Message-Id: <1177156902.2934.96.camel@lappy>
Date:	Sat, 21 Apr 2007 14:01:36 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	miklos@...redi.hu, neilb@...e.de, dgc@....com,
	tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com
Subject: Re: [PATCH 10/10] mm: per device dirty threshold

On Sat, 2007-04-21 at 02:55 -0700, Andrew Morton wrote:
> On Fri, 20 Apr 2007 17:52:04 +0200 Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
> > Scale writeback cache per backing device, proportional to its writeout speed.
> > 
> > By decoupling the BDI dirty thresholds a number of problems we currently have
> > will go away, namely:
> > 
> >  - mutual interference starvation (for any number of BDIs);
> >  - deadlocks with stacked BDIs (loop, FUSE and local NFS mounts).
> > 
> > It might be that all dirty pages are for a single BDI while other BDIs are
> > idling. By giving each BDI a 'fair' share of the dirty limit, each one can have
> > dirty pages outstanding and make progress.
> > 
> > A global threshold also creates a deadlock for stacked BDIs; when A writes to
> > B, and A generates enough dirty pages to get throttled, B will never start
> > writeback until the dirty pages go away. Again, by giving each BDI its own
> > 'independent' dirty limit, this problem is avoided.
> > 
> > So the problem is to determine how to distribute the total dirty limit across
> > the BDIs fairly and efficiently. A DBI that has a large dirty limit but does
> > not have any dirty pages outstanding is a waste.
> > 
> > What is done is to keep a floating proportion between the DBIs based on
> > writeback completions. This way faster/more active devices get a larger share
> > than slower/idle devices.
> 
> This is a pretty major improvement to various nasty corner-cases, if it
> works.
> 
> Does it work?  Please describe the testing you did, and the results.

The testing I did was several dd instances racing each other to various
devices; usually one in a loop and the other a single, timed, instance.

I tested, disk vs disk, disk vs usbstick, disk vs nfs-mount.

Using the debug patch from the last series; the one which exposed the
actual ratio assigned and the total; I monitored (where possible) that
the ratio was around the relative writeout speeds.

The main indicator was that the writes should complete in roughly the
same time as if they were done on an idle system.

the disk vs usbstick gave the most dramatic improvement; on mainline the
usbstick is totally starved by a heavy disk writer, with these patches
it takes about the same time as it would on an idle system

Along with the first series was a number of results; those still stand.

> Has this been confirmed to fix Miklos's FUSE and loopback problems?

I must defer to Miklos for that.

> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > ---
> >  include/linux/backing-dev.h |   51 ++++++++++++
> >  mm/backing-dev.c            |    3 
> >  mm/page-writeback.c         |  181 ++++++++++++++++++++++++++++++++++++--------
> >  3 files changed, 206 insertions(+), 29 deletions(-)
> > 
> > Index: linux-2.6/include/linux/backing-dev.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/backing-dev.h	2007-04-20 15:28:17.000000000 +0200
> > +++ linux-2.6/include/linux/backing-dev.h	2007-04-20 15:33:59.000000000 +0200
> > @@ -28,6 +28,7 @@ typedef int (congested_fn)(void *, int);
> >  enum bdi_stat_item {
> >  	BDI_RECLAIMABLE,
> >  	BDI_WRITEBACK,
> > +	BDI_WRITEOUT,
> >  	NR_BDI_STAT_ITEMS
> >  };
> 
> Whoa, head is now swimming.  What's the difference between "writeback" and
> "writeout"?

writeback is the number of pages in the writeback state.

writeout is a relative proportion (against all other BDIs) of completed
writeouts.

