Message-ID: <20090420212225.GA5819@linux>
Date:	Mon, 20 Apr 2009 23:22:27 +0200
From:	Andrea Righi <righi.andrea@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Paul Menage <menage@...gle.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	agk@...rceware.org, akpm@...ux-foundation.org, axboe@...nel.dk,
	baramsori72@...il.com, Carl Henrik Lunde <chlunde@...g.uio.no>,
	dave@...ux.vnet.ibm.com, Divyesh Shah <dpshah@...gle.com>,
	eric.rannaud@...il.com, fernando@....ntt.co.jp,
	Hirokazu Takahashi <taka@...inux.co.jp>,
	Li Zefan <lizf@...fujitsu.com>, matt@...ehost.com,
	dradford@...ehost.com, ngupta@...gle.com, randy.dunlap@...cle.com,
	roberto@...it.it, Ryo Tsuruta <ryov@...inux.co.jp>,
	Satoshi UCHIDA <s-uchida@...jp.nec.com>,
	subrata@...ux.vnet.ibm.com, yoshikawa.takuya@....ntt.co.jp,
	Nauman Rafique <nauman@...gle.com>, fchecconi@...il.com,
	paolo.valente@...more.it, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] io-throttle controller infrastructure

On Mon, Apr 20, 2009 at 10:59:04AM -0700, Paul E. McKenney wrote:
> On Sat, Apr 18, 2009 at 11:38:29PM +0200, Andrea Righi wrote:
> > This is the core of the io-throttle kernel infrastructure. It creates
> > the basic interfaces to the cgroup subsystem and implements the I/O
> > measurement and throttling functionality.
> 
> A few questions interspersed below.
> 
> 							Thanx, Paul
> 
> > Signed-off-by: Gui Jianfeng <guijianfeng@...fujitsu.com>
> > Signed-off-by: Andrea Righi <righi.andrea@...il.com>
> > ---
> >  block/Makefile                  |    1 +
> >  block/blk-io-throttle.c         |  822 +++++++++++++++++++++++++++++++++++++++
> >  include/linux/blk-io-throttle.h |  144 +++++++
> >  include/linux/cgroup_subsys.h   |    6 +
> >  init/Kconfig                    |   12 +
> >  5 files changed, 985 insertions(+), 0 deletions(-)
> >  create mode 100644 block/blk-io-throttle.c
> >  create mode 100644 include/linux/blk-io-throttle.h
> > 
> > diff --git a/block/Makefile b/block/Makefile
> > index e9fa4dd..42b6a46 100644
> > --- a/block/Makefile
> > +++ b/block/Makefile
> > @@ -13,5 +13,6 @@ obj-$(CONFIG_IOSCHED_AS)	+= as-iosched.o
> >  obj-$(CONFIG_IOSCHED_DEADLINE)	+= deadline-iosched.o
> >  obj-$(CONFIG_IOSCHED_CFQ)	+= cfq-iosched.o
> > 
> > +obj-$(CONFIG_CGROUP_IO_THROTTLE)	+= blk-io-throttle.o
> >  obj-$(CONFIG_BLOCK_COMPAT)	+= compat_ioctl.o
> >  obj-$(CONFIG_BLK_DEV_INTEGRITY)	+= blk-integrity.o
> > diff --git a/block/blk-io-throttle.c b/block/blk-io-throttle.c
> > new file mode 100644
> > index 0000000..c8214fc
> > --- /dev/null
> > +++ b/block/blk-io-throttle.c
> > @@ -0,0 +1,822 @@
> > +/*
> > + * blk-io-throttle.c
> > + *
> > + * 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.
> > + *
> > + * You should have received a copy of the GNU General Public
> > + * License along with this program; if not, write to the
> > + * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
> > + * Boston, MA 021110-1307, USA.
> > + *
> > + * Copyright (C) 2008 Andrea Righi <righi.andrea@...il.com>
> > + */
> > +
> > +#include <linux/init.h>
> > +#include <linux/module.h>
> > +#include <linux/res_counter.h>
> > +#include <linux/memcontrol.h>
> > +#include <linux/slab.h>
> > +#include <linux/gfp.h>
> > +#include <linux/err.h>
> > +#include <linux/genhd.h>
> > +#include <linux/hardirq.h>
> > +#include <linux/list.h>
> > +#include <linux/seq_file.h>
> > +#include <linux/spinlock.h>
> > +#include <linux/blk-io-throttle.h>
> > +#include <linux/mm.h>
> > +#include <linux/page_cgroup.h>
> > +#include <linux/sched.h>
> > +#include <linux/bio.h>
> > +
> > +/*
> > + * Statistics for I/O bandwidth controller.
> > + */
> > +enum iothrottle_stat_index {
> > +	/* # of times the cgroup has been throttled for bw limit */
> > +	IOTHROTTLE_STAT_BW_COUNT,
> > +	/* # of jiffies spent to sleep for throttling for bw limit */
