linux-kernel - Re: [PATCH v3 2/3] x86: probe memory block size advisement value during mm init

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Message-ID: <c0decc3e-8b3a-40e1-a997-f4b6f500f546@redhat.com>
Date: Tue, 29 Oct 2024 14:52:17 +0100
From: David Hildenbrand <david@...hat.com>
To: Gregory Price <gourry@...rry.net>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
 linux-mm@...ck.org, linux-cxl@...ck.org, Jonathan.Cameron@...wei.com,
 dan.j.williams@...el.com, rrichter@....com, Terry.Bowman@....com,
 dave.jiang@...el.com, ira.weiny@...el.com, alison.schofield@...el.com,
 dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
 rafael@...nel.org, lenb@...nel.org, osalvador@...e.de,
 gregkh@...uxfoundation.org, akpm@...ux-foundation.org, rppt@...nel.org
Subject: Re: [PATCH v3 2/3] x86: probe memory block size advisement value
 during mm init

On 29.10.24 14:48, Gregory Price wrote:
> On Tue, Oct 29, 2024 at 01:40:38PM +0100, David Hildenbrand wrote:
>> On 22.10.24 23:34, 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@...hat.com>
>>> Signed-off-by: Gregory Price <gourry@...rry.net>
>>> ---
>>>    arch/x86/mm/init_64.c | 14 +++++++++-----
>>>    1 file changed, 9 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
>>> index ff253648706f..93d669f467f7 100644
>>> --- a/arch/x86/mm/init_64.c
>>> +++ b/arch/x86/mm/init_64.c
>>> @@ -1452,13 +1452,17 @@ 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_probe_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) {
>          ^^^^^^^^^^^^^^^^^^^^^^^^
>>
>>
>> Acked-by: David Hildenbrand <david@...hat.com>
> 
> Will pick this up but wanted to point out the silly bug above.
> This version completely ignores the advise lol.
> 
> Changing to below
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 93d669f467f7..01876629f21f 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1465,7 +1465,7 @@ static unsigned long probe_memory_block_size(void)
>                  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;

Heh, yes, I think I suggested that in my quick draft.

-- 
Cheers,

David / dhildenb