> > @@ -43,6 +44,13 @@ struct backing_dev_info {
> >  	void *unplug_io_data;
> >  
> >  	struct percpu_counter bdi_stat[NR_BDI_STAT_ITEMS];
> > +
> > +	/*
> > +	 * data used for scaling the writeback cache
> > +	 */
> > +	spinlock_t lock;	/* protect the cycle count */
> > +	unsigned long cycles;	/* writeout cycles */
> > +	int dirty_exceeded;
> >  };
> >  
> >  void bdi_init(struct backing_dev_info *bdi);
> > @@ -54,6 +62,12 @@ static inline void __mod_bdi_stat(struct
> >  	percpu_counter_mod(&bdi->bdi_stat[item], amount);
> >  }
> >  
> > +static inline void __mod_bdi_stat64(struct backing_dev_info *bdi,
> > +		enum bdi_stat_item item, s64 amount)
> > +{
> > +	percpu_counter_mod64(&bdi->bdi_stat[item], amount);
> > +}
> > +
> >  static inline void __inc_bdi_stat(struct backing_dev_info *bdi,
> >  		enum bdi_stat_item item)
> >  {
> > @@ -86,12 +100,49 @@ static inline void dec_bdi_stat(struct b
> >  	local_irq_restore(flags);
> >  }
> >  
> > +static inline s64 __bdi_stat(struct backing_dev_info *bdi,
> > +		enum bdi_stat_item item)
> > +{
> > +	return percpu_counter_read(&bdi->bdi_stat[item]);
> > +}
> > +
> >  static inline s64 bdi_stat(struct backing_dev_info *bdi,
> >  		enum bdi_stat_item item)
> >  {
> >  	return percpu_counter_read_positive(&bdi->bdi_stat[item]);
> >  }
> 
> So here, the __ means "it doesn't do the force-it-positive" treatment.
> 
> > +static inline s64 __bdi_stat_sum(struct backing_dev_info *bdi,
> > +		enum bdi_stat_item item)
> > +{
> > +	return percpu_counter_sum(&bdi->bdi_stat[item]);
> > +}
> > +
> > +static inline s64 bdi_stat_sum(struct backing_dev_info *bdi,
> > +		enum bdi_stat_item item)
> > +{
> > +	s64 sum;
> > +	unsigned long flags;
> > +
> > +	local_irq_save(flags);
> > +	sum = __bdi_stat_sum(bdi, item);
> > +	local_irq_restore(flags);
> > +
> > +	return sum;
> > +}
> 
> And here __ means "not safe to use if this counter is updated from
> interrupt context".
> 
> At least, I think that's what it all means.  The lack of code comments
> casts some doubt.
> 
> 
> The interfaces here could do with a little more thought wrt regularity,
> naming and commenting, methinks.

good points, shall consider.

> 
> > +/*
> > + * maximal error of a stat counter.
> > + */
> > +static inline unsigned long bdi_stat_delta(void)
> > +{
> > +#ifdef CONFIG_SMP
> > +	return NR_CPUS * FBC_BATCH;
> 
> This is enormously wrong for CONFIG_NR_CPUS=1024 on a 2-way.
> 
> > +#else
> > +	return 1UL;
> 
> The UL is pretty pointless IMO.  The compiler will happily convert "1" to
> unsigned long here.  And if we later change the return type to signed char,
> we don't have to remember to edit this line too.

me and my pedantry.

> > +#endif
> > +}
> >
> >  /*
> >   * Flags in backing_dev_info::capability
> >   * - The first two flags control whether dirty pages will contribute to the
> > Index: linux-2.6/mm/page-writeback.c
> > ===================================================================
> > --- linux-2.6.orig/mm/page-writeback.c	2007-04-20 15:28:10.000000000 +0200
> > +++ linux-2.6/mm/page-writeback.c	2007-04-20 15:35:01.000000000 +0200
> > @@ -49,8 +49,6 @@
> >   */
> >  static long ratelimit_pages = 32;
> >  
> > -static int dirty_exceeded __cacheline_aligned_in_smp;	/* Dirty mem may be over limit */
> > -
> >  /*
> >   * When balance_dirty_pages decides that the caller needs to perform some
> >   * non-background writeback, this is how many pages it will attempt to write.
> > @@ -103,6 +101,88 @@ EXPORT_SYMBOL(laptop_mode);
> >  static void background_writeout(unsigned long _min_pages);
> >  
> >  /*
> > + * Scale the writeback cache size proportional to the relative writeout speeds.
> > + *
> > + * We do this by tracking a floating average per BDI and a global floating
> > + * average. We optimize away the '/= 2' for the global average by noting that:
> > + *
> > + *  if (++i > thresh) i /= 2:
> > + *
> > + * Can be approximated by:
> > + *
> > + *   thresh/2 + (++i % thresh/2)
> > + *
> > + * Furthermore, when we choose thresh to be 2^n it can be written in terms of
> > + * binary operations and wraparound artifacts disappear.
> > + *
> > + * Also note that this yields a natural counter of the elapsed periods:
> > + *
> > + *   i / thresh
> > + *
> > + * Its monotonous increasing property can be applied to mitigate the wrap-
> > + * around issue.
> > + */

