Date:	Thu, 18 Aug 2011 17:51:51 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Kautuk Consul <consul.kautuk@...il.com>
Cc:	Mel Gorman <mgorman@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Jan Kara <jack@...e.cz>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH] writeback: Per-block device
 bdi->dirty_writeback_interval and bdi->dirty_expire_interval.

[correct email to Mel and add Jan/Dave]

> On Thu, Aug 11, 2011 at 05:50:56PM +0530, Kautuk Consul wrote:
> > Hi,
> > 
> > Currently the /proc/sys/vm/dirty_writeback_centisecs and
> > /proc/sys/vm/dirty_expire_centisecs values are
> > global to the system.
> > All the BDI flush-* threads are controlled by these central values.
> 
> Yes.
> 
> > However, the user/admin might want to set different writeback speeds
> > for different block devices based on
> > their page write-back performance.
> 
> How can the above two sysctl values impact "writeback speeds"?
> In particular, what's the "speed" you mean?
> 
> > For example, the user might want to write-back pages in smaller
> > intervals to a block device which has a
> > faster known writeback speed.
> 
> That's not a complete rational. What does the user ultimately want by
> setting a smaller interval? What would be the problems to the other
> slow devices if the user does so by simply setting a small value
> _globally_?
> 
> We need strong use cases for doing such user interface changes.
> Would you detail the problem and the pains that can only (or best)
> be addressed by this patch?
> 
> Thanks,
> Fengguang
> 
> > This patch creates 3 new counters (in centisecs) for all the BDI
> > threads that were controlled centrally by these
> > 2 counters:
> > i)   /sys/block/<block_dev>/bdi/dirty_writeback_interval,
> > ii)  /sys/block/<block_dev>/bdi/dirty_expire_interval,
> > iii) /proc/sys/vm/sync_supers_centisecs.
> > 
> > Although these new counters can be tuned individually, I have taken
> > care that they be centrally reset by changes
> > to the /proc/sys/vm/dirty_expire_centisecs and
> > /proc/sys/vm/dirty_writeback_centisecs so that the earlier
> > functionality is not broken by distributions using these central values.
> > After resetting all values centrally, these values can be tuned
> > individually without altering the central values.
> > 
> > Signed-off-by: Kautuk Consul <consul.kautuk@...il.com>
> > ---
> > 
> > diff -uprN a/fs/fs-writeback.c b/fs/fs-writeback.c
> > --- a/fs/fs-writeback.c	2011-08-05 10:29:21.000000000 +0530
> > +++ b/fs/fs-writeback.c	2011-08-09 09:15:37.093041675 +0530
> > @@ -638,8 +638,8 @@ static inline bool over_bground_thresh(v
> >   * just walks the superblock inode list, writing back any inodes which are
> >   * older than a specific point in time.
> >   *
> > - * Try to run once per dirty_writeback_interval.  But if a writeback event
> > - * takes longer than a dirty_writeback_interval interval, then leave a
> > + * Try to run once per bdi->dirty_writeback_interval.  But if a writeback event
> > + * takes longer than a bdi->dirty_writeback_interval interval, then leave a
> >   * one-second gap.
> >   *
> >   * older_than_this takes precedence over nr_to_write.  So we'll only write back
> > @@ -663,7 +663,7 @@ static long wb_writeback(struct bdi_writ
> >  	if (wbc.for_kupdate) {
> >  		wbc.older_than_this = &oldest_jif;
> >  		oldest_jif = jiffies -
> > -				msecs_to_jiffies(dirty_expire_interval * 10);
> > +				msecs_to_jiffies(wb->bdi->dirty_expire_interval * 10);
> >  	}
> >  	if (!wbc.range_cyclic) {
> >  		wbc.range_start = 0;
