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Message-ID: <20231001190846.GY3303@kernel.org>
Date: Sun, 1 Oct 2023 22:08:46 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Yajun Deng <yajun.deng@...ux.dev>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memblock: don't run loop in memblock_add_range() twice
On Fri, Sep 29, 2023 at 06:10:50PM +0800, Yajun Deng wrote:
>
> On 2023/9/29 17:04, Mike Rapoport wrote:
> > On Thu, Sep 28, 2023 at 04:47:59PM +0800, Yajun Deng wrote:
> > > On 2023/9/28 14:16, Mike Rapoport wrote:
> > > > On Wed, Sep 27, 2023 at 09:37:52AM +0800, Yajun Deng wrote:
> > > > > There is round twice in memblock_add_range(). The first counts the number
> > > > > of regions needed to accommodate the new area. The second actually inserts
> > > > > them. But the first round isn't really needed, we just need to check the
> > > > > counts before inserting them.
> > > > >
> > > > > Check the count before calling memblock_insert_region(). If the count is
> > > > > equal to the maximum value, it needs to resize the array. Otherwise,
> > > > > insert it directly.
> > > > >
> > > > > To avoid nested calls to memblock_add_range(), we need to call
> > > > > memblock_reserve() out of memblock_double_array().
> > > > memblock_add_range() does an extra loop once in a while, but I don't think
> > > > removing it will have any actual effect on the boot time.
> > >
> > > Yes, it has no obvious actual effect on the boot time, but it does reduce
> > > the number of unnecessary loop.
> > >
> > > The actual effect on the boot time should not be the only criterion for
> > > whether a patch is accepted or not.
> > >
> > > Since the comment in the previous code, it tells the user that it would be
> > > executed twice, this can be misleading to users.
> > >
> > > So the new code will be simpler and clearer. It not just change the code,
> > > but also remove the comment
> > Adding return-by-pointer parameters to memblock_double_array() and pulling
> > memblock_reserve() out of this function is in no way simpler and clearer
> > that having an extra loop.
>
> If memblock_reserve() in memblock_double_array(), there will be nested
> calls to memblock_add_range().
>
> memblock_add_range(A)->memblock_double_array(A)->memblock_reserve(B)->memblock_add_range(B)
>
> ->memblock_insert_region(B)->memblock_merge_regions(B)->memblock_insert_region(A)->memblock_merge_regions(A)
>
> It's hard to see that and debug.
>
> If memblock_reserve() out of memblock_double_array(), there wouldn't have a
> nested calls.
With memblock_reserve() out of memblock_double_array(), the latter stops
being self-contained, which makes the code less readable and less
maintainable.
> memblock_add_range(A)->memblock_double_array(A)->memblock_insert_region(A)->memblock_merge_regions(A)->
>
> memblock_reserve(B)->memblock_add_range(B)->memblock_insert_region(B)->memblock_merge_regions(B)
>
> We should make memblock_add_range is done, and do another
> memblock_add_range.
Sorry, I do not follow you here.
> > If the comment is wrong, just fix the comment.
> > > about "executed twice", it obviously tells the user only resize the array
> > > if it is equal to the maximum value
> > >
> > > and doesn't need to be executed twice.
