Re: [PATCH v5 2/3] x86: probe memory block size advisement value during mm init

[Mike Rapoport]

On Fri, Nov 01, 2024 at 09:47:04AM -0400, Gregory Price wrote:
> Systems with hotplug may provide an advisement value on what the
> memblock size should be.  Probe this value when the rest of the
> configuration values are considered.
> 
> The new heuristic is as follows
> 
> 1) set_memory_block_size_order value if already set (cmdline param)
> 2) minimum block size if memory is less than large block limit
> 3) if no hotplug advice: Max block size if system is bare-metal,
>    otherwise use end of memory alignment.
> 4) if hotplug advice: lesser of advice and end of memory alignment.
> 
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Acked-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>

> ---
>  arch/x86/mm/init_64.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index ff253648706f..f1a495e998ce 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1452,16 +1452,21 @@ static unsigned long probe_memory_block_size(void)
>  	}
>  
>  	/*
> -	 * Use max block size to minimize overhead on bare metal, where
> -	 * alignment for memory hotplug isn't a concern.
> +	 * When hotplug alignment is not a concern, maximize blocksize
> +	 * to minimize overhead. Otherwise, align to the lesser of advice
> +	 * alignment and end of memory alignment.
>  	 */
> -	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
> +	bz = memory_block_advised_max_size();
> +	if (!bz) {
>  		bz = MAX_BLOCK_SIZE;
> -		goto done;
> +		if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
> +			goto done;
> +	} else {
> +		bz = max(min(bz, MAX_BLOCK_SIZE), MIN_MEMORY_BLOCK_SIZE);
>  	}
>  
>  	/* Find the largest allowed block size that aligns to memory end */
> -	for (bz = MAX_BLOCK_SIZE; bz > MIN_MEMORY_BLOCK_SIZE; bz >>= 1) {
> +	for (; bz > MIN_MEMORY_BLOCK_SIZE; bz >>= 1) {
>  		if (IS_ALIGNED(boot_mem_end, bz))
>  			break;
>  	}
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.