Re: [PATCH] mm: optimize PageHighMem() check

[Vineet Gupta]

On Saturday 03 October 2015 02:23 AM, Andrew Morton wrote:
> On Fri, 2 Oct 2015 12:45:53 +0530 Vineet Gupta <vgupta@xxxxxxxxxxxx> wrote:
>
>> On Friday 02 October 2015 04:55 AM, Andrew Morton wrote:
>>> On Tue, 29 Sep 2015 13:24:20 +0530 Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx> wrote:
>>>
>>>>> This came up when implementing HIHGMEM/PAE40 for ARC.
>>>>> The kmap() / kmap_atomic() generated code seemed needlessly bloated due
>>>>> to the way PageHighMem() macro is implemented.
>>>>> It derives the exact zone for page and then does pointer subtraction
>>>>> with first zone to infer the zone_type.
>>>>> The pointer arithmatic in turn generates the code bloat.
>>>>>
>>>>> PageHighMem(page)
>>>>>   is_highmem(page_zone(page))
>>>>>      zone_off = (char *)zone - (char *)zone->zone_pgdat->node_zones
>>>>>
>>>>> Instead use is_highmem_idx() to work on zone_type available in page flags
>>>>>
>>>>>    ----- Before -----
>>>>> 80756348:	mov_s      r13,r0
>>>>> 8075634a:	ld_s       r2,[r13,0]
>>>>> 8075634c:	lsr_s      r2,r2,30
>>>>> 8075634e:	mpy        r2,r2,0x2a4
>>>>> 80756352:	add_s      r2,r2,0x80aef880
>>>>> 80756358:	ld_s       r3,[r2,28]
>>>>> 8075635a:	sub_s      r2,r2,r3
>>>>> 8075635c:	breq       r2,0x2a4,80756378 <kmap+0x48>
>>>>> 80756364:	breq       r2,0x548,80756378 <kmap+0x48>
>>>>>
>>>>>    ----- After  -----
>>>>> 80756330:	mov_s      r13,r0
>>>>> 80756332:	ld_s       r2,[r13,0]
>>>>> 80756334:	lsr_s      r2,r2,30
>>>>> 80756336:	sub_s      r2,r2,1
>>>>> 80756338:	brlo       r2,2,80756348 <kmap+0x30>
>>>>>
>>>>> For x86 defconfig build (32 bit only) it saves around 900 bytes.
>>>>> For ARC defconfig with HIGHMEM, it saved around 2K bytes.
>>>>>
>>>>>    ---->8-------
>>>>> ./scripts/bloat-o-meter x86/vmlinux-defconfig-pre x86/vmlinux-defconfig-post
>>>>> add/remove: 0/0 grow/shrink: 0/36 up/down: 0/-934 (-934)
>>>>> function                                     old     new   delta
>>>>> saveable_page                                162     154      -8
>>>>> saveable_highmem_page                        154     146      -8
>>>>> skb_gro_reset_offset                         147     131     -16
>>>>> ...
>>>>> ...
>>>>> __change_page_attr_set_clr                  1715    1678     -37
>>>>> setup_data_read                              434     394     -40
>>>>> mon_bin_event                               1967    1927     -40
>>>>> swsusp_save                                 1148    1105     -43
>>>>> _set_pages_array                             549     493     -56
>>>>>    ---->8-------
>>>>>
>>>>> e.g. For ARC kmap()
>>>>>
>>> is_highmem() is deranged.  Can't we use a bit in zone->flags or
>>> something?
>> It won't be "a" bit since zone_type is an enum.
> Yes it will!
>
> static inline int is_highmem(struct zone *zone)
> {
> 	return test_bit(ZONE_HIGHMEM, &zone->flags);
> }

Point is do we want to fix this specific case, or do we want to improve zone_idx()
in general.

#define zone_idx(zone)        ((zone) - (zone)->zone_pgdat->node_zones

If former, I can split up zone->flags into 31:1 bit-field. Otherwise I will split
it into 24:8 to hold both zone_flags and zone_type

>> ...
>>
>> However this patch still is independent of that since we have struct page as
>> starting point and zone_type is available from there directly w/o monkeying around
>> with any zone structs.
> yup.

Do u want me to carry this as part of the the ARC HIGHMEM/PAE40 patchset or would
you rather prefer merging it via mm tree.

Thx,
-Vineet
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