Whaha, and here I thought this was an adequate comment :-/
Obviously it sucked, since you are rather confused.

> > +static int vm_cycle_shift __read_mostly;
> > +static struct percpu_counter vm_writeout_total;
> > +
> > +/*
> > + * Sync up the per BDI average to the global cycle.
> > + */
> > +static void bdi_writeout_norm(struct backing_dev_info *bdi)
> > +{
> > +	int bits = vm_cycle_shift;
> > +	unsigned long cycle = 1UL << bits;
> > +	unsigned long mask = ~(cycle - 1);
> > +	unsigned long global_cycle = percpu_counter_read(&vm_writeout_total);
> > +	unsigned long flags;
> > +
> > +	global_cycle <<= 1;
> > +	global_cycle &= mask;
> > +
> > +	if ((bdi->cycles & mask) == global_cycle)
> > +		return;
> > +
> > +	spin_lock_irqsave(&bdi->lock, flags);
> > +	bdi->cycles &= mask;
> > +	while (bdi->cycles != global_cycle) {
> > +		unsigned long val = __bdi_stat(bdi, BDI_WRITEOUT);
> > +		unsigned long half = (val + 1) >> 1;
> > +
> > +		if (!val)
> > +			break;
> > +
> > +		__mod_bdi_stat64(bdi, BDI_WRITEOUT, -half);
> > +		bdi->cycles += cycle;
> > +	}
> > +	bdi->cycles = global_cycle;
> > +	spin_unlock_irqrestore(&bdi->lock, flags);
> > +}
> 
> Here we get to the real critical substance of the patchset, and I don't
> have a clue what it's doing nor how it's doing it.  And I bet nobody else
> does either.

I shall send a comment patch; but let me try to explain:

I am trying to keep a floating proportion between the BDIs based on
writeout events. That is, each device is given a share equal to its
proportion of completed writebacks (writeback, we are in the process of
writing vs. writeout, we have written). This proportion is measured in a
'time'-span measured itself in writeouts.

Example:

  device A completes 4, device B completes 12 and, device C 16 writes.
This gives a 4:12:16 of 32 ratio. Now, assume the 'time'-span is 32
writes. This will force the counters to get halved: 2:6:8 of 16.

Now the devices complete: A:8 B:8 C:0, another 16, making 32 again.
2+8=10 : 6+8=14 : 8+0=8 of 32, or, because its a full period:
5:7:4 of 16.

That is basically what happens; the implementation tries to be a little
smart about it, because it wants to avoid having to traverse all BDIs
when a period expires.

see how the total runs up to 32, gets halved, runs up to 32 again, gets
halved, etc..

That is the
  if (++i > thresh) i /= 2;
from that comment above, which we approximate by:
  thresh/2 + (++i % thresh/2)

the thresh = 2^n -> bit operations part is clear I hope.

now we note that the total (i), is ever increasing, when we look at
  i / (thresh/2)
we see that that is the number of periods expired.

If we then keep track of in which period each BDI is, we can 'normalize'
the (per bdi) counter whenever we detect that the total went into
another period. This is what bdi_writeout_norm() does. bdi->cycle is the
local period (shifted left a bit to align with the global period bits so
that wrap around is handled naturally), and global_cycle the global
period.

