lkml.org 
[lkml]   [2021]   [Jun]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v2 1/3] powerpc/mm/hash: Avoid resizing-down HPT on first memory hotplug
    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]
    \
     
     \ /
      Last update: 2021-06-07 07:25    [W:2.607 / U:0.088 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site