LinuxLists.cc - [PATCH 1/2] rbtree_latch: quit searching when reaching to maximum depth

2020-05-15 12:49:24

Subject: [PATCH 1/2] rbtree_latch: quit searching when reaching to maximum depth

2020-05-15 12:50:14

Subject: [PATCH 2/2] rbtree_latch: don't need to check seq when it found a node

latch_tree_find() should be protected by caller via RCU or so.
When it find a node in an attempt, the node must be a valid one.

Cc: Peter Zijlstra <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Signed-off-by: Lai Jiangshan <[email protected]>
---
include/linux/rbtree_latch.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/rbtree_latch.h b/include/linux/rbtree_latch.h
index b012bd95eabf..09a3c05d1c5b 100644
--- a/include/linux/rbtree_latch.h
+++ b/include/linux/rbtree_latch.h
@@ -208,7 +208,7 @@ latch_tree_find(void *key, struct latch_tree_root *root,
do {
seq = raw_read_seqcount_latch(&root->seq);
node = __lt_find(key, root, seq & 1, ops->comp);
- } while (read_seqcount_retry(&root->seq, seq));
+ } while (!node && read_seqcount_retry(&root->seq, seq));

return node;
}
--
2.20.1

2020-05-15 13:03:04

by Peter Zijlstra

[permalink] [raw]

Subject: Re: [PATCH 1/2] rbtree_latch: quit searching when reaching to maximum depth

On Fri, May 15, 2020 at 12:47:06PM +0000, Lai Jiangshan wrote:
> lib/rbtree.c has ensured that there is not possible to
> inadvertently cause (temporary) loops in the tree structure
> as seen in program order of the modifier. But loop is still
> possible to be seen in searcher due to CPU's reordering.
>
> for example:
> modifier searcher
>
> left rotate at parent
> parent->rb_right is node
> search to parent
> parent->rb_right is node
> +->see node->rb_left changed
> WRITE_ONCE(parent->rb_right, tmp);-+ | node->rb_left is parennt
> no smp_wmb(), some arch can | |
> reorder these two writes | | loop long between
> WRITE_ONCE(node->rb_left, parent);-+-+ parent and node
> |
> +--->finally see
> parent->rb_right
>
> The long loop won't stop until the modifer's CPU flushes
> its writes. Too avoid it, we should limit the searching depth.

Cute, have you actually observed this? Did you have performance issues?

> There are no more than (1<<BITS_PER_LONG)-1 nodes in the tree.
> And the max_depth of a tree is no more than 2*lg(node_count+1),
> which is no mare than 2*BITS_PER_LONG.
>
> So the serarch should stop when diving down up to
> 2*BITS_PER_LONG depth.

Arguably you can have a larger key space, but I think due to memory
constraints this limit still isn't wrong. But I do feel you need a
comment with that.

2020-05-15 13:10:29

by Peter Zijlstra

[permalink] [raw]

Subject: Re: [PATCH 2/2] rbtree_latch: don't need to check seq when it found a node

On Fri, May 15, 2020 at 12:47:07PM +0000, Lai Jiangshan wrote:
> latch_tree_find() should be protected by caller via RCU or so.
> When it find a node in an attempt, the node must be a valid one.
>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Paul E. McKenney <[email protected]>
> Cc: Oleg Nesterov <[email protected]>
> Cc: Michel Lespinasse <[email protected]>
> Cc: Andrea Arcangeli <[email protected]>
> Cc: David Woodhouse <[email protected]>
> Cc: Rik van Riel <[email protected]>
> Cc: Mathieu Desnoyers <[email protected]>
> Signed-off-by: Lai Jiangshan <[email protected]>
> ---
> include/linux/rbtree_latch.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/rbtree_latch.h b/include/linux/rbtree_latch.h
> index b012bd95eabf..09a3c05d1c5b 100644
> --- a/include/linux/rbtree_latch.h
> +++ b/include/linux/rbtree_latch.h
> @@ -208,7 +208,7 @@ latch_tree_find(void *key, struct latch_tree_root *root,
> do {
> seq = raw_read_seqcount_latch(&root->seq);
> node = __lt_find(key, root, seq & 1, ops->comp);
> - } while (read_seqcount_retry(&root->seq, seq));
> + } while (!node && read_seqcount_retry(&root->seq, seq));

So in the case where we search for key=N and race with { erase(N);
insert(N) }, we can now return the old N, as opposed to the new N.

