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Date: Fri, 22 Mar 2013 11:47:45 +0200 (EET)
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave@...1.net>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Al Viro <viro@...iv.linux.org.uk>,
Hugh Dickins <hughd@...gle.com>,
Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
Andi Kleen <ak@...ux.intel.com>,
Matthew Wilcox <matthew.r.wilcox@...el.com>,
"Kirill A. Shutemov" <kirill@...temov.name>,
Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2, RFC 04/30] radix-tree: implement preload for multiple
contiguous elements
Dave Hansen wrote:
> On 03/14/2013 10:50 AM, Kirill A. Shutemov wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> >
> > Currently radix_tree_preload() only guarantees enough nodes to insert
> > one element. It's a hard limit. You cannot batch a number insert under
> > one tree_lock.
> >
> > This patch introduces radix_tree_preload_count(). It allows to
> > preallocate nodes enough to insert a number of *contiguous* elements.
>
> You don't need to write a paper on how radix trees work, but it might be
> nice to include a wee bit of text in here about how the existing preload
> works, and how this new guarantee works.
Reasonable, will do.
> > diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
> > index ffc444c..81318cb 100644
> > --- a/include/linux/radix-tree.h
> > +++ b/include/linux/radix-tree.h
> > @@ -83,6 +83,8 @@ do { \
> > (root)->rnode = NULL; \
> > } while (0)
> >
> > +#define RADIX_TREE_PRELOAD_NR 512 /* For THP's benefit */
>
> This eventually boils down to making the radix_tree_preload array
> larger. Do we really want to do this unconditionally if it's only for
> THP's benefit?
It will be useful not only for THP. Batching can be useful to solve
scalability issues.
> > /**
> > * Radix-tree synchronization
> > *
> > @@ -231,6 +233,7 @@ unsigned long radix_tree_next_hole(struct radix_tree_root *root,
> > unsigned long radix_tree_prev_hole(struct radix_tree_root *root,
> > unsigned long index, unsigned long max_scan);
> > int radix_tree_preload(gfp_t gfp_mask);
> > +int radix_tree_preload_count(unsigned size, gfp_t gfp_mask);
> > void radix_tree_init(void);
> > void *radix_tree_tag_set(struct radix_tree_root *root,
> > unsigned long index, unsigned int tag);
> > diff --git a/lib/radix-tree.c b/lib/radix-tree.c
> > index e796429..9bef0ac 100644
> > --- a/lib/radix-tree.c
> > +++ b/lib/radix-tree.c
> > @@ -81,16 +81,24 @@ static struct kmem_cache *radix_tree_node_cachep;
> > * The worst case is a zero height tree with just a single item at index 0,
> > * and then inserting an item at index ULONG_MAX. This requires 2 new branches
> > * of RADIX_TREE_MAX_PATH size to be created, with only the root node shared.
> > + *
> > + * Worst case for adding N contiguous items is adding entries at indexes
> > + * (ULONG_MAX - N) to ULONG_MAX. It requires nodes to insert single worst-case
> > + * item plus extra nodes if you cross the boundary from one node to the next.
> > + *
> > * Hence:
> > */
> > -#define RADIX_TREE_PRELOAD_SIZE (RADIX_TREE_MAX_PATH * 2 - 1)
> > +#define RADIX_TREE_PRELOAD_MIN (RADIX_TREE_MAX_PATH * 2 - 1)
> > +#define RADIX_TREE_PRELOAD_MAX \
> > + (RADIX_TREE_PRELOAD_MIN + \
> > + DIV_ROUND_UP(RADIX_TREE_PRELOAD_NR - 1, RADIX_TREE_MAP_SIZE))
> >
> > /*
> > * Per-cpu pool of preloaded nodes
> > */
> > struct radix_tree_preload {
> > int nr;
> > - struct radix_tree_node *nodes[RADIX_TREE_PRELOAD_SIZE];
> > + struct radix_tree_node *nodes[RADIX_TREE_PRELOAD_MAX];
> > };
>
> For those of us too lazy to go compile a kernel and figure this out in
> practice, how much bigger does this make the nodes[] array?
We have three possible RADIX_TREE_MAP_SHIFT:
#ifdef __KERNEL__
#define RADIX_TREE_MAP_SHIFT (CONFIG_BASE_SMALL ? 4 : 6)
#else
#define RADIX_TREE_MAP_SHIFT 3 /* For more stressful testing */
#endif
On 64-bit system:
For RADIX_TREE_MAP_SHIFT=3, old array size is 43, new is 107.
For RADIX_TREE_MAP_SHIFT=4, old array size is 31, new is 63.
For RADIX_TREE_MAP_SHIFT=6, old array size is 21, new is 30.
On 32-bit system:
For RADIX_TREE_MAP_SHIFT=3, old array size is 21, new is 84.
For RADIX_TREE_MAP_SHIFT=4, old array size is 15, new is 46.
For RADIX_TREE_MAP_SHIFT=6, old array size is 11, new is 19.
On most machines we will have RADIX_TREE_MAP_SHIFT=6.
>
> > static DEFINE_PER_CPU(struct radix_tree_preload, radix_tree_preloads) = { 0, };
> >
> > @@ -257,29 +265,34 @@ radix_tree_node_free(struct radix_tree_node *node)
> >
> > /*
> > * Load up this CPU's radix_tree_node buffer with sufficient objects to
> > - * ensure that the addition of a single element in the tree cannot fail. On
> > - * success, return zero, with preemption disabled. On error, return -ENOMEM
> > + * ensure that the addition of *contiguous* elements in the tree cannot fail.
> > + * On success, return zero, with preemption disabled. On error, return -ENOMEM
> > * with preemption not disabled.
> > *
> > * To make use of this facility, the radix tree must be initialised without
> > * __GFP_WAIT being passed to INIT_RADIX_TREE().
> > */
> > -int radix_tree_preload(gfp_t gfp_mask)
> > +int radix_tree_preload_count(unsigned size, gfp_t gfp_mask)
> > {
> > struct radix_tree_preload *rtp;
> > struct radix_tree_node *node;
> > int ret = -ENOMEM;
> > + int alloc = RADIX_TREE_PRELOAD_MIN +
> > + DIV_ROUND_UP(size - 1, RADIX_TREE_MAP_SIZE);
>
> Any chance I could talk you in to giving 'alloc' a better name? Maybe
> "preload_target" or "preload_fill_to".
Ok.
> > + if (size > RADIX_TREE_PRELOAD_NR)
> > + return -ENOMEM;
>
> I always wonder if these deep, logical -ENOMEMs deserve a WARN_ONCE().
> We really don't expect to hit this unless something really funky is
> going on, right?
Correct. Will add WARN.
> > preempt_disable();
> > rtp = &__get_cpu_var(radix_tree_preloads);
> > - while (rtp->nr < ARRAY_SIZE(rtp->nodes)) {
> > + while (rtp->nr < alloc) {
> > preempt_enable();
> > node = kmem_cache_alloc(radix_tree_node_cachep, gfp_mask);
> > if (node == NULL)
> > goto out;
> > preempt_disable();
> > rtp = &__get_cpu_var(radix_tree_preloads);
> > - if (rtp->nr < ARRAY_SIZE(rtp->nodes))
> > + if (rtp->nr < alloc)
> > rtp->nodes[rtp->nr++] = node;
> > else
> > kmem_cache_free(radix_tree_node_cachep, node);
> > @@ -288,6 +301,11 @@ int radix_tree_preload(gfp_t gfp_mask)
> > out:
> > return ret;
> > }
--
Kirill A. Shutemov
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