> > +	IOTHROTTLE_STAT_BW_SLEEP,
> > +	/* # of times the cgroup has been throttled for iops limit */
> > +	IOTHROTTLE_STAT_IOPS_COUNT,
> > +	/* # of jiffies spent to sleep for throttling for iops limit */
> > +	IOTHROTTLE_STAT_IOPS_SLEEP,
> > +	/* total number of bytes read and written */
> > +	IOTHROTTLE_STAT_BYTES_TOT,
> > +	/* total number of I/O operations */
> > +	IOTHROTTLE_STAT_IOPS_TOT,
> > +
> > +	IOTHROTTLE_STAT_NSTATS,
> > +};
> > +
> > +struct iothrottle_stat_cpu {
> > +	unsigned long long count[IOTHROTTLE_STAT_NSTATS];
> > +} ____cacheline_aligned_in_smp;
> > +
> > +struct iothrottle_stat {
> > +	struct iothrottle_stat_cpu cpustat[NR_CPUS];
> > +};
> > +
> > +static void iothrottle_stat_add(struct iothrottle_stat *stat,
> > +			enum iothrottle_stat_index type, unsigned long long val)
> > +{
> > +	int cpu = get_cpu();
> > +
> > +	stat->cpustat[cpu].count[type] += val;
> > +	put_cpu();
> > +}
> > +
> > +static void iothrottle_stat_add_sleep(struct iothrottle_stat *stat,
> > +			int type, unsigned long long sleep)
> > +{
> > +	int cpu = get_cpu();
> > +
> > +	switch (type) {
> > +	case IOTHROTTLE_BANDWIDTH:
> > +		stat->cpustat[cpu].count[IOTHROTTLE_STAT_BW_COUNT]++;
> > +		stat->cpustat[cpu].count[IOTHROTTLE_STAT_BW_SLEEP] += sleep;
> > +		break;
> > +	case IOTHROTTLE_IOPS:
> > +		stat->cpustat[cpu].count[IOTHROTTLE_STAT_IOPS_COUNT]++;
> > +		stat->cpustat[cpu].count[IOTHROTTLE_STAT_IOPS_SLEEP] += sleep;
> > +		break;
> > +	}
> > +	put_cpu();
> > +}
> > +
> > +static unsigned long long iothrottle_read_stat(struct iothrottle_stat *stat,
> > +				enum iothrottle_stat_index idx)
> > +{
> > +	int cpu;
> > +	unsigned long long ret = 0;
> > +
> > +	for_each_possible_cpu(cpu)
> > +		ret += stat->cpustat[cpu].count[idx];
> > +	return ret;
> > +}
> > +
> > +struct iothrottle_sleep {
> > +	unsigned long long bw_sleep;
> > +	unsigned long long iops_sleep;
> > +};
> > +
> > +/*
> > + * struct iothrottle_node - throttling rule of a single block device
> > + * @node: list of per block device throttling rules
> > + * @dev: block device number, used as key in the list
> > + * @bw: max i/o bandwidth (in bytes/s)
> > + * @iops: max i/o operations per second
> > + * @stat: throttling statistics
> > + *
> > + * Define a i/o throttling rule for a single block device.
> > + *
> > + * NOTE: limiting rules always refer to dev_t; if a block device is unplugged
> > + * the limiting rules defined for that device persist and they are still valid
> > + * if a new device is plugged and it uses the same dev_t number.
> > + */
> > +struct iothrottle_node {
> > +	struct list_head node;
> > +	dev_t dev;
> > +	struct res_counter bw;
> > +	struct res_counter iops;
> > +	struct iothrottle_stat stat;
> > +};
> > +
> > +/**
> > + * struct iothrottle - throttling rules for a cgroup
> > + * @css: pointer to the cgroup state
> > + * @list: list of iothrottle_node elements
> > + *
> > + * Define multiple per-block device i/o throttling rules.
> > + * Note: the list of the throttling rules is protected by RCU locking:
> > + *	 - hold cgroup_lock() for update.
> > + *	 - hold rcu_read_lock() for read.
> > + */
> > +struct iothrottle {
> > +	struct cgroup_subsys_state css;
> > +	struct list_head list;
> > +};
> > +static struct iothrottle init_iothrottle;
> > +
> > +static inline struct iothrottle *cgroup_to_iothrottle(struct cgroup *cgrp)
> > +{
> > +	return container_of(cgroup_subsys_state(cgrp, iothrottle_subsys_id),
> > +			    struct iothrottle, css);
> > +}
> > +
> > +/*
> > + * Note: called with rcu_read_lock() held.
> > + */
> > +static inline struct iothrottle *task_to_iothrottle(struct task_struct *task)
> > +{
> > +	return container_of(task_subsys_state(task, iothrottle_subsys_id),
> > +			    struct iothrottle, css);
> 
> OK, task_subsys_state() has an rcu_dereference(), so...

