Re: Why __alloc_contig_migrate_range calls migrate_prep() at first?

[Wang Sheng-Hui <shhuiw@foxmail.com>]

On 6/1/2016 3:40 PM, Minchan Kim wrote:
> On Wed, Jun 01, 2016 at 11:42:29AM +0800, Wang Sheng-Hui wrote:
>> Dear,
>>
>> Sorry to trouble you.
>>
>> I noticed cma_alloc would turn to  __alloc_contig_migrate_range for allocating pages.
>> But  __alloc_contig_migrate_range calls  migrate_prep() at first, even if the requested page
>> is single and free, lru_add_drain_all still run (called by  migrate_prep())?
>>
>> Image a large chunk of free contig pages for CMA, various drivers may request a single page from
>> the CMA area, we'll get  lru_add_drain_all run for each page.
>>
>> Should we detect if the required pages are free before migrate_prep(), or detect at least for single 
>> page allocation?
> That makes sense to me.
>
> How about calling migrate_prep once migrate_pages fails in the first trial?

Minchan,

I tried your patch in my env, and the number of calling migrate_prep() dropped a lot.

In my case, CMA reserved 512MB, and the linux will call migrate_prep() 40~ times during bootup,
most are single page allocation request to CMA.
With your patch, migrate_prep() is not called for the single pages allocation requests as the free
pages in CMA area is enough.

Will you please push the patch to upstream?

Thanks,
Sheng-Hui

>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9d666df5ef95..c504c1a623d2 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6623,8 +6623,6 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>  	unsigned int tries = 0;
>  	int ret = 0;
>  
> -	migrate_prep();
> -
>  	while (pfn < end || !list_empty(&cc->migratepages)) {
>  		if (fatal_signal_pending(current)) {
>  			ret = -EINTR;
> @@ -6650,6 +6648,8 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>  
>  		ret = migrate_pages(&cc->migratepages, alloc_migrate_target,
>  				    NULL, 0, cc->mode, MR_CMA);
> +		if (ret)
> +			migrate_prep();
>  	}
>  	if (ret < 0) {
>  		putback_movable_pages(&cc->migratepages);
>
>
>> ------------------
>> Regards,
>> Wang Sheng-HuiN??2aeir,?zC?u(C)?2AE {\b-?ei1>>\x1c(R)&TH?)iAEic?O^n?r?????Ycj$ 1/2 ?$c,\x05c1?-e?~?'.)iAA,yem??yA\f%?{+-?j+?de?x|j)Z?.?thfc?U\x1dc{d 1/2 ?$c,\x1e??JPY?o???a



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