> > >
> > > > > Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
> > > > > ---
> > > > > mm/memblock.c | 117 ++++++++++++++++++++++++--------------------------
> > > > > 1 file changed, 57 insertions(+), 60 deletions(-)
> > > > >
> > > > > diff --git a/mm/memblock.c b/mm/memblock.c
> > > > > index 5a88d6d24d79..3f44c84f5d0b 100644
> > > > > --- a/mm/memblock.c
> > > > > +++ b/mm/memblock.c
> > > > > @@ -400,6 +400,8 @@ void __init memblock_discard(void)
> > > > > * @type: memblock type of the regions array being doubled
> > > > > * @new_area_start: starting address of memory range to avoid overlap with
> > > > > * @new_area_size: size of memory range to avoid overlap with
> > > > > + * @new_reserve_base: starting address of new array
> > > > > + * @new_reserve_size: size of new array
> > > > > *
> > > > > * Double the size of the @type regions array. If memblock is being used to
> > > > > * allocate memory for a new reserved regions array and there is a previously
> > > > > @@ -412,7 +414,9 @@ void __init memblock_discard(void)
> > > > > */
> > > > > static int __init_memblock memblock_double_array(struct memblock_type *type,
> > > > > phys_addr_t new_area_start,
> > > > > - phys_addr_t new_area_size)
> > > > > + phys_addr_t new_area_size,
> > > > > + phys_addr_t *new_reserve_base,
> > > > > + phys_addr_t *new_reserve_size)
> > > > > {
> > > > > struct memblock_region *new_array, *old_array;
> > > > > phys_addr_t old_alloc_size, new_alloc_size;
> > > > > @@ -490,11 +494,13 @@ static int __init_memblock memblock_double_array(struct memblock_type *type,
> > > > > memblock_free(old_array, old_alloc_size);
> > > > > /*
> > > > > - * Reserve the new array if that comes from the memblock. Otherwise, we
> > > > > - * needn't do it
> > > > > + * Keep the address and size if that comes from the memblock. Otherwise,
> > > > > + * we needn't do it.
> > > > > */
> > > > > - if (!use_slab)
> > > > > - BUG_ON(memblock_reserve(addr, new_alloc_size));
> > > > > + if (!use_slab) {
> > > > > + *new_reserve_base = addr;
> > > > > + *new_reserve_size = new_alloc_size;
> > > > > + }
> > > > > /* Update slab flag */
> > > > > *in_slab = use_slab;
> > > > > @@ -588,11 +594,12 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
> > > > > phys_addr_t base, phys_addr_t size,
> > > > > int nid, enum memblock_flags flags)
> > > > > {
> > > > > - bool insert = false;
> > > > > phys_addr_t obase = base;
> > > > > phys_addr_t end = base + memblock_cap_size(base, &size);
> > > > > - int idx, nr_new, start_rgn = -1, end_rgn;
> > > > > + phys_addr_t new_base = 0, new_size;
> > > > > + int idx, start_rgn = -1, end_rgn;
> > > > > struct memblock_region *rgn;
> > > > > + unsigned long ocnt = type->cnt;
> > > > > if (!size)
> > > > > return 0;
> > > > > @@ -608,25 +615,6 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
> > > > > return 0;
> > > > > }
> > > > > - /*
> > > > > - * The worst case is when new range overlaps all existing regions,
> > > > > - * then we'll need type->cnt + 1 empty regions in @type. So if
> > > > > - * type->cnt * 2 + 1 is less than or equal to type->max, we know
> > > > > - * that there is enough empty regions in @type, and we can insert
> > > > > - * regions directly.
> > > > > - */
> > > > > - if (type->cnt * 2 + 1 <= type->max)
> > > > > - insert = true;
> > > > > -
> > > > > -repeat:
> > > > > - /*
> > > > > - * The following is executed twice. Once with %false @insert and
> > > > > - * then with %true. The first counts the number of regions needed
> > > > > - * to accommodate the new area. The second actually inserts them.
> > > > > - */
> > > > > - base = obase;
> > > > > - nr_new = 0;
> > > > > -
> > > > > for_each_memblock_type(idx, type, rgn) {
> > > > > phys_addr_t rbase = rgn->base;
> > > > > phys_addr_t rend = rbase + rgn->size;
> > > > > @@ -644,15 +632,23 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
> > > > > WARN_ON(nid != memblock_get_region_node(rgn));
> > > > > #endif
> > > > > WARN_ON(flags != rgn->flags);
> > > > > - nr_new++;
> > > > > - if (insert) {
> > > > > - if (start_rgn == -1)
> > > > > - start_rgn = idx;
> > > > > - end_rgn = idx + 1;
> > > > > - memblock_insert_region(type, idx++, base,
> > > > > - rbase - base, nid,
> > > > > - flags);
> > > > > - }
> > > > > +
> > > > > + /*
> > > > > + * If type->cnt is equal to type->max, it means there's
> > > > > + * not enough empty region and the array needs to be
> > > > > + * resized. Otherwise, insert it directly.
