FreeBSD/Linux Kernel Cross Reference
sys/kernel/latencytop.c

     /*
      * latencytop.c: Latency display infrastructure
      *
      * (C) Copyright 2008 Intel Corporation
      * Author: Arjan van de Ven <arjan@linux.intel.com>
      *
      * This program is free software; you can redistribute it and/or
      * modify it under the terms of the GNU General Public License
      * as published by the Free Software Foundation; version 2
     * of the License.
     */
    
    /*
     * CONFIG_LATENCYTOP enables a kernel latency tracking infrastructure that is
     * used by the "latencytop" userspace tool. The latency that is tracked is not
     * the 'traditional' interrupt latency (which is primarily caused by something
     * else consuming CPU), but instead, it is the latency an application encounters
     * because the kernel sleeps on its behalf for various reasons.
     *
     * This code tracks 2 levels of statistics:
     * 1) System level latency
     * 2) Per process latency
     *
     * The latency is stored in fixed sized data structures in an accumulated form;
     * if the "same" latency cause is hit twice, this will be tracked as one entry
     * in the data structure. Both the count, total accumulated latency and maximum
     * latency are tracked in this data structure. When the fixed size structure is
     * full, no new causes are tracked until the buffer is flushed by writing to
     * the /proc file; the userspace tool does this on a regular basis.
     *
     * A latency cause is identified by a stringified backtrace at the point that
     * the scheduler gets invoked. The userland tool will use this string to
     * identify the cause of the latency in human readable form.
     *
     * The information is exported via /proc/latency_stats and /proc/<pid>/latency.
     * These files look like this:
     *
     * Latency Top version : v0.1
     * 70 59433 4897 i915_irq_wait drm_ioctl vfs_ioctl do_vfs_ioctl sys_ioctl
     * |    |    |    |
     * |    |    |    +----> the stringified backtrace
     * |    |    +---------> The maximum latency for this entry in microseconds
     * |    +--------------> The accumulated latency for this entry (microseconds)
     * +-------------------> The number of times this entry is hit
     *
     * (note: the average latency is the accumulated latency divided by the number
     * of times)
     */
    
    #include <linux/latencytop.h>
    #include <linux/kallsyms.h>
    #include <linux/seq_file.h>
    #include <linux/notifier.h>
    #include <linux/spinlock.h>
    #include <linux/proc_fs.h>
    #include <linux/export.h>
    #include <linux/sched.h>
    #include <linux/list.h>
    #include <linux/stacktrace.h>
    
    static DEFINE_RAW_SPINLOCK(latency_lock);
    
    #define MAXLR 128
    static struct latency_record latency_record[MAXLR];
    
    int latencytop_enabled;
    
    void clear_all_latency_tracing(struct task_struct *p)
    {
            unsigned long flags;
    
            if (!latencytop_enabled)
                    return;
    
            raw_spin_lock_irqsave(&latency_lock, flags);
            memset(&p->latency_record, 0, sizeof(p->latency_record));
            p->latency_record_count = 0;
            raw_spin_unlock_irqrestore(&latency_lock, flags);
    }
    
    static void clear_global_latency_tracing(void)
    {
            unsigned long flags;
    
            raw_spin_lock_irqsave(&latency_lock, flags);
            memset(&latency_record, 0, sizeof(latency_record));
            raw_spin_unlock_irqrestore(&latency_lock, flags);
    }
    
    static void __sched
    account_global_scheduler_latency(struct task_struct *tsk, struct latency_record *lat)
    {
            int firstnonnull = MAXLR + 1;
            int i;
    
            if (!latencytop_enabled)
                    return;
    
            /* skip kernel threads for now */
           if (!tsk->mm)
                   return;
   
           for (i = 0; i < MAXLR; i++) {
                   int q, same = 1;
   
                   /* Nothing stored: */
                   if (!latency_record[i].backtrace[0]) {
                           if (firstnonnull > i)
                                   firstnonnull = i;
                           continue;
                   }
                   for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
                           unsigned long record = lat->backtrace[q];
   
                           if (latency_record[i].backtrace[q] != record) {
                                   same = 0;
                                   break;
                           }
   
                           /* 0 and ULONG_MAX entries mean end of backtrace: */
                           if (record == 0 || record == ULONG_MAX)
                                   break;
                   }
                   if (same) {
                           latency_record[i].count++;
                           latency_record[i].time += lat->time;
                           if (lat->time > latency_record[i].max)
                                   latency_record[i].max = lat->time;
                           return;
                   }
           }
   
           i = firstnonnull;
           if (i >= MAXLR - 1)
                   return;
   