Do you mean the comment is obvious and it can be just removed?

> 
> > +}
> > +
> > +/*
> > + * Note: called with rcu_read_lock() or iot->lock held.
> > + */
> > +static struct iothrottle_node *
> > +iothrottle_search_node(const struct iothrottle *iot, dev_t dev)
> > +{
> > +	struct iothrottle_node *n;
> > +
> > +	if (list_empty(&iot->list))
> > +		return NULL;
> > +	list_for_each_entry_rcu(n, &iot->list, node)
> > +		if (n->dev == dev)
> > +			return n;
> > +	return NULL;
> > +}
> > +
> > +/*
> > + * Note: called with iot->lock held.
> 
> Should this be a WARN_ON() or something similar?  The machine is unable
> to enforce a comment.  ;-)

Right. :) Actually this is an old and never fixed comment... the
iot->list is always modified only under cgroup_lock(), so there's no
need to introduce another lock in struct iothrottle.

Anyway, adding a WARN_ON() seems a good idea, maybe adding a
WARN_ON_ONCE(cgroup_is_locked()) and define cgroup_is_locked() of
course, because cgroup_mutex is not exported outside kernel/cgroup.c.

I'll fix the comment and add the check.

> 
> > + */
> > +static inline void iothrottle_insert_node(struct iothrottle *iot,
> > +						struct iothrottle_node *n)
> > +{
> > +	list_add_rcu(&n->node, &iot->list);
> > +}
> > +
> > +/*
> > + * Note: called with iot->lock held.
> 
> Ditto.

OK, see above.

> 
> > + */
> > +static inline void
> > +iothrottle_replace_node(struct iothrottle *iot, struct iothrottle_node *old,
> > +			struct iothrottle_node *new)
> > +{
> > +	list_replace_rcu(&old->node, &new->node);
> > +}
> > +
> > +/*
> > + * Note: called with iot->lock held.
> 
> Ditto.

OK, see above.