> > > > > + */
> > > > > + if ((type->cnt == type->max) &&
> > > > > + memblock_double_array(type, obase, size,
> > > > > + &new_base, &new_size))
> > > > > + return -ENOMEM;
> > > > > +
> > > > > + if (start_rgn == -1)
> > > > > + start_rgn = idx;
> > > > > + end_rgn = idx + 1;
> > > > > + memblock_insert_region(type, idx++, base,
> > > > > + rbase - base, nid,
> > > > > + flags);
> > > > > }
> > > > > /* area below @rend is dealt with, forget about it */
> > > > > base = min(rend, end);
> > > > > @@ -660,33 +656,28 @@ static int __init_memblock memblock_add_range(struct memblock_type *type,
> > > > > /* insert the remaining portion */
> > > > > if (base < end) {
> > > > > - nr_new++;
> > > > > - if (insert) {
> > > > > - if (start_rgn == -1)
> > > > > - start_rgn = idx;
> > > > > - end_rgn = idx + 1;
> > > > > - memblock_insert_region(type, idx, base, end - base,
> > > > > - nid, flags);
> > > > > - }
> > > > > + if ((type->cnt == type->max) &&
> > > > > + memblock_double_array(type, obase, size,
> > > > > + &new_base, &new_size))
> > > > > + return -ENOMEM;
> > > > > +
> > > > > + if (start_rgn == -1)
> > > > > + start_rgn = idx;
> > > > > + end_rgn = idx + 1;
> > > > > + memblock_insert_region(type, idx, base, end - base,
> > > > > + nid, flags);
> > > > > }
> > > > > - if (!nr_new)
> > > > > + if (ocnt == type->cnt)
> > > > > return 0;
> > > > > - /*
> > > > > - * If this was the first round, resize array and repeat for actual
> > > > > - * insertions; otherwise, merge and return.
> > > > > - */
> > > > > - if (!insert) {
> > > > > - while (type->cnt + nr_new > type->max)
> > > > > - if (memblock_double_array(type, obase, size) < 0)
> > > > > - return -ENOMEM;
> > > > > - insert = true;
> > > > > - goto repeat;
> > > > > - } else {
> > > > > - memblock_merge_regions(type, start_rgn, end_rgn);
> > > > > - return 0;
> > > > > - }
> > > > > + memblock_merge_regions(type, start_rgn, end_rgn);
> > > > > +
> > > > > + /* Reserve the new array */
> > > > > + if (new_base)
> > > > > + memblock_reserve(new_base, new_size);
> > > > > +
> > > > > + return 0;
> > > > > }
> > > > > /**
> > > > > @@ -755,6 +746,7 @@ static int __init_memblock memblock_isolate_range(struct memblock_type *type,
> > > > > int *start_rgn, int *end_rgn)
> > > > > {
> > > > > phys_addr_t end = base + memblock_cap_size(base, &size);
> > > > > + phys_addr_t new_base = 0, new_size;
> > > > > int idx;
> > > > > struct memblock_region *rgn;
> > > > > @@ -764,10 +756,15 @@ static int __init_memblock memblock_isolate_range(struct memblock_type *type,
> > > > > return 0;
> > > > > /* we'll create at most two more regions */
> > > > > - while (type->cnt + 2 > type->max)
> > > > > - if (memblock_double_array(type, base, size) < 0)
> > > > > + if (type->cnt + 2 > type->max) {
> > > > > + if (memblock_double_array(type, base, size,
> > > > > + &new_base, &new_size))
> > > > > return -ENOMEM;
> > > > > + if (new_base)
> > > > > + memblock_reserve(new_base, new_size);
> > > > > + }
> > > > > +
> > > > > for_each_memblock_type(idx, type, rgn) {
> > > > > phys_addr_t rbase = rgn->base;
> > > > > phys_addr_t rend = rbase + rgn->size;
> > > > > --
> > > > > 2.25.1
> > > > >
--
Sincerely yours,
Mike.
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