But given that is entirely subject to timing anyway, that is irrelevant.
We change the boundary case in the timing.

Acked-by: Peter Zijlstra (Intel) <[email protected]>

2020-05-15 13:16:48

by Mathieu Desnoyers

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Subject: Re: [PATCH 1/2] rbtree_latch: quit searching when reaching to maximum depth

----- On May 15, 2020, at 8:47 AM, Lai Jiangshan [email protected] wrote:
[...]
> So the serarch should stop when diving down up to
> 2*BITS_PER_LONG depth.

serarch -> search

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

2020-05-15 14:41:42

by Lai Jiangshan

[permalink] [raw]

Subject: Re: [PATCH 1/2] rbtree_latch: quit searching when reaching to maximum depth

On Fri, May 15, 2020 at 9:04 PM Peter Zijlstra <[email protected]> wrote:
>
> On Fri, May 15, 2020 at 12:47:06PM +0000, Lai Jiangshan wrote:
> > lib/rbtree.c has ensured that there is not possible to
> > inadvertently cause (temporary) loops in the tree structure
> > as seen in program order of the modifier. But loop is still
> > possible to be seen in searcher due to CPU's reordering.
> >
> > for example:
> > modifier searcher
> >
> > left rotate at parent
> > parent->rb_right is node
> > search to parent
> > parent->rb_right is node
> > +->see node->rb_left changed
> > WRITE_ONCE(parent->rb_right, tmp);-+ | node->rb_left is parennt
> > no smp_wmb(), some arch can | |
> > reorder these two writes | | loop long between
> > WRITE_ONCE(node->rb_left, parent);-+-+ parent and node
> > |
> > +--->finally see
> > parent->rb_right
> >
> > The long loop won't stop until the modifer's CPU flushes
> > its writes. Too avoid it, we should limit the searching depth.
>
> Cute, have you actually observed this? Did you have performance issues?

I can only test it on x86 by now, which implies smp_wmb() between
writes. I haven't observed any thing wrong. I'm just imaging
it on some other ARCHs.

I accidentally found this part of code when I searched for
whether there is any attempt again to use rbtree with RCU, and
whether there are the cases besides speculative page fault.

>
> > There are no more than (1<<BITS_PER_LONG)-1 nodes in the tree.
> > And the max_depth of a tree is no more than 2*lg(node_count+1),
> > which is no mare than 2*BITS_PER_LONG.
> >
> > So the serarch should stop when diving down up to
> > 2*BITS_PER_LONG depth.
>
> Arguably you can have a larger key space, but I think due to memory
> constraints this limit still isn't wrong. But I do feel you need a
> comment with that.

Sure, I will add some comments about why "2*BITS_PER_LONG" in code.

But how it could be larger key space? there are not more than
(1<<BITS_PER_LONG) bytes in the kernel dereferencable address
space, and (1<<BITS_PER_LONG)/sizeof(rb_node) must be less than
(1<<BITS_PER_LONG)-1.