> 
> > + */
> > +static inline void
> > +iothrottle_delete_node(struct iothrottle *iot, struct iothrottle_node *n)
> > +{
> > +	list_del_rcu(&n->node);
> > +}
> > +
> > +/*
> > + * Note: called from kernel/cgroup.c with cgroup_lock() held.
> > + */
> > +static struct cgroup_subsys_state *
> > +iothrottle_create(struct cgroup_subsys *ss, struct cgroup *cgrp)
> > +{
> > +	struct iothrottle *iot;
> > +
> > +	if (unlikely((cgrp->parent) == NULL)) {
> > +		iot = &init_iothrottle;
> > +	} else {
> > +		iot = kzalloc(sizeof(*iot), GFP_KERNEL);
> > +		if (unlikely(!iot))
> > +			return ERR_PTR(-ENOMEM);
> > +	}
> > +	INIT_LIST_HEAD(&iot->list);
> > +
> > +	return &iot->css;
> > +}
> > +
> > +/*
> > + * Note: called from kernel/cgroup.c with cgroup_lock() held.
> > + */
> > +static void iothrottle_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp)
> > +{
> > +	struct iothrottle_node *n, *p;
> > +	struct iothrottle *iot = cgroup_to_iothrottle(cgrp);
> > +
> > +	free_css_id(&iothrottle_subsys, &iot->css);
> > +	/*
> > +	 * don't worry about locking here, at this point there must be not any
> > +	 * reference to the list.
> > +	 */
> > +	if (!list_empty(&iot->list))
> > +		list_for_each_entry_safe(n, p, &iot->list, node)
> > +			kfree(n);
> > +	kfree(iot);
> > +}
> > +
> > +/*
> > + * NOTE: called with rcu_read_lock() held.
> > + *
> > + * do not care too much about locking for single res_counter values here.
> > + */
> > +static void iothrottle_show_limit(struct seq_file *m, dev_t dev,
> > +			struct res_counter *res)
> > +{
> > +	if (!res->limit)
> > +		return;
> > +	seq_printf(m, "%u %u %llu %llu %lli %llu %li\n",
> > +		MAJOR(dev), MINOR(dev),
> > +		res->limit, res->policy,
> > +		(long long)res->usage, res->capacity,
> > +		jiffies_to_clock_t(res_counter_ratelimit_delta_t(res)));
> 
> OK, looks like the rcu_dereference() in the list_for_each_entry_rcu() in
> the caller suffices here.  But thought I should ask the question anyway,
> even though at first glance it does look correct.
> 
> > +}
> > +
> > +/*
> > + * NOTE: called with rcu_read_lock() held.
> > + *
> > + */
> > +static void iothrottle_show_failcnt(struct seq_file *m, dev_t dev,
> > +				struct iothrottle_stat *stat)
> > +{
> > +	unsigned long long bw_count, bw_sleep, iops_count, iops_sleep;
> > +
> > +	bw_count = iothrottle_read_stat(stat, IOTHROTTLE_STAT_BW_COUNT);
> > +	bw_sleep = iothrottle_read_stat(stat, IOTHROTTLE_STAT_BW_SLEEP);
> > +	iops_count = iothrottle_read_stat(stat, IOTHROTTLE_STAT_IOPS_COUNT);
> > +	iops_sleep = iothrottle_read_stat(stat, IOTHROTTLE_STAT_IOPS_SLEEP);
> > +
> > +	seq_printf(m, "%u %u %llu %li %llu %li\n", MAJOR(dev), MINOR(dev),
> > +		bw_count, jiffies_to_clock_t(bw_sleep),
> > +		iops_count, jiffies_to_clock_t(iops_sleep));
> 
> Ditto.
> 
> > +}
> > +
> > +/*
> > + * NOTE: called with rcu_read_lock() held.
> > + */
> > +static void iothrottle_show_stat(struct seq_file *m, dev_t dev,
> > +				struct iothrottle_stat *stat)
> > +{
> > +	unsigned long long bytes, iops;
> > +
> > +	bytes = iothrottle_read_stat(stat, IOTHROTTLE_STAT_BYTES_TOT);
> > +	iops = iothrottle_read_stat(stat, IOTHROTTLE_STAT_IOPS_TOT);
> > +
> > +	seq_printf(m, "%u %u %llu %llu\n", MAJOR(dev), MINOR(dev), bytes, iops);
> 
> Ditto.
> 
> > +}