> > @@ -811,15 +811,16 @@ static long wb_check_old_data_flush(stru
> >  {
> >  	unsigned long expired;
> >  	long nr_pages;
> > +	struct backing_dev_info *bdi = wb->bdi;
> > 
> >  	/*
> >  	 * When set to zero, disable periodic writeback
> >  	 */
> > -	if (!dirty_writeback_interval)
> > +	if (!bdi->dirty_writeback_interval)
> >  		return 0;
> > 
> >  	expired = wb->last_old_flush +
> > -			msecs_to_jiffies(dirty_writeback_interval * 10);
> > +			msecs_to_jiffies(bdi->dirty_writeback_interval * 10);
> >  	if (time_before(jiffies, expired))
> >  		return 0;
> > 
> > @@ -923,8 +924,8 @@ int bdi_writeback_thread(void *data)
> >  			continue;
> >  		}
> > 
> > -		if (wb_has_dirty_io(wb) && dirty_writeback_interval)
> > -			schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
> > +		if (wb_has_dirty_io(wb) && bdi->dirty_writeback_interval)
> > +			schedule_timeout(msecs_to_jiffies(bdi->dirty_writeback_interval * 10));
> >  		else {
> >  			/*
> >  			 * We have nothing to do, so can go sleep without any
> > diff -uprN a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> > --- a/include/linux/backing-dev.h	2011-08-05 10:29:21.000000000 +0530
> > +++ b/include/linux/backing-dev.h	2011-08-09 09:15:37.094041619 +0530
> > @@ -76,6 +76,8 @@ struct backing_dev_info {
> > 
> >  	unsigned int min_ratio;
> >  	unsigned int max_ratio, max_prop_frac;
> > +	unsigned int dirty_writeback_interval;
> > +	unsigned int dirty_expire_interval;
> > 
> >  	struct bdi_writeback wb;  /* default writeback info for this bdi */
> >  	spinlock_t wb_lock;	  /* protects work_list */
> > @@ -333,4 +335,5 @@ static inline int bdi_sched_wait(void *w
> >  	return 0;
> >  }
> > 
> > +extern unsigned int shortest_dirty_writeback_interval;
> >  #endif		/* _LINUX_BACKING_DEV_H */
> > diff -uprN a/include/linux/writeback.h b/include/linux/writeback.h
> > --- a/include/linux/writeback.h	2011-08-05 10:29:21.000000000 +0530
> > +++ b/include/linux/writeback.h	2011-08-09 10:09:23.581268260 +0530
> > @@ -100,6 +100,7 @@ extern unsigned long dirty_background_by
> >  extern int vm_dirty_ratio;
> >  extern unsigned long vm_dirty_bytes;
> >  extern unsigned int dirty_writeback_interval;
> > +extern unsigned int sync_supers_interval;
> >  extern unsigned int dirty_expire_interval;
> >  extern int vm_highmem_is_dirtyable;
> >  extern int block_dump;
> > @@ -123,6 +124,10 @@ extern int dirty_bytes_handler(struct ct
> >  struct ctl_table;
> >  int dirty_writeback_centisecs_handler(struct ctl_table *, int,
> >  				      void __user *, size_t *, loff_t *);
> > +int sync_supers_centisecs_handler(struct ctl_table *, int,
> > +				      void __user *, size_t *, loff_t *);
> > +int dirty_expire_centisecs_handler(struct ctl_table *, int,
> > +				      void __user *, size_t *, loff_t *);
> > 
> >  void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty);
> >  unsigned long bdi_dirty_limit(struct backing_dev_info *bdi,
> > diff -uprN a/kernel/sysctl.c b/kernel/sysctl.c
> > --- a/kernel/sysctl.c	2011-08-05 10:29:21.000000000 +0530
> > +++ b/kernel/sysctl.c	2011-08-09 12:39:43.453087554 +0530
> > @@ -1076,12 +1076,19 @@ static struct ctl_table vm_table[] = {
> >  		.mode		= 0644,
> >  		.proc_handler	= dirty_writeback_centisecs_handler,
> >  	},
> > +    {
> > +        .procname   = "sync_supers_centisecs",
> > +        .data       = &sync_supers_interval,
> > +        .maxlen     = sizeof(sync_supers_interval),
> > +        .mode       = 0644,
> > +        .proc_handler   = sync_supers_centisecs_handler,
> > +    },
> >  	{
> >  		.procname	= "dirty_expire_centisecs",
> >  		.data		= &dirty_expire_interval,
