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Date:   Fri, 29 Sep 2023 12:04:06 +0300
From:   Mike Rapoport <rppt@kernel.org>
To:     Yajun Deng <yajun.deng@linux.dev>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] memblock: don't run loop in memblock_add_range() twice
Message-ID: <20230929090406.GV3303@kernel.org>
References: <20230927013752.2515238-1-yajun.deng@linux.dev>
 <20230928061619.GS3303@kernel.org>
 <3ee9c8e4-870c-4ab0-906a-7d214031d1a6@linux.dev>
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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 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@linux.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.