linux-kernel - Re: [PATCH v3] drivers/base/memory.c: cache blocks in radix tree to accelerate lookup

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Date:   Thu, 9 Jan 2020 10:48:26 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Scott Cheloha <cheloha@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Hildenbrand <david@...hat.com>, nathanl@...ux.ibm.com,
        ricklind@...ux.vnet.ibm.com,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [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@...ux.vnet.ibm.com>
> Acked-by: David Hildenbrand <david@...hat.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@...e.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