Patchwork ext4 extent status tree LRU locking

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Submitter Zheng Liu
Date June 14, 2013, 2:09 p.m.
Message ID <20130614140940.GA20401@gmail.com>
Download mbox | patch
Permalink /patch/251433/
State Superseded
Headers show

Comments

Theodore Ts'o - June 14, 2013, 2:02 p.m.
On Fri, Jun 14, 2013 at 10:09:40PM +0800, Zheng Liu wrote:
> This commit tries to fix this problem.  Now a new member called
> i_touch_when is added into ext4_inode_info to record the last access
> time for an inode.  Meanwhile we never need to keep a proper in-order
> LRU list.  So this can avoid to burns some CPU time.  When we try to
> reclaim some entries from extent status tree, we use list_sort() to get
> a proper in-order list.  Then we traverse this list to discard some
> entries.

I think this approach is very sound.  I have one reservation, however.
If there are a large number of inodes on this list, list_sort() could
burn a lot of CPU time, especially if the shrinker is called a very
large number of times in a short time period (i.e., when the system is
thrashing, or when one or more memory containers are under a large
amount of memory pressure).

I have a suggestion for how to address this: Keep a timestamp of when
the list last has been sorted in struct ext4_super_info.  When
iterating over the list, looking for a candidate inode, if inode's
i_touch_when is greater than the last time sorted, skip the inode.  If
there are no inodes left in the list, or more than some threshold
inodes have been skipped, only then resort the list (and update the
timestamp in the ext4_super_info structure).

What do you think?

					- Ted
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Zheng Liu - June 14, 2013, 2:09 p.m.
On Tue, Jun 11, 2013 at 04:22:16PM -0700, Dave Hansen wrote:
> I've got a test case which I intended to use to stress the VM a bit.  It
> fills memory up with page cache a couple of times.  It essentially runs
> 30 or so cp's in parallel.
> 
> 98% of my CPU is system time, and 96% of _that_ is being spent on the
> spinlock in ext4_es_lru_add().  I think the LRU list head and its lock
> end up being *REALLY* hot cachelines and are *the* bottleneck on this
> test.  Note that this is _before_ we go in to reclaim and actually start
> calling in to the shrinker.  There is zero memory pressure in this test.
> 
> I'm not sure the benefits of having a proper in-order LRU during reclaim
> outweigh such a drastic downside for the common case.

Hi Dave,

I paste a patch that tries to fix this problem that you reported.  I do
the following test in my sand box, and it seeems that this patch could
fix the problem.  This patch is not very mature.  But I hope to be sure
that my direction is right.  So I really appreciate if you could confirm
that this patch can fix the problem in your test case.

I try to reproduce the test as you described.  I creaete a ext4 file
system that is a loopback device in a x86_64 server with 16 CPUs, 24G
memory.  Then I create 160 64G sparse files, and copy them to /dev/null.
I use 'perf record/report' to observe the overhead, and the results are
as below:

unpatched kernel
----------------
65.32%               cp  [kernel.kallsyms]            [k] _raw_spin_lock                                                                                                       
                         |
                         --- _raw_spin_lock
                            |          
                            |--99.30%-- ext4_es_lru_add
                            |          ext4_es_lookup_extent
                            |          ext4_map_blocks
                            |          _ext4_get_block
                            |          ext4_get_block
                            |          do_mpage_readpage

The result shows that ext4_es_lru_add() burns about 65% CPU time.

patched kernel
--------------
2.26%               cp  [kernel.kallsyms]          [k] _raw_spin_lock                                                                                                         
                         |
                         --- _raw_spin_lock
                            |          
                            |--96.36%-- rmqueue_bulk.clone.0
                            |          get_page_from_freelist
                            |          |          
                            |          |--60.13%-- __alloc_pages_nodemask

After applied the patch, ext4_es_lru_add() is never a bottleneck.

I am not sure whether I have reproduced your test.  So that would be
great if you can confirm that this patch can fix the problem.  The patch
bases against the master branch of ext4 tree.

Any comment or suggestion is welcome.

Thanks,
                                                - Zheng


Subject: [PATCH] ext4: improve extent cache shrink mechanism to avoid to burn CPU time

From: Zheng Liu <wenqing.lz@taobao.com>

Now we maintain an proper in-order LRU list in ext4 to reclaim entries
from extent status tree when we are under heavy memory pressure.  For
keeping this order, a spin lock is used to protect this list.  But this
lock burns a lot of CPU time.