> >  		.maxlen		= sizeof(dirty_expire_interval),
> >  		.mode		= 0644,
> > -		.proc_handler	= proc_dointvec_minmax,
> > +		.proc_handler	= dirty_expire_centisecs_handler,
> >  		.extra1		= &zero,
> >  	},
> >  	{
> > diff -uprN a/mm/backing-dev.c b/mm/backing-dev.c
> > --- a/mm/backing-dev.c	2011-08-05 10:29:21.000000000 +0530
> > +++ b/mm/backing-dev.c	2011-08-09 12:08:06.287079027 +0530
> > @@ -39,6 +39,10 @@ DEFINE_SPINLOCK(bdi_lock);
> >  LIST_HEAD(bdi_list);
> >  LIST_HEAD(bdi_pending_list);
> > 
> > +/* Same value as the dirty_writeback_interval as this is what our
> > + * initial shortest_dirty_writeback_interval. */
> > +unsigned int shortest_dirty_writeback_interval = 5 * 100;
> > +
> >  static struct task_struct *sync_supers_tsk;
> >  static struct timer_list sync_supers_timer;
> > 
> > @@ -204,12 +208,50 @@ static ssize_t max_ratio_store(struct de
> >  }
> >  BDI_SHOW(max_ratio, bdi->max_ratio)
> > 
> > +static ssize_t dirty_writeback_interval_store(struct device *dev,
> > +		struct device_attribute *attr, const char *buf, size_t count)
> > +{
> > +	struct backing_dev_info *bdi = dev_get_drvdata(dev);
> > +	char *end;
> > +	unsigned int interval;
> > +	ssize_t ret = -EINVAL;
> > +
> > +	interval = simple_strtoul(buf, &end, 10);
> > +	if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
> > +		bdi->dirty_writeback_interval = interval;
> > +		shortest_dirty_writeback_interval =
> > +						min(shortest_dirty_writeback_interval,interval);
> > +		ret = count;
> > +	}
> > +	return ret;
> > +}
> > +BDI_SHOW(dirty_writeback_interval, bdi->dirty_writeback_interval)
> > +
> > +static ssize_t dirty_expire_interval_store (struct device *dev,
> > +		struct device_attribute *attr, const char *buf, size_t count)
> > +{
> > +	struct backing_dev_info *bdi = dev_get_drvdata(dev);
> > +	char *end;
> > +	unsigned int interval;
> > +	ssize_t ret = -EINVAL;
> > +
> > +	interval = simple_strtoul(buf, &end, 10);
> > +	if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
> > +		bdi->dirty_expire_interval = interval;
> > +		ret = count;
> > +	}
> > +	return ret;
> > +}
> > +BDI_SHOW(dirty_expire_interval, bdi->dirty_expire_interval)
> > +
> >  #define __ATTR_RW(attr) __ATTR(attr, 0644, attr##_show, attr##_store)
> > 
> >  static struct device_attribute bdi_dev_attrs[] = {
> >  	__ATTR_RW(read_ahead_kb),
> >  	__ATTR_RW(min_ratio),
> >  	__ATTR_RW(max_ratio),
> > +	__ATTR_RW(dirty_writeback_interval),
> > +	__ATTR_RW(dirty_expire_interval),
> >  	__ATTR_NULL,
> >  };
> > 
> > @@ -291,7 +333,7 @@ void bdi_arm_supers_timer(void)
> >  	if (!dirty_writeback_interval)
> >  		return;
> > 
> > -	next = msecs_to_jiffies(dirty_writeback_interval * 10) + jiffies;
> > +	next = msecs_to_jiffies(sync_supers_interval* 10) + jiffies;
> >  	mod_timer(&sync_supers_timer, round_jiffies_up(next));
> >  }
> > 
> > @@ -336,7 +378,7 @@ void bdi_wakeup_thread_delayed(struct ba
> >  {
> >  	unsigned long timeout;
> > 
> > -	timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
> > +	timeout = msecs_to_jiffies(bdi->dirty_writeback_interval * 10);
> >  	mod_timer(&bdi->wb.wakeup_timer, jiffies + timeout);
> >  }
> > 
> > @@ -348,7 +390,19 @@ static unsigned long bdi_longest_inactiv
> >  {
> >  	unsigned long interval;
> > 
> > -	interval = msecs_to_jiffies(dirty_writeback_interval * 10);
> > +	interval = msecs_to_jiffies(shortest_dirty_writeback_interval * 10);
> > +	return max(5UL * 60 * HZ, interval);
> > +}
> > +
> > +/*
> > + * Calculate the longest interval (jiffies) this bdi thread is allowed to be
> > + * inactive.