> <continues to wonder wtf "writeout" is.  Perhaps knowing that would help>
> 
> I dunno.  I'm sure it's very good code but I don't have the time nor
> inclination to reverse engineer the design from the implementation.
> 
> This is a very important part of the kernel - one of the few most important
> parts, really.  See all the crap going around about CPU schedulers at
> present?  Well hoo-boy, if we get this part of code even a little bit
> wrong, they won't know what hit them.
> 
> So please, spend quite a lot of time thinking about how we can make this
> code as comprehensible and approachable and maintainable as possible. 
> Often this is done with comments ;)
> 
> > +static void __bdi_writeout_inc(struct backing_dev_info *bdi)
> > +{
> > +	bdi_writeout_norm(bdi);
> 
> I'm assuming that "norm" here means "normalise".  There's a hint for me.
> 
> > +	__inc_bdi_stat(bdi, BDI_WRITEOUT);
> > +	percpu_counter_mod(&vm_writeout_total, 1);
> > +}
> > +
> > +void get_writeout_scale(struct backing_dev_info *bdi, long *scale, long *div)
> > +{
> > +	int bits = vm_cycle_shift - 1;
> > +	unsigned long cycle = 1UL << bits;
> > +	unsigned long mask = cycle - 1;
> > +	unsigned long total = percpu_counter_read(&vm_writeout_total);
> > +
> > +	if (bdi_cap_writeback_dirty(bdi)) {
> > +		bdi_writeout_norm(bdi);
> > +		*scale = bdi_stat(bdi, BDI_WRITEOUT);
> > +	} else
> > +		*scale = 0;
> > +
> > +	*div = cycle + (total & mask);
> > +}
> 
> I suppose that if I stared at this for long enough I could work out what
> it's doing, and why it's doing it.  But given that it needs comments
> telling others that, there isn't much point in me blowing the time to do
> so.

Right, so with the above clear (I hope), bdi_writeout_inc() tracks the
per bdi and global writeout events. We need to normalize the counter
before incrementing because the global period might have expired due to
another BDI's activity.

get_writeout_scale() does the magic of getting the current ratio.
Remember the example; say that the current state for A, B and, C is:
  3:8:7 of 18
and we're currently interested in A's share.

the 18 (*div) is obtained from 16 (cycle) + 2 (total & 15),
the 3 (*scale) is read from the per BDI counter (again, after
normalizing it, for the global period might have been advance since we
last showed interest in it).

(nr * 3) / 18 gives A's share of nr.

If anything remains unclear, please holler.

> It should have static scope.

ok

> Are all the per-bdi counters being exposed in sysfs?  I think not. 
> Probably they should be?

Like stated in that sysfs email, I'd rather have we had a per bdi sysfs
spot, the current location is only for disks.

> > +/*
> >   * Work out the current dirty-memory clamping and background writeout
> >   * thresholds.
> >   *
> > @@ -158,8 +238,8 @@ static unsigned long determine_dirtyable
> >  }
> >  
> >  static void
> > -get_dirty_limits(long *pbackground, long *pdirty,
> > -					struct address_space *mapping)
> > +get_dirty_limits(long *pbackground, long *pdirty, long *pbdi_dirty,
> > +		 struct backing_dev_info *bdi)
> >  {
> >  	int background_ratio;		/* Percentages */
> >  	int dirty_ratio;
> > @@ -193,6 +273,30 @@ get_dirty_limits(long *pbackground, long
> >  	}
> >  	*pbackground = background;
> >  	*pdirty = dirty;
> > +
> > +	if (bdi) {
> > +		long long tmp = dirty;
> > +		long reserve;
> > +		long scale, div;
> > +
> > +		get_writeout_scale(bdi, &scale, &div);
> > +
> > +		tmp *= scale;
> > +		do_div(tmp, div);
> > +
> > +		reserve = dirty -
> > +			(global_page_state(NR_FILE_DIRTY) +
> > +			 global_page_state(NR_WRITEBACK) +
> > +			 global_page_state(NR_UNSTABLE_NFS));
> > +
> > +		if (reserve < 0)
> > +			reserve = 0;
> > +
> > +		reserve += bdi_stat(bdi, BDI_RECLAIMABLE) +
> > +			bdi_stat(bdi, BDI_WRITEBACK);
> > +
> > +		*pbdi_dirty = min((long)tmp, reserve);
> 
> min_t is preferred
> 
> tmp isn't a particularly good identifier

ok, shall fix.

