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_slowpath   - # of locking operations via the slowpath
  26  *   pv_lock_stealing   - # of lock stealing operations
  27  *   pv_spurious_wakeup - # of spurious wakeups in non-head vCPUs
  28  *   pv_wait_again      - # of wait's after a queue head vCPU kick
  29  *   pv_wait_early      - # of early vCPU wait's
  30  *   pv_wait_head       - # of vCPU wait's at the queue head
  31  *   pv_wait_node       - # of vCPU wait's at a non-head queue node
  32  *
  33  * Writing to the "reset_counters" file will reset all the above counter
  34  * values.
  35  *
  36  * These statistical counters are implemented as per-cpu variables which are
  37  * summed and computed whenever the corresponding debugfs files are read. This
  38  * minimizes added overhead making the counters usable even in a production
  39  * environment.
  40  *
  41  * There may be slight difference between pv_kick_wake and pv_kick_unlock.
  42  */
  43 enum qlock_stats {
  44         qstat_pv_hash_hops,
  45         qstat_pv_kick_unlock,
  46         qstat_pv_kick_wake,
  47         qstat_pv_latency_kick,
  48         qstat_pv_latency_wake,
  49         qstat_pv_lock_slowpath,
  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_num,      /* Total number of statistical counters */
  57         qstat_reset_cnts = qstat_num,
  58 };
  59
  60 #ifdef CONFIG_QUEUED_LOCK_STAT
  61 /*
  62  * Collect pvqspinlock statistics
  63  */
  64 #include <linux/debugfs.h>
  65 #include <linux/sched.h>
  66 #include <linux/sched/clock.h>
  67 #include <linux/fs.h>
  68
  69 static const char * const qstat_names[qstat_num + 1] = {
  70         [qstat_pv_hash_hops]       = "pv_hash_hops",
  71         [qstat_pv_kick_unlock]     = "pv_kick_unlock",
  72         [qstat_pv_kick_wake]       = "pv_kick_wake",
  73         [qstat_pv_spurious_wakeup] = "pv_spurious_wakeup",
  74         [qstat_pv_latency_kick]    = "pv_latency_kick",
  75         [qstat_pv_latency_wake]    = "pv_latency_wake",
  76         [qstat_pv_lock_slowpath]   = "pv_lock_slowpath",
  77         [qstat_pv_lock_stealing]   = "pv_lock_stealing",
  78         [qstat_pv_wait_again]      = "pv_wait_again",
  79         [qstat_pv_wait_early]      = "pv_wait_early",
  80         [qstat_pv_wait_head]       = "pv_wait_head",
  81         [qstat_pv_wait_node]       = "pv_wait_node",
  82         [qstat_reset_cnts]         = "reset_counters",
  83 };
  84
  85 /*
  86  * Per-cpu counters
  87  */
  88 static DEFINE_PER_CPU(unsigned long, qstats[qstat_num]);
  89 static DEFINE_PER_CPU(u64, pv_kick_time);
  90
  91 /*
  92  * Function to read and return the qlock statistical counter values
  93  *
  94  * The following counters are handled specially:
  95  * 1. qstat_pv_latency_kick
  96  *    Average kick latency (ns) = pv_latency_kick/pv_kick_unlock
  97  * 2. qstat_pv_latency_wake
  98  *    Average wake latency (ns) = pv_latency_wake/pv_kick_wake
  99  * 3. qstat_pv_hash_hops
 100  *    Average hops/hash = pv_hash_hops/pv_kick_unlock
 101  */
 102 static ssize_t qstat_read(struct file *file, char __user *user_buf,
 103                           size_t count, loff_t *ppos)
 104 {
 105         char buf[64];
 106         int cpu, counter, len;
 107         u64 stat = 0, kicks = 0;
 108
 109         /*
 110          * Get the counter ID stored in file->f_inode->i_private
 111          */
 112         counter = (long)file_inode(file)->i_private;
 113
 114         if (counter >= qstat_num)
 115                 return -EBADF;
 116
 117         for_each_possible_cpu(cpu) {
 118                 stat += per_cpu(qstats[counter], cpu);
 119                 /*
 120                  * Need to sum additional counter for some of them
 121                  */
 122                 switch (counter) {
 123
 124                 case qstat_pv_latency_kick:
 125                 case qstat_pv_hash_hops:
 126                         kicks += per_cpu(qstats[qstat_pv_kick_unlock], cpu);
 127                         break;
 128
 129                 case qstat_pv_latency_wake:
 130                         kicks += per_cpu(qstats[qstat_pv_kick_wake], cpu);
 131                         break;
 132                 }
 133         }
 134
 135         if (counter == qstat_pv_hash_hops) {
 136                 u64 frac = 0;
 137
 138                 if (kicks) {
 139                         frac = 100ULL * do_div(stat, kicks);
 140                         frac = DIV_ROUND_CLOSEST_ULL(frac, kicks);
 141                 }
 142
 143                 /*
 144                  * Return a X.XX decimal number
 145                  */
 146                 len = snprintf(buf, sizeof(buf) - 1, "%llu.%02llu\n", stat, frac);
 147         } else {
 148                 /*
 149                  * Round to the nearest ns
 150                  */
 151                 if ((counter == qstat_pv_latency_kick) ||
 152                     (counter == qstat_pv_latency_wake)) {
 153                         if (kicks)
 154                                 stat = DIV_ROUND_CLOSEST_ULL(stat, kicks);
 155                 }
 156                 len = snprintf(buf, sizeof(buf) - 1, "%llu\n", stat);
 157         }
 158
 159         return simple_read_from_buffer(user_buf, count, ppos, buf, len);
 160 }
 161
 162 /*
 163  * Function to handle write request
 164  *
 165  * When counter = reset_cnts, reset all the counter values.
