Re: [PATCH 0/7] minmax: reduce compilation time

[Jens Axboe]

On 7/27/24 10:31 AM, Lorenzo Stoakes wrote:
> On Sat, Jul 27, 2024 at 09:38:54AM GMT, Jens Axboe wrote:
>> On 7/27/24 9:30 AM, Jens Axboe wrote:
>>> On 7/26/24 4:48 PM, Linus Torvalds wrote:
>>>> I didn't even look at what the issue was with the
>>>> bio_for_each_segment() expansion, in the hope that Jens will make that
>>>> one look better.
>>>
>>> I did take a quick look, pretty obviously bvec_iter_bvec() which makes
>>> it horrible, which came from Kent's immutable work quite a while ago.
>>> Not sure yet what to do about it, will spend some time on this next
>>> week.
>>
>> Maybe something like this, totally untested...
>>
>> diff --git a/include/linux/bvec.h b/include/linux/bvec.h
>> index f41c7f0ef91e..9ccccddadde2 100644
>> --- a/include/linux/bvec.h
>> +++ b/include/linux/bvec.h
>> @@ -130,12 +130,15 @@ struct bvec_iter_all {
>>  	(mp_bvec_iter_page((bvec), (iter)) +			\
>>  	 mp_bvec_iter_page_idx((bvec), (iter)))
>>
>> -#define bvec_iter_bvec(bvec, iter)				\
>> -((struct bio_vec) {						\
>> -	.bv_page	= bvec_iter_page((bvec), (iter)),	\
>> -	.bv_len		= bvec_iter_len((bvec), (iter)),	\
>> -	.bv_offset	= bvec_iter_offset((bvec), (iter)),	\
>> -})
>> +static inline struct bio_vec bvec_iter_bvec(struct bio_vec *bv,
>> +					    struct bvec_iter iter)
>> +{
>> +	return (struct bio_vec) {
>> +		.bv_page	= bvec_iter_page(bv, iter),
>> +		.bv_len		= bvec_iter_len(bv, iter),
>> +		.bv_offset	= bvec_iter_offset(bv, iter)
>> +	};
>> +}
>>
>>  static inline bool bvec_iter_advance(const struct bio_vec *bv,
>>  		struct bvec_iter *iter, unsigned bytes)
>>
>> --
>> Jens Axboe
>>
> 
> I tried this patch, doesn't seem to make a huge difference, going from
> 3,958,564 bytes with longest line of 82 kB to 3,943,824 bytes with a
> longest line of 77kB.
> 
> It seems that the .bv_len = ... expansion is what's doing it, so I tried
> patching mp_bvec_iter_len() as well to do a silly ?: thing (sorry), which
> takes us down to 3,880,309 with longest line of 20kB.

Right, I did compile it after the fact and applied the same thing to
mp_bvec_iter_len().

> This is starting to feel like whack-a-mole isn't it? I looked at the next
> longest line, which originates from include/linux/pid_namespace.h believe
> it or not where some compiler cleverness + a loop is resulting in _another_
> combinatorial explosion.

Oh it's certainly whack-a-mole, doesn't mean it's not worth doing for
the low hanging stuff :-)

> Patch attached including Jens's change + mine.

bvec side matches what I have here, fwiw, except I also did
mp_bvec_iter_len(). Didn't see big expansion there, but might as well
keep them consistent.

-- 
Jens Axboe