This commit tries to fix this problem.  Now a new member called
i_touch_when is added into ext4_inode_info to record the last access
time for an inode.  Meanwhile we never need to keep a proper in-order
LRU list.  So this can avoid to burns some CPU time.  When we try to
reclaim some entries from extent status tree, we use list_sort() to get
a proper in-order list.  Then we traverse this list to discard some
entries.

In this commit, we break the loop if s_extent_cache_cnt == 0 because
that means that all extents in extent status tree have been reclaimed.

Reported-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
---
 fs/ext4/ext4.h           |    1 +
 fs/ext4/extents_status.c |   43 ++++++++++++++++++++++++++++++++++---------
 fs/ext4/inode.c          |    4 ++++
 fs/ext4/super.c          |    1 +
 4 files changed, 40 insertions(+), 9 deletions(-)
Dave Hansen - June 14, 2013, 3:57 p.m.
On 06/14/2013 07:09 AM, Zheng Liu wrote:
> -	INIT_LIST_HEAD(&scanned);
> -
>  	spin_lock(&sbi->s_es_lru_lock);
> +	list_sort(NULL, &sbi->s_es_lru, ext4_inode_touch_time_cmp);
>  	list_for_each_safe(cur, tmp, &sbi->s_es_lru) {

How long can this list get?  I have the feeling this might get a bit
painful, especially on a NUMA machine.

But, it definitely eliminates the spinlock contention that I was seeing.
 The top ext4 function in my profiles is way down under 1% of CPU time
now.  Thanks for the quick response, and please let me know if you need
any further testing.
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Dave Hansen - June 14, 2013, 4:55 p.m.
On 06/14/2013 10:11 AM, Zheng Liu wrote:
> On Fri, Jun 14, 2013 at 08:57:44AM -0700, Dave Hansen wrote:
>> > On 06/14/2013 07:09 AM, Zheng Liu wrote:
>>> > > -	INIT_LIST_HEAD(&scanned);
>>> > > -
>>> > >  	spin_lock(&sbi->s_es_lru_lock);
>>> > > +	list_sort(NULL, &sbi->s_es_lru, ext4_inode_touch_time_cmp);
>>> > >  	list_for_each_safe(cur, tmp, &sbi->s_es_lru) {
>> > 
>> > How long can this list get?  I have the feeling this might get a bit
>> > painful, especially on a NUMA machine.
> I guess that you worry about the time of sorting a lru list, right?

Yeah, just worried about the amount of time it takes to examine (and
possibly write to) every item on the list.
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Zheng Liu - June 14, 2013, 5 p.m.
On Fri, Jun 14, 2013 at 10:02:15AM -0400, Theodore Ts'o wrote:
> On Fri, Jun 14, 2013 at 10:09:40PM +0800, Zheng Liu wrote:
> > This commit tries to fix this problem.  Now a new member called
> > i_touch_when is added into ext4_inode_info to record the last access
> > time for an inode.  Meanwhile we never need to keep a proper in-order
> > LRU list.  So this can avoid to burns some CPU time.  When we try to
> > reclaim some entries from extent status tree, we use list_sort() to get
> > a proper in-order list.  Then we traverse this list to discard some
> > entries.
> 
> I think this approach is very sound.  I have one reservation, however.
> If there are a large number of inodes on this list, list_sort() could
> burn a lot of CPU time, especially if the shrinker is called a very
> large number of times in a short time period (i.e., when the system is
> thrashing, or when one or more memory containers are under a large
> amount of memory pressure).
> 
> I have a suggestion for how to address this: Keep a timestamp of when
> the list last has been sorted in struct ext4_super_info.  When
> iterating over the list, looking for a candidate inode, if inode's
> i_touch_when is greater than the last time sorted, skip the inode.  If
> there are no inodes left in the list, or more than some threshold
> inodes have been skipped, only then resort the list (and update the
> timestamp in the ext4_super_info structure).
> 
> What do you think?

Thanks for your suggestion.  I fully agree with you.  What I concern is
how to define this threshold.  A fixed value is very simple but too
inflexible.  If this value is too big, this lru list could never be
sorted.  So my proposal is that we set this value accroding to the
number of used inodes.  For example, if the number of used inodes is
10,000, this threshold could be set to 10 (10,000 x 0.1%).  Meanwhile,
we need to provide a sysfs interface to let user set this value.  Does
it make sense?