 166  * Since the counter updates aren't atomic, the resetting is done twice
 167  * to make sure that the counters are very likely to be all cleared.
 168  */
 169 static ssize_t qstat_write(struct file *file, const char __user *user_buf,
 170                            size_t count, loff_t *ppos)
 171 {
 172         int cpu;
 173
 174         /*
 175          * Get the counter ID stored in file->f_inode->i_private
 176          */
 177         if ((long)file_inode(file)->i_private != qstat_reset_cnts)
 178                 return count;
 179
 180         for_each_possible_cpu(cpu) {
 181                 int i;
 182                 unsigned long *ptr = per_cpu_ptr(qstats, cpu);
 183
 184                 for (i = 0 ; i < qstat_num; i++)
 185                         WRITE_ONCE(ptr[i], 0);
 186         }
 187         return count;
 188 }
 189
 190 /*
 191  * Debugfs data structures
 192  */
 193 static const struct file_operations fops_qstat = {
 194         .read = qstat_read,
 195         .write = qstat_write,
 196         .llseek = default_llseek,
 197 };
 198
 199 /*
 200  * Initialize debugfs for the qspinlock statistical counters
 201  */
 202 static int __init init_qspinlock_stat(void)
 203 {
 204         struct dentry *d_qstat = debugfs_create_dir("qlockstat", NULL);
 205         int i;
 206
 207         if (!d_qstat)
 208                 goto out;
 209
 210         /*
 211          * Create the debugfs files
 212          *
 213          * As reading from and writing to the stat files can be slow, only
 214          * root is allowed to do the read/write to limit impact to system
 215          * performance.
 216          */
 217         for (i = 0; i < qstat_num; i++)
 218                 if (!debugfs_create_file(qstat_names[i], 0400, d_qstat,
 219                                          (void *)(long)i, &fops_qstat))
 220                         goto fail_undo;
 221
 222         if (!debugfs_create_file(qstat_names[qstat_reset_cnts], 0200, d_qstat,
 223                                  (void *)(long)qstat_reset_cnts, &fops_qstat))
 224                 goto fail_undo;
 225
 226         return 0;
 227 fail_undo:
 228         debugfs_remove_recursive(d_qstat);
 229 out:
 230         pr_warn("Could not create 'qlockstat' debugfs entries\n");
 231         return -ENOMEM;
 232 }
 233 fs_initcall(init_qspinlock_stat);
 234
 235 /*
 236  * Increment the PV qspinlock statistical counters
 237  */
 238 static inline void qstat_inc(enum qlock_stats stat, bool cond)
 239 {
 240         if (cond)
 241                 this_cpu_inc(qstats[stat]);
 242 }
 243
 244 /*
 245  * PV hash hop count
 246  */
 247 static inline void qstat_hop(int hopcnt)
 248 {
 249         this_cpu_add(qstats[qstat_pv_hash_hops], hopcnt);
 250 }
 251
 252 /*
 253  * Replacement function for pv_kick()
 254  */
 255 static inline void __pv_kick(int cpu)
 256 {
 257         u64 start = sched_clock();
 258
 259         per_cpu(pv_kick_time, cpu) = start;
 260         pv_kick(cpu);
 261         this_cpu_add(qstats[qstat_pv_latency_kick], sched_clock() - start);
 262 }
 263
 264 /*
 265  * Replacement function for pv_wait()
 266  */
 267 static inline void __pv_wait(u8 *ptr, u8 val)
 268 {
 269         u64 *pkick_time = this_cpu_ptr(&pv_kick_time);
 270
 271         *pkick_time = 0;
 272         pv_wait(ptr, val);
 273         if (*pkick_time) {
 274                 this_cpu_add(qstats[qstat_pv_latency_wake],
 275                              sched_clock() - *pkick_time);
 276                 qstat_inc(qstat_pv_kick_wake, true);
 277         }
 278 }
 279
 280 #define pv_kick(c)      __pv_kick(c)
 281 #define pv_wait(p, v)   __pv_wait(p, v)
 282
 283 #else /* CONFIG_QUEUED_LOCK_STAT */
 284
 285 static inline void qstat_inc(enum qlock_stats stat, bool cond)  { }
 286 static inline void qstat_hop(int hopcnt)                        { }
 287
 288 #endif /* CONFIG_QUEUED_LOCK_STAT */