linux/kernel/locking/lock_events.c

/* SPDX-License-Identifier: GPL-2.0 */
/*
 * 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; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * Authors: Waiman Long <waiman.long@hpe.com>
 */

/*
 * Collect locking event counts
 */
#include <linux/debugfs.h>
#include <linux/sched.h>
#include <linux/sched/clock.h>
#include <linux/fs.h>

#include "lock_events.h"

#undef  LOCK_EVENT
#define LOCK_EVENT(name)	[LOCKEVENT_ ## name] = #name,

#define LOCK_EVENTS_DIR		"lock_event_counts"

/*
 * When CONFIG_LOCK_EVENT_COUNTS is enabled, event counts of different
 * types of locks will be reported under the <debugfs>/lock_event_counts/
 * directory. See lock_events_list.h for the list of available locking
 * events.
 *
 * Writing to the special ".reset_counts" file will reset all the above
 * locking event counts. This is a very slow operation and so should not
 * be done frequently.
 *
 * These event counts are implemented as per-cpu variables which are
 * summed and computed whenever the corresponding debugfs files are read. This
 * minimizes added overhead making the counts usable even in a production
 * environment.
 */
static const char * const lockevent_names[lockevent_num + 1] = {

#include "lock_events_list.h"

	[LOCKEVENT_reset_cnts] = ".reset_counts",
};

/*
 * Per-cpu counts
 */
DEFINE_PER_CPU(unsigned long, lockevents[lockevent_num]);

/*
 * The lockevent_read() function can be overridden.
 */
ssize_t __weak lockevent_read(struct file *file, char __user *user_buf,
			      size_t count, loff_t *ppos)
{
	char buf[64];
	int cpu, id, len;
	u64 sum = 0;

	/*
	 * Get the counter ID stored in file->f_inode->i_private
	 */
	id = (long)file_inode(file)->i_private;

	if (id >= lockevent_num)
		return -EBADF;

	for_each_possible_cpu(cpu)
		sum += per_cpu(lockevents[id], cpu);
	len = snprintf(buf, sizeof(buf) - 1, "%llu\n", sum);

	return simple_read_from_buffer(user_buf, count, ppos, buf, len);
}

/*
 * Function to handle write request
 *
 * When idx = reset_cnts, reset all the counts.
 */
static ssize_t lockevent_write(struct file *file, const char __user *user_buf,
			   size_t count, loff_t *ppos)
{
	int cpu;

	/*
	 * Get the counter ID stored in file->f_inode->i_private
	 */
	if ((long)file_inode(file)->i_private != LOCKEVENT_reset_cnts)
		return count;

	for_each_possible_cpu(cpu) {
		int i;
		unsigned long *ptr = per_cpu_ptr(lockevents, cpu);

		for (i = 0 ; i < lockevent_num; i++)
			WRITE_ONCE(ptr[i], 0);
	}
	return count;
}

/*
 * Debugfs data structures
 */
static const struct file_operations fops_lockevent = {
	.read = lockevent_read,
	.write = lockevent_write,
	.llseek = default_llseek,
};

#ifdef CONFIG_PARAVIRT_SPINLOCKS
#include <asm/paravirt.h>

static bool __init skip_lockevent(const char *name)
{
	static int pv_on __initdata = -1;

	if (pv_on < 0)
		pv_on = !pv_is_native_spin_unlock();
	/*
	 * Skip PV qspinlock events on bare metal.
	 */
	if (!pv_on && !memcmp(name, "pv_", 3))
		return true;
	return false;
}
#else
static inline bool skip_lockevent(const char *name)
{
	return false;
}
#endif

/*
 * Initialize debugfs for the locking event counts.
 */
static int __init init_lockevent_counts(void)
{
	struct dentry *d_counts = debugfs_create_dir(LOCK_EVENTS_DIR, NULL);
	int i;

	if (!d_counts)
		goto out;

	/*
	 * Create the debugfs files
	 *
	 * As reading from and writing to the stat files can be slow, only
	 * root is allowed to do the read/write to limit impact to system
	 * performance.
	 */
	for (i = 0; i < lockevent_num; i++) {
		if (skip_lockevent(lockevent_names[i]))
			continue;
		if (!debugfs_create_file(lockevent_names[i], 0400, d_counts,
					 (void *)(long)i, &fops_lockevent))
			goto fail_undo;
	}

	if (!debugfs_create_file(lockevent_names[LOCKEVENT_reset_cnts], 0200,
				 d_counts, (void *)(long)LOCKEVENT_reset_cnts,
				 &fops_lockevent))
		goto fail_undo;

	return 0;
fail_undo:
	debugfs_remove_recursive(d_counts);
out:
	pr_warn("Could not create '%s' debugfs entries\n", LOCK_EVENTS_DIR);
	return -ENOMEM;
}
fs_initcall(init_lockevent_counts);