Re: 答复: [PATCH] mm: optimization on page allocation when CMA enabled

[Zhaoyang Huang]

On Wed, May 3, 2023 at 6:01 AM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote:
>
> On Tue, May 02, 2023 at 12:12:28PM +0000, 黄朝阳 (Zhaoyang Huang) wrote:
> > > Hi Zhaoyang!
> > >
> > > On Fri, Apr 28, 2023 at 07:00:41PM +0800, zhaoyang.huang wrote:
> > > > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > > >
> > > > Please be notice bellowing typical scenario that commit 168676649
> > > > introduce, that is, 12MB free cma pages 'help' GFP_MOVABLE to keep
> > > > draining/fragmenting U&R page blocks until they shrink to 12MB without
> > > > enter slowpath which against current reclaiming policy. This commit change
> > > the criteria from hard coded '1/2'
> > > > to watermark check which leave U&R free pages stay around WMARK_LOW
> > > > when being fallback.
> > >
> > > Can you, please, explain the problem you're solving in more details?
> > I am trying to solve a OOM problem caused by slab allocation fail as all free pages are MIGRATE_CMA by applying 168676649, which could help to reduce the fault ration from 12/20 to 2/20. I noticed it introduce the phenomenon which I describe above.
> > >
> > > If I understand your code correctly, you're effectively reducing the use of cma
> > > areas for movable allocations. Why it's good?
> > Not exactly. In fact, this commit lead to the use of cma early than it is now, which could help to protect U&R be 'stolen' by GFP_MOVABLE. Imagine this scenario, 30MB total free pages composed of 10MB CMA and 20MB U&R, while zone's watermark low is 25MB. An GFP_MOVABLE allocation can keep stealing U&R pages(don't meet 1/2 criteria) without enter slowpath(zone_watermark_ok(WMARK_LOW) is true) until they shrink to 15MB. In my opinion, it makes more sense to have CMA take its duty to help movable allocation when U&R lower to certain zone's watermark instead of when their size become smaller than CMA.
> > > Also, this is a hot path, please, make sure you're not adding much overhead.
> > I would like to take more thought.
>
> Got it, thank you for the explanation!
>
> How about the following approach (completely untested)?
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6da423ec356f..4b50f497c09d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2279,12 +2279,13 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
>         if (IS_ENABLED(CONFIG_CMA)) {
>                 /*
>                  * Balance movable allocations between regular and CMA areas by
> -                * allocating from CMA when over half of the zone's free memory
> -                * is in the CMA area.
> +                * allocating from CMA when over half of the zone's easily
> +                * available free memory is in the CMA area.
>                  */
>                 if (alloc_flags & ALLOC_CMA &&
>                     zone_page_state(zone, NR_FREE_CMA_PAGES) >
> -                   zone_page_state(zone, NR_FREE_PAGES) / 2) {
> +                   (zone_page_state(zone, NR_FREE_PAGES) -
> +                    zone->_watermark[WMARK_LOW]) / 2) {
IMO, we should focus on non-cma area which trigger use of cma when
they are lower than corresponding watermark(there is still
WMARK_MIN/HIGH to deal with within slowpath)
>                         page = __rmqueue_cma_fallback(zone, order);
>                         if (page)
>                                 return page;
>
> Basically the idea is to keep free space equally split between cma and non-cma areas.
> Will it work for you?
I don't think it makes sense to 'equally split' cma and non-cma areas
over free space while cma could occupy various proportions in a single
zone. This fixed 1/2 could lead to different situation on 20% or 50%
cma occupation.
>
> Thanks!