linux-kernel - Re: [PATCH] drivers/base/memory.c: memory subsys init: skip search for missing blocks

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Message-ID: <c702b775-4a38-bc97-c67f-83f986bbe5fa@redhat.com>
Date:   Fri, 1 Nov 2019 20:00:44 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Scott Cheloha <cheloha@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     nathanl@...ux.ibm.com, ricklind@...ux.vnet.ibm.com
Subject: Re: [PATCH] drivers/base/memory.c: memory subsys init: skip search
 for missing blocks

On 01.11.19 19:10, Scott Cheloha wrote:
> Add a flag to init_memory_block() to  enable/disable searching for a
> matching block before creating a device for the block and adding it to
> the memory subsystem's bus.
> 
> When the memory subsystem is being initialized there is no need to check
> if a given block has already been added to its bus.  The bus is new, so the
> block in question cannot yet have a corresponding device.
> 
> The search for a missing block is O(n) so this saves substantial time at
> boot if there are many such blocks to add.
> 
> Signed-off-by: Scott Cheloha <cheloha@...ux.vnet.ibm.com>
> ---
>   drivers/base/memory.c | 18 +++++++++++-------
>   1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
> index 55907c27075b..1160df4a8feb 100644
> --- a/drivers/base/memory.c
> +++ b/drivers/base/memory.c
> @@ -643,17 +643,21 @@ int register_memory(struct memory_block *memory)
>   }
>   
>   static int init_memory_block(struct memory_block **memory,
> -			     unsigned long block_id, unsigned long state)
> +			     unsigned long block_id, unsigned long state,
> +			     bool may_exist)
>   {
>   	struct memory_block *mem;
>   	unsigned long start_pfn;
>   	int ret = 0;
>   
> -	mem = find_memory_block_by_id(block_id);
> -	if (mem) {
> -		put_device(&mem->dev);
> -		return -EEXIST;
> +	if (may_exist) {
> +		mem = find_memory_block_by_id(block_id);
> +		if (mem) {
> +			put_device(&mem->dev);
> +			return -EEXIST;
> +		}

No, I don't really like that. Can we please speed up the lookup via a 
radix tree as noted in the comment of "find_memory_block()".

/*
  * 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.
  */

This will speed up all users of walk_memory_blocks() similarly.
Especially, it will also speed up link_mem_sections() used during boot, 
which relies on walk_memory_blocks().

-- 

Thanks,

David / dhildenb