> > +	}
> >  }
> >  
> >  /*
> > @@ -204,9 +308,11 @@ get_dirty_limits(long *pbackground, long
> >   */
> >  static void balance_dirty_pages(struct address_space *mapping)
> >  {
> > -	long nr_reclaimable;
> > +	long bdi_nr_reclaimable;
> > +	long bdi_nr_writeback;
> >  	long background_thresh;
> >  	long dirty_thresh;
> > +	long bdi_thresh;
> >  	unsigned long pages_written = 0;
> >  	unsigned long write_chunk = sync_writeback_pages();
> >  
> > @@ -221,15 +327,15 @@ static void balance_dirty_pages(struct a
> >  			.range_cyclic	= 1,
> >  		};
> >  
> > -		get_dirty_limits(&background_thresh, &dirty_thresh, mapping);
> > -		nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> > -					global_page_state(NR_UNSTABLE_NFS);
> > -		if (nr_reclaimable + global_page_state(NR_WRITEBACK) <=
> > -			dirty_thresh)
> > +		get_dirty_limits(&background_thresh, &dirty_thresh,
> > +				&bdi_thresh, bdi);
> > +		bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
> > +		bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
> > +		if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
> >  				break;
> >  
> > -		if (!dirty_exceeded)
> > -			dirty_exceeded = 1;
> > +		if (!bdi->dirty_exceeded)
> > +			bdi->dirty_exceeded = 1;
> >  
> >  		/* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
> >  		 * Unstable writes are a feature of certain networked
> > @@ -237,16 +343,27 @@ static void balance_dirty_pages(struct a
> >  		 * written to the server's write cache, but has not yet
> >  		 * been flushed to permanent storage.
> >  		 */
> > -		if (nr_reclaimable) {
> > +		if (bdi_nr_reclaimable) {
> >  			writeback_inodes(&wbc);
> > -			get_dirty_limits(&background_thresh,
> > -					 	&dirty_thresh, mapping);
> > -			nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> > -					global_page_state(NR_UNSTABLE_NFS);
> > -			if (nr_reclaimable +
> > -				global_page_state(NR_WRITEBACK)
> > -					<= dirty_thresh)
> > -						break;
> > +
> > +			get_dirty_limits(&background_thresh, &dirty_thresh,
> > +				       &bdi_thresh, bdi);
> > +
> > +			if (bdi_thresh < 2*bdi_stat_delta()) {
> > +				bdi_nr_reclaimable =
> > +					bdi_stat_sum(bdi, BDI_RECLAIMABLE);
> > +				bdi_nr_writeback =
> > +					bdi_stat_sum(bdi, BDI_WRITEBACK);
> > +			} else {
> > +				bdi_nr_reclaimable =
> > +					bdi_stat(bdi, BDI_RECLAIMABLE);
> > +				bdi_nr_writeback =
> > +					bdi_stat(bdi, BDI_WRITEBACK);
> > +			}
> > +
> > +			if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
> > +				break;
> > +
> >  			pages_written += write_chunk - wbc.nr_to_write;
> >  			if (pages_written >= write_chunk)
> >  				break;		/* We've done our duty */
> > @@ -254,9 +371,9 @@ static void balance_dirty_pages(struct a
> >  		congestion_wait(WRITE, HZ/10);
> >  	}
> >  
> > -	if (nr_reclaimable + global_page_state(NR_WRITEBACK)
> > -		<= dirty_thresh && dirty_exceeded)
> > -			dirty_exceeded = 0;
> > +	if (bdi_nr_reclaimable + bdi_nr_writeback < bdi_thresh &&
> > +			bdi->dirty_exceeded)
> > +		bdi->dirty_exceeded = 0;
> >  
> >  	if (writeback_in_progress(bdi))
> >  		return;		/* pdflush is already working this queue */
> > @@ -270,7 +387,9 @@ static void balance_dirty_pages(struct a
> >  	 * background_thresh, to keep the amount of dirty memory low.
> >  	 */
> >  	if ((laptop_mode && pages_written) ||
> > -	     (!laptop_mode && (nr_reclaimable > background_thresh)))
> > +			(!laptop_mode && (global_page_state(NR_FILE_DIRTY)
> > +					  + global_page_state(NR_UNSTABLE_NFS)
> > +					  > background_thresh)))
> >  		pdflush_operation(background_writeout, 0);
> >  }
> 
> Did you test laptop mode?