> > + */
> > +static unsigned long bdi_longest_inactive_this(struct backing_dev_info *bdi)
> > +{
> > +	unsigned long interval;
> > +
> > +	interval = msecs_to_jiffies(bdi->dirty_writeback_interval * 10);
> >  	return max(5UL * 60 * HZ, interval);
> >  }
> > 
> > @@ -422,7 +476,7 @@ static int bdi_forker_thread(void *ptr)
> >  			 */
> >  			if (bdi->wb.task && !have_dirty_io &&
> >  			    time_after(jiffies, bdi->wb.last_active +
> > -						bdi_longest_inactive())) {
> > +						bdi_longest_inactive_this(bdi))) {
> >  				task = bdi->wb.task;
> >  				bdi->wb.task = NULL;
> >  				spin_unlock(&bdi->wb_lock);
> > @@ -469,7 +523,7 @@ static int bdi_forker_thread(void *ptr)
> >  			break;
> > 
> >  		case NO_ACTION:
> > -			if (!wb_has_dirty_io(me) || !dirty_writeback_interval)
> > +			if (!wb_has_dirty_io(me) || !me->bdi->dirty_writeback_interval)
> >  				/*
> >  				 * There are no dirty data. The only thing we
> >  				 * should now care about is checking for
> > @@ -479,7 +533,7 @@ static int bdi_forker_thread(void *ptr)
> >  				 */
> >  				schedule_timeout(bdi_longest_inactive());
> >  			else
> > -				schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
> > +				schedule_timeout(msecs_to_jiffies(me->bdi->dirty_writeback_interval * 10));
> >  			try_to_freeze();
> >  			/* Back to the main loop */
> >  			continue;
> > @@ -641,6 +695,8 @@ int bdi_init(struct backing_dev_info *bd
> >  	bdi->min_ratio = 0;
> >  	bdi->max_ratio = 100;
> >  	bdi->max_prop_frac = PROP_FRAC_BASE;
> > +	bdi->dirty_writeback_interval = dirty_writeback_interval;
> > +	bdi->dirty_expire_interval = dirty_expire_interval;
> >  	spin_lock_init(&bdi->wb_lock);
> >  	INIT_LIST_HEAD(&bdi->bdi_list);
> >  	INIT_LIST_HEAD(&bdi->work_list);
> > diff -uprN a/mm/page-writeback.c b/mm/page-writeback.c
> > --- a/mm/page-writeback.c	2011-08-05 10:29:21.000000000 +0530
> > +++ b/mm/page-writeback.c	2011-08-09 13:09:37.985919961 +0530
> > @@ -92,6 +92,11 @@ unsigned long vm_dirty_bytes;
> >  unsigned int dirty_writeback_interval = 5 * 100; /* centiseconds */
> > 
> >  /*
> > + * The interval between sync_supers thread writebacks
> > + */
> > +unsigned int sync_supers_interval = 5 * 100; /* centiseconds */
> > +
> > +/*
> >   * The longest time for which data is allowed to remain dirty
> >   */
> >  unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */
> > @@ -686,8 +691,60 @@ void throttle_vm_writeout(gfp_t gfp_mask
> >  int dirty_writeback_centisecs_handler(ctl_table *table, int write,
> >  	void __user *buffer, size_t *length, loff_t *ppos)
> >  {
> > +	struct backing_dev_info *bdi;
> > +
> > +	proc_dointvec(table, write, buffer, length, ppos);
> > +
> > +	if (write) {
> > +		/* Traverse all the BDIs registered to the BDI list and reset their
> > +		 * bdi->dirty_writeback_interval to this value. */
> > +	    spin_lock_bh(&bdi_lock);
> > +		list_for_each_entry(bdi, &bdi_list, bdi_list)
> > +			bdi->dirty_writeback_interval = dirty_writeback_interval;
> > +	    spin_unlock_bh(&bdi_lock);
> > +
> > +		sync_supers_interval =
> > +			shortest_dirty_writeback_interval = dirty_writeback_interval;
> > +
> > +	}
> > +
> > +	bdi_arm_supers_timer();
> > +
> > +	return 0;
> > +}
> > +
> > +/*
> > + * sysctl handler for /proc/sys/vm/sync_supers_centisecs
> > + */
> > +int sync_supers_centisecs_handler(ctl_table *table, int write,
> > +	void __user *buffer, size_t *length, loff_t *ppos)
> > +{
> >  	proc_dointvec(table, write, buffer, length, ppos);
> > +
> >  	bdi_arm_supers_timer();
> > +
> > +	return 0;
> > +}
> > +
> > +/*
> > + * sysctl handler for /proc/sys/vm/dirty_expire_centisecs
> > + */
> > +int dirty_expire_centisecs_handler(ctl_table *table, int write,
> > +	void __user *buffer, size_t *length, loff_t *ppos)
> > +{
> > +	struct backing_dev_info *bdi;
> > +
> > +	proc_dointvec_minmax(table, write, buffer, length, ppos);
> > +
> > +	if (write) {
> > +		/* Traverse all the BDIs registered to the BDI list and reset their
> > +		 * bdi->dirty_expire_interval to this value. */
> > +	    spin_lock_bh(&bdi_lock);
> > +		list_for_each_entry(bdi, &bdi_list, bdi_list)
> > +			bdi->dirty_expire_interval = dirty_expire_interval;
> > +	    spin_unlock_bh(&bdi_lock);
> > +	}
> > +
> >  	return 0;
> >  }
> > --
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