2020-05-15 15:06:56

by Peter Zijlstra

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Subject: Re: [PATCH 1/2] rbtree_latch: quit searching when reaching to maximum depth

On Fri, May 15, 2020 at 10:39:25PM +0800, Lai Jiangshan wrote:
> On Fri, May 15, 2020 at 9:04 PM Peter Zijlstra <[email protected]> wrote:
> > On Fri, May 15, 2020 at 12:47:06PM +0000, Lai Jiangshan wrote:
> > > lib/rbtree.c has ensured that there is not possible to
> > > inadvertently cause (temporary) loops in the tree structure
> > > as seen in program order of the modifier. But loop is still
> > > possible to be seen in searcher due to CPU's reordering.
> > >
> > > for example:
> > > modifier searcher
> > >
> > > left rotate at parent
> > > parent->rb_right is node
> > > search to parent
> > > parent->rb_right is node
> > > +->see node->rb_left changed
> > > WRITE_ONCE(parent->rb_right, tmp);-+ | node->rb_left is parennt
> > > no smp_wmb(), some arch can | |
> > > reorder these two writes | | loop long between
> > > WRITE_ONCE(node->rb_left, parent);-+-+ parent and node
> > > |
> > > +--->finally see
> > > parent->rb_right
> > >
> > > The long loop won't stop until the modifer's CPU flushes
> > > its writes. Too avoid it, we should limit the searching depth.
> >
> > Cute, have you actually observed this? Did you have performance issues?
>
> I can only test it on x86 by now, which implies smp_wmb() between
> writes. I haven't observed any thing wrong. I'm just imaging
> it on some other ARCHs.

Note that smp_wmb() doesn't imply flushing the store-buffers. Nor does
the TSO memory model of x86 (it's the main feature that distinguishes
TSO from SC).

x86's MFENCE is a completion barrier and does imply so though.

> I accidentally found this part of code when I searched for
> whether there is any attempt again to use rbtree with RCU, and
> whether there are the cases besides speculative page fault.

It got mentioned earlier in the contect of a stream of changes, an
uninterrupted modifier can basically starve a search.

But I don't think that's a problem with the current users.

> > > There are no more than (1<<BITS_PER_LONG)-1 nodes in the tree.
> > > And the max_depth of a tree is no more than 2*lg(node_count+1),
> > > which is no mare than 2*BITS_PER_LONG.
> > >
> > > So the serarch should stop when diving down up to
> > > 2*BITS_PER_LONG depth.
> >
> > Arguably you can have a larger key space, but I think due to memory
> > constraints this limit still isn't wrong. But I do feel you need a
> > comment with that.
>
> Sure, I will add some comments about why "2*BITS_PER_LONG" in code.
>
> But how it could be larger key space? there are not more than
> (1<<BITS_PER_LONG) bytes in the kernel dereferencable address
> space, and (1<<BITS_PER_LONG)/sizeof(rb_node) must be less than
> (1<<BITS_PER_LONG)-1.

Well, the key space is determined by the comparator operators that are
provided, which can easily compare values that are larger than 64bit.

But yes, the address space implies limits regardless of the actual
key-space. Note that BITS_PER_LONG does not related to the actual memory
space for things like i386-PAE and ARMv7-LPEA.

2020-05-15 16:02:16

by Lai Jiangshan

[permalink] [raw]

Subject: [PATCH V2 1/2] rbtree_latch: quit searching when reaching to maximum depth

lib/rbtree.c has ensured that there is not possible to
inadvertently cause (temporary) loops in the tree structure
as seen in program order of the modifier. But loop is still
possible to be seen in searcher due to CPU's reordering.

for example:
modifier searcher

left rotate at parent
parent->rb_right is node
search to parent
parent->rb_right is node
+->see node->rb_left changed
WRITE_ONCE(parent->rb_right, tmp);-+ | node->rb_left is parennt
no smp_wmb(), some ARCHs can | |
reorder these two writes | | loop long between
WRITE_ONCE(node->rb_left, parent);-+-+ parent and node
|
+--->finally see
parent->rb_right

The long loop won't stop until the modifer's CPU flushes
its writes. Too avoid it, we should limit the searching depth.
There are no more than (1<<BITS_PER_LONG)-1 nodes in the tree.
And the max_depth of a tree is no more than 2*lg(node_count+1),
which is no mare than 2*BITS_PER_LONG.

So the searching should stop when diving down up to
2*BITS_PER_LONG depth.

