Messages in this thread | | | Date | Mon, 7 Jun 2021 15:02:38 +1000 | From | David Gibson <> | Subject | Re: [PATCH v2 1/3] powerpc/mm/hash: Avoid resizing-down HPT on first memory hotplug |
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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 <leobras.c@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 <david@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 [unhandled content-type:application/pgp-signature] | |