Thanks,
                                                - Zheng
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Zheng Liu - June 14, 2013, 5:11 p.m.
On Fri, Jun 14, 2013 at 08:57:44AM -0700, Dave Hansen wrote:
> On 06/14/2013 07:09 AM, Zheng Liu wrote:
> > -	INIT_LIST_HEAD(&scanned);
> > -
> >  	spin_lock(&sbi->s_es_lru_lock);
> > +	list_sort(NULL, &sbi->s_es_lru, ext4_inode_touch_time_cmp);
> >  	list_for_each_safe(cur, tmp, &sbi->s_es_lru) {
> 
> How long can this list get?  I have the feeling this might get a bit
> painful, especially on a NUMA machine.

I guess that you worry about the time of sorting a lru list, right?
Ted and I also worry about this, especially when shrinker is called()
very frequently.  Ted has presented a solution.  I will give it a try.

> 
> But, it definitely eliminates the spinlock contention that I was seeing.
>  The top ext4 function in my profiles is way down under 1% of CPU time
> now.  Thanks for the quick response, and please let me know if you need
> any further testing.

Thanks for your help.  As I said above, I will try to improve this patch
later.  My server's numa has been turned off in BIOS and I haven't a
privilege to turn it on.  So that would be great if you could help me to
do some further testing.

Thanks in advance,
                                                - Zheng
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Theodore Ts'o - June 14, 2013, 6 p.m.
On Sat, Jun 15, 2013 at 01:00:28AM +0800, Zheng Liu wrote:
> > I have a suggestion for how to address this: Keep a timestamp of when
> > the list last has been sorted in struct ext4_super_info.  When
> > iterating over the list, looking for a candidate inode, if inode's
> > i_touch_when is greater than the last time sorted, skip the inode.  If
> > there are no inodes left in the list, or more than some threshold
> > inodes have been skipped, only then resort the list (and update the
> > timestamp in the ext4_super_info structure).
> > 
> 
> Thanks for your suggestion.  I fully agree with you.  What I concern is
> how to define this threshold.  A fixed value is very simple but too
> inflexible.  If this value is too big, this lru list could never be
> sorted.

In my proposal, if the list is never sorted before, we would
definitely sort the list the first time there is a request to shrink
the number of extent caches.  So the list would always be sorted at
least once.

If an inode gets accessed after the time when the list has been
sorted, then the last_touch time will be greater than the last_sorted
time, right?  And our goal is to use the least recently used inode, so
as long as we know they have been used more recently than the time the
list was sorted, and there are still inodes on the list where the
last_touch time is older than the last_sort time, we don't have a
problem.  So we will evict all of the non-delalloc extent cache items
from that inode, and remove it from the list.

The original reason for having the threshold at all is so that if we
skip a huge number of inodes on the list, this would take a long time.
But thinking about this some more, we don't even need a threshold.
What we do instead is if the the last_touch time is newer than the
last_sort time, we just move the inode to the end of the list.  That
means the end of the list will be unsorted, but that's OK, because the
oldest inodes are still at the beginning of the list.  Once the head
of the list is newer than the last_sorted time, then we know we need
to resort the entire list.

Does that make sense to you?

					- Ted
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Patch

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 019db3c..1c31f27 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -874,6 +874,7 @@  struct ext4_inode_info {
 	rwlock_t i_es_lock;
 	struct list_head i_es_lru;
 	unsigned int i_es_lru_nr;	/* protected by i_es_lock */
+	unsigned long i_touch_when;	/* jiffies of last accessing */
 
 	/* ialloc */
 	ext4_group_t	i_last_alloc_group;
diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
index e6941e6..0b867b8 100644
--- a/fs/ext4/extents_status.c
+++ b/fs/ext4/extents_status.c
@@ -10,6 +10,7 @@ 
  * Ext4 extents status tree core functions.
  */
 #include <linux/rbtree.h>
+#include <linux/list_sort.h>
 #include "ext4.h"
 #include "extents_status.h"
 #include "ext4_extents.h"
@@ -291,7 +292,6 @@  out:
 
 	read_unlock(&EXT4_I(inode)->i_es_lock);
 
-	ext4_es_lru_add(inode);
 	trace_ext4_es_find_delayed_extent_range_exit(inode, es);
 }
 
@@ -672,7 +672,6 @@  int ext4_es_insert_extent(struct inode *inode, ext4_lblk_t lblk,
 error:
 	write_unlock(&EXT4_I(inode)->i_es_lock);
 
-	ext4_es_lru_add(inode);
 	ext4_es_print_tree(inode);
 
 	return err;
@@ -734,7 +733,6 @@  out:
 
 	read_unlock(&EXT4_I(inode)->i_es_lock);
 