Cc: Peter Zijlstra <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Signed-off-by: Lai Jiangshan <[email protected]>
---
include/linux/rbtree_latch.h | 39 ++++++++++++++++++++++++++++++++++++
1 file changed, 39 insertions(+)

diff --git a/include/linux/rbtree_latch.h b/include/linux/rbtree_latch.h
index 7d012faa509a..638942f53c0a 100644
--- a/include/linux/rbtree_latch.h
+++ b/include/linux/rbtree_latch.h
@@ -102,11 +102,47 @@ __lt_erase(struct latch_tree_node *ltn, struct latch_tree_root *ltr, int idx)
rb_erase(&ltn->node[idx], &ltr->tree[idx]);
}

+/*
+ * Beware when rbtree is being searched in RCU read sites.
+ *
+ * lib/rbtree.c has ensured that there is not possible to
+ * inadvertently cause (temporary) loops in the tree structure
+ * as seen in program order of the modifier. But loop is still
+ * possible to be seen in searcher due to CPU's reordering.
+ *
+ * for example:
+ * modifier searcher
+ *
+ * left rotate at parent search to parent
+ * parent->rb_right is node parent->rb_right is node
+ * +->see node->rb_left changed
+ * WRITE_ONCE(parent->rb_right, tmp);-+ | node->rb_left is parennt
+ * no smp_wmb(), some ARCHs can | |
+ * reorder these two writes | | loop long between
+ * WRITE_ONCE(node->rb_left, parent);-+-+ parent and node
+ * |
+ * +--->finally see
+ * parent->rb_right
+ *
+ * The long loop won't stop until the searcher finally see the changes
+ * from the modifier. Too avoid it, we should limit the searching depth.
+ *
+ * Limited by the address space of the kernel, there are no more than
+ * (1<<BITS_PER_LONG)-1 nodes in the tree. And the max_depth of a tree is
+ * no more than 2*lg(node_count+1), which is no mare than 2*BITS_PER_LONG.
+ *
+ * So the searching should stop when diving down up to
+ * 2*BITS_PER_LONG depth.
+ *
+ * Note: the above problem is not subject to the TSO memory model, such as
+ * x86, which can dispense with the depth check.
+ */
static __always_inline struct latch_tree_node *
__lt_find(void *key, struct latch_tree_root *ltr, int idx,
int (*comp)(void *key, struct latch_tree_node *node))
{
struct rb_node *node = rcu_dereference_raw(ltr->tree[idx].rb_node);
+ int depth = 2 * BITS_PER_LONG;
struct latch_tree_node *ltn;
int c;

@@ -120,6 +156,9 @@ __lt_find(void *key, struct latch_tree_root *ltr, int idx,
node = rcu_dereference_raw(node->rb_right);
else
return ltn;
+
+ if (!IS_ENABLED(CONFIG_X86) && (--depth < 0))
+ break;
}

return NULL;
--
2.20.1

2020-05-23 00:59:36

by Lai Jiangshan

[permalink] [raw]

Subject: Re: [PATCH V2 1/2] rbtree_latch: quit searching when reaching to maximum depth

On Sat, May 16, 2020 at 12:01 AM Lai Jiangshan <[email protected]> wrote:
>
> lib/rbtree.c has ensured that there is not possible to
> inadvertently cause (temporary) loops in the tree structure
> as seen in program order of the modifier. But loop is still
> possible to be seen in searcher due to CPU's reordering.
>
> for example:
> modifier searcher
>
> left rotate at parent
> parent->rb_right is node
> search to parent
> parent->rb_right is node
> +->see node->rb_left changed
> WRITE_ONCE(parent->rb_right, tmp);-+ | node->rb_left is parennt
> no smp_wmb(), some ARCHs can | |
> reorder these two writes | | loop long between
> WRITE_ONCE(node->rb_left, parent);-+-+ parent and node
> |
> +--->finally see
> parent->rb_right
>
> The long loop won't stop until the modifer's CPU flushes
> its writes. Too avoid it, we should limit the searching depth.
> There are no more than (1<<BITS_PER_LONG)-1 nodes in the tree.
> And the max_depth of a tree is no more than 2*lg(node_count+1),
> which is no mare than 2*BITS_PER_LONG.
>
> So the searching should stop when diving down up to
> 2*BITS_PER_LONG depth.
>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Paul E. McKenney <[email protected]>
> Cc: Oleg Nesterov <[email protected]>
> Cc: Michel Lespinasse <[email protected]>
> Cc: Andrea Arcangeli <[email protected]>
> Cc: Rik van Riel <[email protected]>
> Cc: Mathieu Desnoyers <[email protected]>
> Signed-off-by: Lai Jiangshan <[email protected]>
> ---