> > +
> > +static int iothrottle_read(struct cgroup *cgrp, struct cftype *cft,
> > +				struct seq_file *m)
> > +{
> > +	struct iothrottle *iot = cgroup_to_iothrottle(cgrp);
> > +	struct iothrottle_node *n;
> > +
> > +	rcu_read_lock();
> > +	if (list_empty(&iot->list))
> > +		goto unlock_and_return;
> > +	list_for_each_entry_rcu(n, &iot->list, node) {
> > +		BUG_ON(!n->dev);
> > +		switch (cft->private) {
> > +		case IOTHROTTLE_BANDWIDTH:
> > +			iothrottle_show_limit(m, n->dev, &n->bw);
> > +			break;
> > +		case IOTHROTTLE_IOPS:
> > +			iothrottle_show_limit(m, n->dev, &n->iops);
> > +			break;
> > +		case IOTHROTTLE_FAILCNT:
> > +			iothrottle_show_failcnt(m, n->dev, &n->stat);
> > +			break;
> > +		case IOTHROTTLE_STAT:
> > +			iothrottle_show_stat(m, n->dev, &n->stat);
> > +			break;
> > +		}
> > +	}
> > +unlock_and_return:
> > +	rcu_read_unlock();
> > +	return 0;
> > +}
> > +
> > +static dev_t devname2dev_t(const char *buf)
> > +{
> > +	struct block_device *bdev;
> > +	dev_t dev = 0;
> > +	struct gendisk *disk;
> > +	int part;
> > +
> > +	/* use a lookup to validate the block device */
> > +	bdev = lookup_bdev(buf);
> > +	if (IS_ERR(bdev))
> > +		return 0;
> > +	/* only entire devices are allowed, not single partitions */
> > +	disk = get_gendisk(bdev->bd_dev, &part);
> > +	if (disk && !part) {
> > +		BUG_ON(!bdev->bd_inode);
> > +		dev = bdev->bd_inode->i_rdev;
> > +	}
> > +	bdput(bdev);
> > +
> > +	return dev;
> > +}
> > +
> > +/*
> > + * The userspace input string must use one of the following syntaxes:
> > + *
> > + * dev:0			<- delete an i/o limiting rule
> > + * dev:io-limit:0		<- set a leaky bucket throttling rule
> > + * dev:io-limit:1:bucket-size	<- set a token bucket throttling rule
> > + * dev:io-limit:1		<- set a token bucket throttling rule using
> > + *				   bucket-size == io-limit
> > + */
> > +static int iothrottle_parse_args(char *buf, size_t nbytes, int filetype,
> > +			dev_t *dev, unsigned long long *iolimit,
> > +			unsigned long long *strategy,
> > +			unsigned long long *bucket_size)
> > +{
> > +	char *p;
> > +	int count = 0;
> > +	char *s[4];
> > +	int ret;
> > +
> > +	memset(s, 0, sizeof(s));
> > +	*dev = 0;
> > +	*iolimit = 0;
> > +	*strategy = 0;
> > +	*bucket_size = 0;
> > +
> > +	/* split the colon-delimited input string into its elements */
> > +	while (count < ARRAY_SIZE(s)) {
> > +		p = strsep(&buf, ":");
> > +		if (!p)
> > +			break;
> > +		if (!*p)
> > +			continue;
> > +		s[count++] = p;
> > +	}
> > +
> > +	/* i/o limit */
> > +	if (!s[1])
> > +		return -EINVAL;
> > +	ret = strict_strtoull(s[1], 10, iolimit);
> > +	if (ret < 0)
> > +		return ret;
> > +	if (!*iolimit)
> > +		goto out;
> > +	/* throttling strategy (leaky bucket / token bucket) */
> > +	if (!s[2])
> > +		return -EINVAL;
> > +	ret = strict_strtoull(s[2], 10, strategy);
> > +	if (ret < 0)
> > +		return ret;
> > +	switch (*strategy) {
> > +	case RATELIMIT_LEAKY_BUCKET:
> > +		goto out;
> > +	case RATELIMIT_TOKEN_BUCKET:
> > +		break;
> > +	default:
> > +		return -EINVAL;
> > +	}
> > +	/* bucket size */
> > +	if (!s[3])
> > +		*bucket_size = *iolimit;
> > +	else {
> > +		ret = strict_strtoll(s[3], 10, bucket_size);
> > +		if (ret < 0)
> > +			return ret;
> > +	}
> > +	if (*bucket_size <= 0)
> > +		return -EINVAL;