-	ext4_es_lru_add(inode);
 	trace_ext4_es_lookup_extent_exit(inode, es, found);
 	return found;
 }
@@ -878,12 +876,30 @@  int ext4_es_zeroout(struct inode *inode, struct ext4_extent *ex)
 				     EXTENT_STATUS_WRITTEN);
 }
 
+static int ext4_inode_touch_time_cmp(void *priv, struct list_head *a,
+				     struct list_head *b)
+{
+	struct ext4_inode_info *eia, *eib;
+	unsigned long diff;
+
+	eia = list_entry(a, struct ext4_inode_info, i_es_lru);
+	eib = list_entry(b, struct ext4_inode_info, i_es_lru);
+
+	diff = eia->i_touch_when - eib->i_touch_when;
+	if (diff < 0)
+		return -1;
+	if (diff > 0)
+		return 1;
+	return 0;
+}
+
 static int ext4_es_shrink(struct shrinker *shrink, struct shrink_control *sc)
 {
 	struct ext4_sb_info *sbi = container_of(shrink,
 					struct ext4_sb_info, s_es_shrinker);
 	struct ext4_inode_info *ei;
-	struct list_head *cur, *tmp, scanned;
+	struct list_head *cur, *tmp;
+	LIST_HEAD(scanned);
 	int nr_to_scan = sc->nr_to_scan;
 	int ret, nr_shrunk = 0;
 
@@ -893,10 +909,16 @@  static int ext4_es_shrink(struct shrinker *shrink, struct shrink_control *sc)
 	if (!nr_to_scan)
 		return ret;
 
-	INIT_LIST_HEAD(&scanned);
-
 	spin_lock(&sbi->s_es_lru_lock);
+	list_sort(NULL, &sbi->s_es_lru, ext4_inode_touch_time_cmp);
 	list_for_each_safe(cur, tmp, &sbi->s_es_lru) {
+		/*
+		 * If we have already reclaimed all extents from extent
+		 * status tree, just stop the loop immediately.
+		 */
+		if (percpu_counter_read_positive(&sbi->s_extent_cache_cnt) == 0)
+			break;
+
 		list_move_tail(cur, &scanned);
 
 		ei = list_entry(cur, struct ext4_inode_info, i_es_lru);
@@ -947,11 +969,14 @@  void ext4_es_lru_add(struct inode *inode)
 	struct ext4_inode_info *ei = EXT4_I(inode);
 	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
 
+	ei->i_touch_when = jiffies;
+
+	if (!list_empty(&ei->i_es_lru))
+		return;
+
 	spin_lock(&sbi->s_es_lru_lock);
 	if (list_empty(&ei->i_es_lru))
-		list_add_tail(&ei->i_es_lru, &sbi->s_es_lru);
-	else
-		list_move_tail(&ei->i_es_lru, &sbi->s_es_lru);
+		list_add(&ei->i_es_lru, &sbi->s_es_lru);
 	spin_unlock(&sbi->s_es_lru_lock);
 }
 
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 38f03dc..1477406 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -571,6 +571,8 @@  int ext4_map_blocks(handle_t *handle, struct inode *inode,
 		  "logical block %lu\n", inode->i_ino, flags, map->m_len,
 		  (unsigned long) map->m_lblk);
 
+	ext4_es_lru_add(inode);
+
 	/* Lookup extent status tree firstly */
 	if (ext4_es_lookup_extent(inode, map->m_lblk, &es)) {
 		if (ext4_es_is_written(&es) || ext4_es_is_unwritten(&es)) {
@@ -1888,6 +1890,8 @@  static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
 		  "logical block %lu\n", inode->i_ino, map->m_len,
 		  (unsigned long) map->m_lblk);
 
+	ext4_es_lru_add(inode);
+
 	/* Lookup extent status tree firstly */
 	if (ext4_es_lookup_extent(inode, iblock, &es)) {
 
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index a9c1438..b365695 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -849,6 +849,7 @@  static struct inode *ext4_alloc_inode(struct super_block *sb)
 	rwlock_init(&ei->i_es_lock);
 	INIT_LIST_HEAD(&ei->i_es_lru);
 	ei->i_es_lru_nr = 0;
+	ei->i_touch_when = 0;
 	ei->i_reserved_data_blocks = 0;
 	ei->i_reserved_meta_blocks = 0;
 	ei->i_allocated_meta_blocks = 0;