 len = snprintf(buf, sizeof(buf) - 1, "%llu\n", stat);
 166         }
 167
 168         return simple_read_from_buffer(user_buf, count, ppos, buf, len);
 169 }
 170
 171 /*
 172  * Function to handle write request
 173  *
 174  * When counter = reset_cnts, reset all the counter values.
 175  * Since the counter updates aren't atomic, the resetting is done twice
 176  * to make sure that the counters are very likely to be all cleared.
 177  */
 178 static ssize_t qstat_write(struct file *file, const char __user *user_buf,
 179                            size_t count, loff_t *ppos)
 180 {
 181         int cpu;
 182
 183         /*
 184          * Get the counter ID stored in file->f_inode->i_private
 185          */
 186         if ((long)file_inode(file)->i_private != qstat_reset_cnts)
 187                 return count;
 188
 189         for_each_possible_cpu(cpu) {
 190                 int i;
 191                 unsigned long *ptr = per_cpu_ptr(qstats, cpu);
 192
 193                 for (i = 0 ; i < qstat_num; i++)
 194                         WRITE_ONCE(ptr[i], 0);
 195         }
 196         return count;
 197 }
 198
 199 /*
 200  * Debugfs data structures
 201  */
 202 static const struct file_operations fops_qstat = {
 203         .read = qstat_read,
 204         .write = qstat_write,
 205         .llseek = default_llseek,
 206 };
 207
 208 /*
 209  * Initialize debugfs for the qspinlock statistical counters
 210  */
 211 static int __init init_qspinlock_stat(void)
 212 {
 213         struct dentry *d_qstat = debugfs_create_dir("qlockstat", NULL);
 214         int i;
 215
 216         if (!d_qstat)
 217                 goto out;
 218
 219         /*
 220          * Create the debugfs files
 221          *
 222          * As reading from and writing to the stat files can be slow, only
 223          * root is allowed to do the read/write to limit impact to system
 224          * performance.
 225          */
 226         for (i = 0; i < qstat_num; i++)
 227                 if (!debugfs_create_file(qstat_names[i], 0400, d_qstat,
 228                                          (void *)(long)i, &fops_qstat))
 229                         goto fail_undo;
 230
 231         if (!debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
 232                                  (void *)(long)qstat_reset_cnts, &fops_qstat))
 233                 goto fail_undo;
 234
 235         return 0;
 236 fail_undo:
 237         debugfs_remove_recursive(d_qstat);
 238 out:
 239         pr_warn("Could not create 'qlockstat' debugfs entries\n");
 240         return -ENOMEM;
 241 }
 242 fs_initcall(init_qspinlock_stat);
 243
 244 /*
 245  * Increment the PV qspinlock statistical counters
 246  */
 247 static inline void qstat_inc(enum qlock_stats stat, bool cond)
 248 {
 249         if (cond)
 250                 this_cpu_inc(qstats[stat]);
 251 }
 252
 253 /*
 254  * PV hash hop count
 255  */
 256 static inline void qstat_hop(int hopcnt)
 257 {
 258         this_cpu_add(qstats[qstat_pv_hash_hops], hopcnt);
 259 }
 260
 261 /*
 262  * Replacement function for pv_kick()
 263  */
 264 static inline void __pv_kick(int cpu)
 265 {
 266         u64 start = sched_clock();
 267
 268         per_cpu(pv_kick_time, cpu) = start;
 269         pv_kick(cpu);
 270         this_cpu_add(qstats[qstat_pv_latency_kick], sched_clock() - start);
 271 }
 272
 273 /*
 274  * Replacement function for pv_wait()
 275  */
 276 static inline void __pv_wait(u8 *ptr, u8 val)
 277 {
 278         u64 *pkick_time = this_cpu_ptr(&pv_kick_time);
 279
 280         *pkick_time = 0;
 281         pv_wait(ptr, val);
 282         if (*pkick_time) {
 283                 this_cpu_add(qstats[qstat_pv_latency_wake],
 284                              sched_clock() - *pkick_time);
 285                 qstat_inc(qstat_pv_kick_wake, true);
 286         }
 287 }
 288
 289 #define pv_kick(c)      __pv_kick(c)
 290 #define pv_wait(p, v)   __pv_wait(p, v)
 291
 292 #else /* CONFIG_QUEUED_LOCK_STAT */
 293
 294 static inline void qstat_inc(enum qlock_stats stat, bool cond)  { }
 295 static inline void qstat_hop(int hopcnt)                        { }
 296
 297 #endif /* CONFIG_QUEUED_LOCK_STAT */