> > +out:
> > +	/* block device number */
> > +	*dev = devname2dev_t(s[0]);
> > +	return *dev ? 0 : -EINVAL;
> > +}
> > +
> > +static int iothrottle_write(struct cgroup *cgrp, struct cftype *cft,
> > +				const char *buffer)
> > +{
> > +	struct iothrottle *iot;
> > +	struct iothrottle_node *n, *newn = NULL;
> > +	dev_t dev;
> > +	unsigned long long iolimit, strategy, bucket_size;
> > +	char *buf;
> > +	size_t nbytes = strlen(buffer);
> > +	int ret = 0;
> > +
> > +	/*
> > +	 * We need to allocate a new buffer here, because
> > +	 * iothrottle_parse_args() can modify it and the buffer provided by
> > +	 * write_string is supposed to be const.
> > +	 */
> > +	buf = kmalloc(nbytes + 1, GFP_KERNEL);
> > +	if (!buf)
> > +		return -ENOMEM;
> > +	memcpy(buf, buffer, nbytes + 1);
> > +
> > +	ret = iothrottle_parse_args(buf, nbytes, cft->private, &dev, &iolimit,
> > +				&strategy, &bucket_size);
> > +	if (ret)
> > +		goto out1;
> > +	newn = kzalloc(sizeof(*newn), GFP_KERNEL);
> > +	if (!newn) {
> > +		ret = -ENOMEM;
> > +		goto out1;
> > +	}
> > +	newn->dev = dev;
> > +	res_counter_init(&newn->bw, NULL);
> > +	res_counter_init(&newn->iops, NULL);
> > +
> > +	switch (cft->private) {
> > +	case IOTHROTTLE_BANDWIDTH:
> > +		res_counter_ratelimit_set_limit(&newn->iops, 0, 0, 0);
> > +		res_counter_ratelimit_set_limit(&newn->bw, strategy,
> > +				ALIGN(iolimit, 1024), ALIGN(bucket_size, 1024));
> > +		break;
> > +	case IOTHROTTLE_IOPS:
> > +		res_counter_ratelimit_set_limit(&newn->bw, 0, 0, 0);
> > +		/*
> > +		 * scale up iops cost by a factor of 1000, this allows to apply
> > +		 * a more fine grained sleeps, and throttling results more
> > +		 * precise this way.
> > +		 */
> > +		res_counter_ratelimit_set_limit(&newn->iops, strategy,
> > +				iolimit * 1000, bucket_size * 1000);
> > +		break;
> > +	default:
> > +		WARN_ON(1);
> > +		break;
> > +	}
> > +
> > +	if (!cgroup_lock_live_group(cgrp)) {
> > +		ret = -ENODEV;
> > +		goto out1;
> > +	}
> > +	iot = cgroup_to_iothrottle(cgrp);
> > +
> > +	n = iothrottle_search_node(iot, dev);
> > +	if (!n) {
> > +		if (iolimit) {
> > +			/* Add a new block device limiting rule */
> > +			iothrottle_insert_node(iot, newn);
> > +			newn = NULL;
> > +		}
> > +		goto out2;
> > +	}
> > +	switch (cft->private) {
> > +	case IOTHROTTLE_BANDWIDTH:
> > +		if (!iolimit && !n->iops.limit) {
> > +			/* Delete a block device limiting rule */
> > +			iothrottle_delete_node(iot, n);
> > +			goto out2;
> > +		}
> > +		if (!n->iops.limit)
> > +			break;
> > +		/* Update a block device limiting rule */
> > +		newn->iops = n->iops;
> > +		break;
> > +	case IOTHROTTLE_IOPS:
> > +		if (!iolimit && !n->bw.limit) {
> > +			/* Delete a block device limiting rule */
> > +			iothrottle_delete_node(iot, n);
> > +			goto out2;
> > +		}
> > +		if (!n->bw.limit)
> > +			break;
> > +		/* Update a block device limiting rule */
> > +		newn->bw = n->bw;
> > +		break;
> > +	}
> > +	iothrottle_replace_node(iot, n, newn);
> > +	newn = NULL;
> > +out2:
> > +	cgroup_unlock();
> 
> How does the above lock relate to the iot->lock called out in the comment
> headers in the earlier functions?  Hmmm...  Come to think of it, I don't
> see an acquisition of iot->lock anywhere.
> 
> So, what is the story here?

