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SubjectRe: [PATCH v3] drivers/base/memory.c: cache blocks in radix tree to accelerate lookup
On Tue 17-12-19 13:32:38, Scott Cheloha wrote:
> Searching for a particular memory block by id is slow because each block
> device is kept in an unsorted linked list on the subsystem bus.
>
> Lookup is much faster if we cache the blocks in a radix tree. Memory
> subsystem initialization and hotplug/hotunplug is at least a little faster
> for any machine with more than ~100 blocks, and the speedup grows with
> the block count.
>
> Signed-off-by: Scott Cheloha <cheloha@linux.vnet.ibm.com>
> Acked-by: David Hildenbrand <david@redhat.com>

OK, Greg doesn't see demand for abstracting a faster lookup into the
core so making a memory subsystem thing sounds like the way to go.
Please add more information about time savings into the changelog and
then feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>

Do not forget to add Andrew to the Cc when resubmitting.

Thanks!

> ---
> v2 incorporates suggestions from David Hildenbrand.
>
> v3 changes:
> - Rebase atop "drivers/base/memory.c: drop the mem_sysfs_mutex"
>
> - Be conservative: don't use radix_tree_for_each_slot() in
> walk_memory_blocks() yet. It introduces RCU which could
> change behavior. Walking the tree "by hand" with
> find_memory_block_by_id() is slower but keeps the patch
> simple.
>
> drivers/base/memory.c | 36 +++++++++++++++++++++++-------------
> 1 file changed, 23 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> index 799b43191dea..8902930d5ef2 100644
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -19,6 +19,7 @@
> #include <linux/memory.h>
> #include <linux/memory_hotplug.h>
> #include <linux/mm.h>
> +#include <linux/radix-tree.h>
> #include <linux/stat.h>
> #include <linux/slab.h>
>
> @@ -56,6 +57,13 @@ static struct bus_type memory_subsys = {
> .offline = memory_subsys_offline,
> };
>
> +/*
> + * Memory blocks are cached in a local radix tree to avoid
> + * a costly linear search for the corresponding device on
> + * the subsystem bus.
> + */
> +static RADIX_TREE(memory_blocks, GFP_KERNEL);
> +
> static BLOCKING_NOTIFIER_HEAD(memory_chain);
>
> int register_memory_notifier(struct notifier_block *nb)
> @@ -572,20 +580,14 @@ int __weak arch_get_memory_phys_device(unsigned long start_pfn)
> /* A reference for the returned memory block device is acquired. */
> static struct memory_block *find_memory_block_by_id(unsigned long block_id)
> {
> - struct device *dev;
> + struct memory_block *mem;
>
> - dev = subsys_find_device_by_id(&memory_subsys, block_id, NULL);
> - return dev ? to_memory_block(dev) : NULL;
> + mem = radix_tree_lookup(&memory_blocks, block_id);
> + if (mem)
> + get_device(&mem->dev);
> + return mem;
> }
>
> -/*
> - * For now, we have a linear search to go find the appropriate
> - * memory_block corresponding to a particular phys_index. If
> - * this gets to be a real problem, we can always use a radix
> - * tree or something here.
> - *
> - * This could be made generic for all device subsystems.
> - */
> struct memory_block *find_memory_block(struct mem_section *section)
> {
> unsigned long block_id = base_memory_block_id(__section_nr(section));
> @@ -628,9 +630,15 @@ int register_memory(struct memory_block *memory)
> memory->dev.offline = memory->state == MEM_OFFLINE;
>
> ret = device_register(&memory->dev);
> - if (ret)
> + if (ret) {
> put_device(&memory->dev);
> -
> + return ret;
> + }
> + ret = radix_tree_insert(&memory_blocks, memory->dev.id, memory);
> + if (ret) {
> + put_device(&memory->dev);
> + device_unregister(&memory->dev);
> + }
> return ret;
> }
>
> @@ -688,6 +696,8 @@ static void unregister_memory(struct memory_block *memory)
> if (WARN_ON_ONCE(memory->dev.bus != &memory_subsys))
> return;
>
> + WARN_ON(radix_tree_delete(&memory_blocks, memory->dev.id) == NULL);
> +
> /* drop the ref. we got via find_memory_block() */
> put_device(&memory->dev);
> device_unregister(&memory->dev);
> --
> 2.24.0
>

--
Michal Hocko
SUSE Labs

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