           /* Allocted a new one: */
           memcpy(&latency_record[i], lat, sizeof(struct latency_record));
   }
   
   /*
    * Iterator to store a backtrace into a latency record entry
    */
   static inline void store_stacktrace(struct task_struct *tsk,
                                           struct latency_record *lat)
   {
           struct stack_trace trace;
   
           memset(&trace, 0, sizeof(trace));
           trace.max_entries = LT_BACKTRACEDEPTH;
           trace.entries = &lat->backtrace[0];
           save_stack_trace_tsk(tsk, &trace);
   }
   
   /**
    * __account_scheduler_latency - record an occurred latency
    * @tsk - the task struct of the task hitting the latency
    * @usecs - the duration of the latency in microseconds
    * @inter - 1 if the sleep was interruptible, 0 if uninterruptible
    *
    * This function is the main entry point for recording latency entries
    * as called by the scheduler.
    *
    * This function has a few special cases to deal with normal 'non-latency'
    * sleeps: specifically, interruptible sleep longer than 5 msec is skipped
    * since this usually is caused by waiting for events via select() and co.
    *
    * Negative latencies (caused by time going backwards) are also explicitly
    * skipped.
    */
   void __sched
   __account_scheduler_latency(struct task_struct *tsk, int usecs, int inter)
   {
           unsigned long flags;
           int i, q;
           struct latency_record lat;
   
           /* Long interruptible waits are generally user requested... */
           if (inter && usecs > 5000)
                   return;
   
           /* Negative sleeps are time going backwards */
           /* Zero-time sleeps are non-interesting */
           if (usecs <= 0)
                   return;
   
           memset(&lat, 0, sizeof(lat));
           lat.count = 1;
           lat.time = usecs;
           lat.max = usecs;
           store_stacktrace(tsk, &lat);
   
           raw_spin_lock_irqsave(&latency_lock, flags);
   
           account_global_scheduler_latency(tsk, &lat);
   
           for (i = 0; i < tsk->latency_record_count; i++) {
                   struct latency_record *mylat;
                   int same = 1;
   
                   mylat = &tsk->latency_record[i];
                   for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
                           unsigned long record = lat.backtrace[q];
   
                           if (mylat->backtrace[q] != record) {
                                   same = 0;
                                   break;
                           }
   
                           /* 0 and ULONG_MAX entries mean end of backtrace: */
                           if (record == 0 || record == ULONG_MAX)
                                   break;
                   }
                   if (same) {
                           mylat->count++;
                           mylat->time += lat.time;
                           if (lat.time > mylat->max)
                                   mylat->max = lat.time;
                           goto out_unlock;
                   }
           }
   
           /*
            * short term hack; if we're > 32 we stop; future we recycle:
            */
           if (tsk->latency_record_count >= LT_SAVECOUNT)
                   goto out_unlock;
   
           /* Allocated a new one: */
           i = tsk->latency_record_count++;
           memcpy(&tsk->latency_record[i], &lat, sizeof(struct latency_record));
   
   out_unlock:
           raw_spin_unlock_irqrestore(&latency_lock, flags);
   }
   
   static int lstats_show(struct seq_file *m, void *v)
   {
           int i;
   
           seq_puts(m, "Latency Top version : v0.1\n");
   
           for (i = 0; i < MAXLR; i++) {
                   struct latency_record *lr = &latency_record[i];
   
                   if (lr->backtrace[0]) {
                           int q;
                           seq_printf(m, "%i %lu %lu",
                                      lr->count, lr->time, lr->max);
                           for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
                                   unsigned long bt = lr->backtrace[q];
                                   if (!bt)
                                           break;
                                   if (bt == ULONG_MAX)
                                           break;
                                   seq_printf(m, " %ps", (void *)bt);
                           }
                           seq_printf(m, "\n");
                   }
           }
           return 0;
   }
   
   static ssize_t
   lstats_write(struct file *file, const char __user *buf, size_t count,
                loff_t *offs)
   {
           clear_global_latency_tracing();
   
           return count;
   }
   
   static int lstats_open(struct inode *inode, struct file *filp)
   {
           return single_open(filp, lstats_show, NULL);
   }
   
   static const struct file_operations lstats_fops = {
           .open           = lstats_open,
           .read           = seq_read,
           .write          = lstats_write,
           .llseek         = seq_lseek,
           .release        = single_release,
   };
   
   static int __init init_lstats_procfs(void)
   {
           proc_create("latency_stats", 0644, NULL, &lstats_fops);
           return 0;
   }
   device_initcall(init_lstats_procfs);

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