latencytop.c

Go to the documentation of this file.

/*
 * latencytop.c: Latency display infrastructure
 *
 * (C) Copyright 2008 Intel Corporation
 * Author: Arjan van de Ven <[email protected]>
 *
 * 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);
}

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);