linux-kernel - Re: [PATCH v4] drivers/base/memory.c: cache blocks in radix tree to accelerate lookup

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Date:   Fri, 17 Jan 2020 10:35:14 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Scott Cheloha <cheloha@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        nathanl@...ux.ibm.com, ricklind@...ux.vnet.ibm.com,
        Scott Cheloha <cheloha@...ux.ibm.com>,
        Donald Dutile <ddutile@...hat.com>
Subject: Re: [PATCH v4] drivers/base/memory.c: cache blocks in radix tree to
 accelerate lookup

On Thu 16-01-20 17:17:54, David Hildenbrand wrote:
[...]
> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> index c6d288fad493..c75dec35de43 100644
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -19,7 +19,7 @@
>  #include <linux/memory.h>
>  #include <linux/memory_hotplug.h>
>  #include <linux/mm.h>
> -#include <linux/radix-tree.h>
> +#include <linux/xarray.h>
>  #include <linux/stat.h>
>  #include <linux/slab.h>
>  
> @@ -58,11 +58,11 @@ static struct bus_type memory_subsys = {
>  };
>  
>  /*
> - * Memory blocks are cached in a local radix tree to avoid
> + * Memory blocks are cached in a local xarray to avoid
>   * a costly linear search for the corresponding device on
>   * the subsystem bus.
>   */
> -static RADIX_TREE(memory_blocks, GFP_KERNEL);
> +static DEFINE_XARRAY(memory_blocks);
>  
>  static BLOCKING_NOTIFIER_HEAD(memory_chain);
>  
> @@ -566,7 +566,7 @@ static struct memory_block *find_memory_block_by_id(unsigned long block_id)
>  {
>         struct memory_block *mem;
>  
> -       mem = radix_tree_lookup(&memory_blocks, block_id);
> +       mem = xa_load(&memory_blocks, block_id);
>         if (mem)
>                 get_device(&mem->dev);
>         return mem;
> @@ -621,7 +621,8 @@ int register_memory(struct memory_block *memory)
>                 put_device(&memory->dev);
>                 return ret;
>         }
> -       ret = radix_tree_insert(&memory_blocks, memory->dev.id, memory);
> +       ret = xa_err(xa_store(&memory_blocks, memory->dev.id, memory,
> +                             GFP_KERNEL));
>         if (ret) {
>                 put_device(&memory->dev);
>                 device_unregister(&memory->dev);
> @@ -683,7 +684,7 @@ 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);
> +       WARN_ON(xa_erase(&memory_blocks, memory->dev.id) == NULL);
>  
>         /* drop the ref. we got via find_memory_block() */
>         put_device(&memory->dev);

OK, this looks sensible. xa_store shouldn't ever return an existing
device as we do the lookpup beforehand so good. We might need to
reorganize the code if we want to drop the loopup though.

Thanks!
-- 
Michal Hocko
SUSE Labs