As said before, only the comment in struct iothrottle is correct, we use
cgroup_lock() to protect iot->list, so there's no need to introduce
another lock inside struct iothrottle.

And the other comments about iot->lock must be fixed.

> 
> > +	if (n) {
> > +		synchronize_rcu();
> > +		kfree(n);
> > +	}
> > +out1:
> > +	kfree(newn);
> > +	kfree(buf);
> > +	return ret;
> > +}
> > +
> > +static struct cftype files[] = {
> > +	{
> > +		.name = "bandwidth-max",
> > +		.read_seq_string = iothrottle_read,
> > +		.write_string = iothrottle_write,
> > +		.max_write_len = 256,
> > +		.private = IOTHROTTLE_BANDWIDTH,
> > +	},
> > +	{
> > +		.name = "iops-max",
> > +		.read_seq_string = iothrottle_read,
> > +		.write_string = iothrottle_write,
> > +		.max_write_len = 256,
> > +		.private = IOTHROTTLE_IOPS,
> > +	},
> > +	{
> > +		.name = "throttlecnt",
> > +		.read_seq_string = iothrottle_read,
> > +		.private = IOTHROTTLE_FAILCNT,
> > +	},
> > +	{
> > +		.name = "stat",
> > +		.read_seq_string = iothrottle_read,
> > +		.private = IOTHROTTLE_STAT,
> > +	},
> > +};
> > +
> > +static int iothrottle_populate(struct cgroup_subsys *ss, struct cgroup *cgrp)
> > +{
> > +	return cgroup_add_files(cgrp, ss, files, ARRAY_SIZE(files));
> > +}
> > +
> > +struct cgroup_subsys iothrottle_subsys = {
> > +	.name = "blockio",
> > +	.create = iothrottle_create,
> > +	.destroy = iothrottle_destroy,
> > +	.populate = iothrottle_populate,
> > +	.subsys_id = iothrottle_subsys_id,
> > +	.early_init = 1,
> > +	.use_id = 1,
> > +};
> > +
> > +/*
> > + * NOTE: called with rcu_read_lock() held.
> > + */
> > +static void iothrottle_evaluate_sleep(struct iothrottle_sleep *sleep,
> > +				struct iothrottle *iot,
> > +				struct block_device *bdev, ssize_t bytes)
> > +{
> > +	struct iothrottle_node *n;
> > +	dev_t dev;
> > +
> > +	BUG_ON(!iot);
> > +
> > +	/* accounting and throttling is done only on entire block devices */
> > +	dev = MKDEV(MAJOR(bdev->bd_inode->i_rdev), bdev->bd_disk->first_minor);
> > +	n = iothrottle_search_node(iot, dev);
> > +	if (!n)
> > +		return;
> > +
> > +	/* Update statistics */
> > +	iothrottle_stat_add(&n->stat, IOTHROTTLE_STAT_BYTES_TOT, bytes);
> > +	if (bytes)
> > +		iothrottle_stat_add(&n->stat, IOTHROTTLE_STAT_IOPS_TOT, 1);
> > +
> > +	/* Evaluate sleep values */
> > +	sleep->bw_sleep = res_counter_ratelimit_sleep(&n->bw, bytes);
> > +	/*
> > +	 * scale up iops cost by a factor of 1000, this allows to apply
> > +	 * a more fine grained sleeps, and throttling works better in
> > +	 * this way.
> > +	 *
> > +	 * Note: do not account any i/o operation if bytes is negative or zero.
> > +	 */
> > +	sleep->iops_sleep = res_counter_ratelimit_sleep(&n->iops,
> > +						bytes ? 1000 : 0);
> > +}
> > +
> > +/*
> > + * NOTE: called with rcu_read_lock() held.
> > + */
> > +static void iothrottle_acct_stat(struct iothrottle *iot,