admittedly, no. Shall do.

> > @@ -306,7 +425,7 @@ void balance_dirty_pages_ratelimited_nr(
> >  	unsigned long *p;
> >  
> >  	ratelimit = ratelimit_pages;
> > -	if (dirty_exceeded)
> > +	if (mapping->backing_dev_info->dirty_exceeded)
> >  		ratelimit = 8;
> >  
> >  	/*
> > @@ -342,7 +461,7 @@ void throttle_vm_writeout(gfp_t gfp_mask
> >  	}
> >  
> >          for ( ; ; ) {
> > -		get_dirty_limits(&background_thresh, &dirty_thresh, NULL);
> > +		get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);
> >  
> >                  /*
> >                   * Boost the allowable dirty threshold a bit for page
> > @@ -377,7 +496,7 @@ static void background_writeout(unsigned
> >  		long background_thresh;
> >  		long dirty_thresh;
> >  
> > -		get_dirty_limits(&background_thresh, &dirty_thresh, NULL);
> > +		get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);
> >  		if (global_page_state(NR_FILE_DIRTY) +
> >  			global_page_state(NR_UNSTABLE_NFS) < background_thresh
> >  				&& min_pages <= 0)
> > @@ -585,6 +704,8 @@ void __init page_writeback_init(void)
> >  	mod_timer(&wb_timer, jiffies + dirty_writeback_interval);
> >  	writeback_set_ratelimit();
> >  	register_cpu_notifier(&ratelimit_nb);
> > +	vm_cycle_shift = 1 + ilog2(vm_total_pages);
> > +	percpu_counter_init(&vm_writeout_total, 0);
> >  }
> >  
> >  /**
> > @@ -988,8 +1109,10 @@ int test_clear_page_writeback(struct pag
> >  			radix_tree_tag_clear(&mapping->page_tree,
> >  						page_index(page),
> >  						PAGECACHE_TAG_WRITEBACK);
> > -			if (bdi_cap_writeback_dirty(bdi))
> > +			if (bdi_cap_writeback_dirty(bdi)) {
> >  				__dec_bdi_stat(bdi, BDI_WRITEBACK);
> > +				__bdi_writeout_inc(bdi);
> > +			}
> >  		}
> >  		write_unlock_irqrestore(&mapping->tree_lock, flags);
> >  	} else {
> > Index: linux-2.6/mm/backing-dev.c
> > ===================================================================
> > --- linux-2.6.orig/mm/backing-dev.c	2007-04-20 15:20:11.000000000 +0200
> > +++ linux-2.6/mm/backing-dev.c	2007-04-20 15:31:42.000000000 +0200
> > @@ -12,6 +12,9 @@ void bdi_init(struct backing_dev_info *b
> >  	if (!(bdi_cap_writeback_dirty(bdi) || bdi_cap_account_dirty(bdi)))
> >  		return;
> >  
> > +	spin_lock_init(&bdi->lock);
> > +	bdi->cycles = 0;
> > +	bdi->dirty_exceeded = 0;
> >  	for (i = 0; i < NR_BDI_STAT_ITEMS; i++)
> >  		percpu_counter_init(&bdi->bdi_stat[i], 0);
> >  }
> > 
> 
> ho hum, I'll toss it all in -mm, see what happens.

You show great confidence, thanks!

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