linux-kernel - Re: [PATCH v2] x86/mm: use max memory block size on bare metal

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Date:   Wed, 10 Jun 2020 09:20:52 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Sistare <steven.sistare@...cle.com>
Subject: Re: [PATCH v2] x86/mm: use max memory block size on bare metal

On 10.06.20 00:54, Daniel Jordan wrote:
> Some of our servers spend significant time at kernel boot initializing
> memory block sysfs directories and then creating symlinks between them
> and the corresponding nodes.  The slowness happens because the machines
> get stuck with the smallest supported memory block size on x86 (128M),
> which results in 16,288 directories to cover the 2T of installed RAM.
> The search for each memory block is noticeable even with
> commit 4fb6eabf1037 ("drivers/base/memory.c: cache memory blocks in
> xarray to accelerate lookup").
> 
> Commit 078eb6aa50dc ("x86/mm/memory_hotplug: determine block size based
> on the end of boot memory") chooses the block size based on alignment
> with memory end.  That addresses hotplug failures in qemu guests, but
> for bare metal systems whose memory end isn't aligned to even the
> smallest size, it leaves them at 128M.
> 
> Make kernels that aren't running on a hypervisor use the largest
> supported size (2G) to minimize overhead on big machines.  Kernel boot
> goes 7% faster on the aforementioned servers, shaving off half a second.
> 
> Signed-off-by: Daniel Jordan <daniel.m.jordan@...cle.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Sistare <steven.sistare@...cle.com>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org
> ---
> 
> Applies to 5.7 and today's mainline
> 
>  arch/x86/mm/init_64.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 8b5f73f5e207c..906fbdb060748 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -55,6 +55,7 @@
>  #include <asm/uv/uv.h>
>  #include <asm/setup.h>
>  #include <asm/ftrace.h>
> +#include <asm/hypervisor.h>
>  
>  #include "mm_internal.h"
>  
> @@ -1390,6 +1391,15 @@ static unsigned long probe_memory_block_size(void)
>  		goto done;
>  	}
>  
> +	/*
> +	 * Use max block size to minimize overhead on bare metal, where
> +	 * alignment for memory hotplug isn't a concern.
> +	 */
> +	if (hypervisor_is_type(X86_HYPER_NATIVE)) {
> +		bz = MAX_BLOCK_SIZE;
> +		goto done;
> +	}

I'd assume that bioses on physical machines >= 64GB will not align
bigger (>= 2GB) DIMMs to something < 2GB.

Acked-by: David Hildenbrand <david@...hat.com>

> +
>  	/* Find the largest allowed block size that aligns to memory end */
>  	for (bz = MAX_BLOCK_SIZE; bz > MIN_MEMORY_BLOCK_SIZE; bz >>= 1) {
>  		if (IS_ALIGNED(boot_mem_end, bz))
> 


-- 
Thanks,

David / dhildenb