> > +			struct block_device *bdev, int type,
> > +			unsigned long long sleep)
> > +{
> > +	struct iothrottle_node *n;
> > +	dev_t dev = MKDEV(MAJOR(bdev->bd_inode->i_rdev),
> > +			bdev->bd_disk->first_minor);
> > +
> > +	n = iothrottle_search_node(iot, dev);
> > +	if (!n)
> > +		return;
> > +	iothrottle_stat_add_sleep(&n->stat, type, sleep);
> > +}
> > +
> > +static void iothrottle_acct_task_stat(int type, unsigned long long sleep)
> > +{
> > +	/*
> > +	 * XXX: per-task statistics may be inaccurate (this is not a
> > +	 * critical issue, anyway, respect to introduce locking
> > +	 * overhead or increase the size of task_struct).
> > +	 */
> > +	switch (type) {
> > +	case IOTHROTTLE_BANDWIDTH:
> > +		current->io_throttle_bw_cnt++;
> > +		current->io_throttle_bw_sleep += sleep;
> > +		break;
> > +
> > +	case IOTHROTTLE_IOPS:
> > +		current->io_throttle_iops_cnt++;
> > +		current->io_throttle_iops_sleep += sleep;
> > +		break;
> > +	}
> > +}
> > +
> > +/*
> > + * A helper function to get iothrottle from css id.
> > + *
> > + * NOTE: must be called under rcu_read_lock(). The caller must check
> > + * css_is_removed() or some if it's concern.
> > + */
> > +static struct iothrottle *iothrottle_lookup(unsigned long id)
> > +{
> > +	struct cgroup_subsys_state *css;
> > +
> > +	if (!id)
> > +		return NULL;
> > +	css = css_lookup(&iothrottle_subsys, id);
> > +	if (!css)
> > +		return NULL;
> > +	return container_of(css, struct iothrottle, css);
> > +}
> > +
> > +static struct iothrottle *get_iothrottle_from_page(struct page *page)
> > +{
> > +	struct iothrottle *iot;
> > +	unsigned long id;
> > +
> > +	BUG_ON(!page);
> > +	id = page_cgroup_get_owner(page);
> > +
> > +	rcu_read_lock();
> > +	iot = iothrottle_lookup(id);
> > +	if (!iot)
> > +		goto out;
> > +	css_get(&iot->css);
> > +out:
> > +	rcu_read_unlock();
> > +	return iot;
> > +}
> > +
> > +static struct iothrottle *get_iothrottle_from_bio(struct bio *bio)
> > +{
> > +	if (!bio)
> > +		return NULL;
> > +	return get_iothrottle_from_page(bio_page(bio));
> > +}
> > +
> > +int iothrottle_set_page_owner(struct page *page, struct mm_struct *mm)
> > +{
> > +	struct iothrottle *iot;
> > +	unsigned short id = 0;
> > +
> > +	if (iothrottle_disabled())
> > +		return 0;
> > +	if (!mm)
> > +		goto out;
> > +	rcu_read_lock();
> > +	iot = task_to_iothrottle(rcu_dereference(mm->owner));
> 
> Given that task_to_iothrottle() calls task_subsys_state(), which contains
> an rcu_dereference(), why is the rcu_dereference() above required?
> (There might well be a good reason, just cannot see it right offhand.)

The first rcu_dereference() is required to safely get a task_struct from
mm_struct. The second rcu_dereference() inside task_to_iothrottle() is
required to safely get the struct iothrottle from task_struct.

Thanks for your comments!
-Andrea
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