On Fri, Apr 30, 2021 at 11:36:06AM -0300, Leonardo Bras wrote:
> Because hypervisors may need to create HPTs without knowing the guest
> page size, the smallest used page-size (4k) may be chosen, resulting in
> a HPT that is possibly bigger than needed.
> 
> On a guest with bigger page-sizes, the amount of entries for HTP may be
> too high, causing the guest to ask for a HPT resize-down on the first
> hotplug.
> 
> This becomes a problem when HPT resize-down fails, and causes the
> HPT resize to be performed on every LMB added, until HPT size is
> compatible to guest memory size, causing a major slowdown.
> 
> So, avoiding HPT resizing-down on hot-add significantly improves memory
> hotplug times.
> 
> As an example, hotplugging 256GB on a 129GB guest took 710s without this
> patch, and 21s after applied.
> 
> Signed-off-by: Leonardo Bras <leobra...@gmail.com>

Sorry it's taken me so long to look at these

I don't love the extra statefulness that the 'shrinking' parameter
adds, but I can't see an elegant way to avoid it, so:

Reviewed-by: David Gibson <da...@gibson.dropbear.id.au>

> ---
>  arch/powerpc/mm/book3s64/hash_utils.c | 36 ++++++++++++++++-----------
>  1 file changed, 21 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/powerpc/mm/book3s64/hash_utils.c 
> b/arch/powerpc/mm/book3s64/hash_utils.c
> index 581b20a2feaf..608e4ed397a9 100644
> --- a/arch/powerpc/mm/book3s64/hash_utils.c
> +++ b/arch/powerpc/mm/book3s64/hash_utils.c
> @@ -795,7 +795,7 @@ static unsigned long __init htab_get_table_size(void)
>  }
>  
>  #ifdef CONFIG_MEMORY_HOTPLUG
> -static int resize_hpt_for_hotplug(unsigned long new_mem_size)
> +static int resize_hpt_for_hotplug(unsigned long new_mem_size, bool shrinking)
>  {
>       unsigned target_hpt_shift;
>  
> @@ -804,19 +804,25 @@ static int resize_hpt_for_hotplug(unsigned long 
> new_mem_size)
>  
>       target_hpt_shift = htab_shift_for_mem_size(new_mem_size);
>  
> -     /*
> -      * To avoid lots of HPT resizes if memory size is fluctuating
> -      * across a boundary, we deliberately have some hysterisis
> -      * here: we immediately increase the HPT size if the target
> -      * shift exceeds the current shift, but we won't attempt to
> -      * reduce unless the target shift is at least 2 below the
> -      * current shift
> -      */
> -     if (target_hpt_shift > ppc64_pft_size ||
> -         target_hpt_shift < ppc64_pft_size - 1)
> -             return mmu_hash_ops.resize_hpt(target_hpt_shift);
> +     if (shrinking) {
>  
> -     return 0;
> +             /*
> +              * To avoid lots of HPT resizes if memory size is fluctuating
> +              * across a boundary, we deliberately have some hysterisis
> +              * here: we immediately increase the HPT size if the target
> +              * shift exceeds the current shift, but we won't attempt to
> +              * reduce unless the target shift is at least 2 below the
> +              * current shift
> +              */
> +
> +             if (target_hpt_shift >= ppc64_pft_size - 1)
> +                     return 0;
> +
> +     } else if (target_hpt_shift <= ppc64_pft_size) {
> +             return 0;
> +     }
> +
> +     return mmu_hash_ops.resize_hpt(target_hpt_shift);
>  }
>  
>  int hash__create_section_mapping(unsigned long start, unsigned long end,
> @@ -829,7 +835,7 @@ int hash__create_section_mapping(unsigned long start, 
> unsigned long end,
>               return -1;
>       }
>  
> -     resize_hpt_for_hotplug(memblock_phys_mem_size());
> +     resize_hpt_for_hotplug(memblock_phys_mem_size(), false);
>  
>       rc = htab_bolt_mapping(start, end, __pa(start),
>                              pgprot_val(prot), mmu_linear_psize,
> @@ -848,7 +854,7 @@ int hash__remove_section_mapping(unsigned long start, 
> unsigned long end)
>       int rc = htab_remove_mapping(start, end, mmu_linear_psize,
>                                    mmu_kernel_ssize);
>  
> -     if (resize_hpt_for_hotplug(memblock_phys_mem_size()) == -ENOSPC)
> +     if (resize_hpt_for_hotplug(memblock_phys_mem_size(), true) == -ENOSPC)
>               pr_warn("Hash collision while resizing HPT\n");
>  
>       return rc;

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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