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Date: Mon, 2 Oct 2023 17:47:19 +0800
From: Yajun Deng <yajun.deng@...ux.dev>
To: Mike Rapoport <rppt@...nel.org>
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 2023/10/2 03:08, Mike Rapoport wrote:
> 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.
Okay, I will send v2, remove the changes of memblock_double_array().
>> 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
>>>>>>
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