Hello

Could anyone have a review or an ack on the updated patch?
Compared v1:
Applied Mathieu's suggest to change the changelog
Avoid the depth check on x86, so the patch makes no functionality
change on x86

And which tree should the patches route to? It is memory ordering
related.

Thanks
Lai

> include/linux/rbtree_latch.h | 39 ++++++++++++++++++++++++++++++++++++
> 1 file changed, 39 insertions(+)
>
> diff --git a/include/linux/rbtree_latch.h b/include/linux/rbtree_latch.h
> index 7d012faa509a..638942f53c0a 100644
> --- a/include/linux/rbtree_latch.h
> +++ b/include/linux/rbtree_latch.h
> @@ -102,11 +102,47 @@ __lt_erase(struct latch_tree_node *ltn, struct latch_tree_root *ltr, int idx)
> rb_erase(&ltn->node[idx], &ltr->tree[idx]);
> }
>
> +/*
> + * Beware when rbtree is being searched in RCU read sites.
> + *
> + * lib/rbtree.c has ensured that there is not possible to
> + * inadvertently cause (temporary) loops in the tree structure
> + * as seen in program order of the modifier. But loop is still
> + * possible to be seen in searcher due to CPU's reordering.
> + *
> + * for example:
> + * modifier searcher
> + *
> + * left rotate at parent search to parent
> + * parent->rb_right is node parent->rb_right is node
> + * +->see node->rb_left changed
> + * WRITE_ONCE(parent->rb_right, tmp);-+ | node->rb_left is parennt
> + * no smp_wmb(), some ARCHs can | |
> + * reorder these two writes | | loop long between
> + * WRITE_ONCE(node->rb_left, parent);-+-+ parent and node
> + * |
> + * +--->finally see
> + * parent->rb_right
> + *
> + * The long loop won't stop until the searcher finally see the changes
> + * from the modifier. Too avoid it, we should limit the searching depth.
> + *
> + * Limited by the address space of the kernel, there are no more than
> + * (1<<BITS_PER_LONG)-1 nodes in the tree. And the max_depth of a tree is
> + * no more than 2*lg(node_count+1), which is no mare than 2*BITS_PER_LONG.
> + *
> + * So the searching should stop when diving down up to
> + * 2*BITS_PER_LONG depth.
> + *
> + * Note: the above problem is not subject to the TSO memory model, such as
> + * x86, which can dispense with the depth check.
> + */
> static __always_inline struct latch_tree_node *
> __lt_find(void *key, struct latch_tree_root *ltr, int idx,
> int (*comp)(void *key, struct latch_tree_node *node))
> {
> struct rb_node *node = rcu_dereference_raw(ltr->tree[idx].rb_node);
> + int depth = 2 * BITS_PER_LONG;
> struct latch_tree_node *ltn;
> int c;
>
> @@ -120,6 +156,9 @@ __lt_find(void *key, struct latch_tree_root *ltr, int idx,
> node = rcu_dereference_raw(node->rb_right);
> else
> return ltn;
> +
> + if (!IS_ENABLED(CONFIG_X86) && (--depth < 0))
> + break;
> }
>
> return NULL;
> --
> 2.20.1
>