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

[黄朝阳 (Zhaoyang Huang)]

> 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.
> 
> And please use scripts/checkpatch.pl next time, there are many code style
> issues.
ok

> 
> Thanks!
> 
> >
> > DMA32 free:25900kB boost:0kB min:4176kB low:25856kB high:29516kB
> >
> > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > ---
> >  mm/page_alloc.c | 40 ++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 36 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 0745aed..97768fe
> > 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3071,6 +3071,39 @@ static bool
> > unreserve_highatomic_pageblock(const struct alloc_context *ac,
> >
> >  }
> >
> > +#ifdef CONFIG_CMA
> > +static bool __if_use_cma_first(struct zone *zone, unsigned int order,
> > +unsigned int alloc_flags) {
> > +     unsigned long cma_proportion = 0;
> > +     unsigned long cma_free_proportion = 0;
> > +     unsigned long watermark = 0;
> > +     unsigned long wm_fact[ALLOC_WMARK_MASK] = {1, 1, 2};
> > +     long count = 0;
> > +     bool cma_first = false;
> > +
> > +     watermark = wmark_pages(zone, alloc_flags &
> ALLOC_WMARK_MASK);
> > +     /*check if GFP_MOVABLE pass previous watermark check via the help
> of CMA*/
> > +     if (!zone_watermark_ok(zone, order, watermark, 0, alloc_flags &
> (~ALLOC_CMA)))
> > +     {
> > +             alloc_flags &= ALLOC_WMARK_MASK;
> > +             /* WMARK_LOW failed lead to using cma first, this helps
> U&R stay
> > +              * around low when being drained by GFP_MOVABLE
> > +              */
> > +             if (alloc_flags <= ALLOC_WMARK_LOW)
> > +                     cma_first = true;
> > +             /*check proportion for WMARK_HIGH*/
> > +             else {
> > +                     count =
> atomic_long_read(&zone->managed_pages);
> > +                     cma_proportion = zone->cma_pages * 100 / count;
> > +                     cma_free_proportion = zone_page_state(zone,
> NR_FREE_CMA_PAGES) * 100
> > +                             /  zone_page_state(zone,
> NR_FREE_PAGES);
> > +                     cma_first = (cma_free_proportion >=
> wm_fact[alloc_flags] * cma_proportion
> > +                                     || cma_free_proportion >=
> 50);
> > +             }
> > +     }
> > +     return cma_first;
> > +}
> > +#endif
> >  /*
> >   * Do the hard work of removing an element from the buddy allocator.
> >   * Call me with the zone->lock already held.
> > @@ -3087,10 +3120,9 @@ static bool
> unreserve_highatomic_pageblock(const struct alloc_context *ac,
> >                * allocating from CMA when over half of the zone's 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) {
> > -                     page = __rmqueue_cma_fallback(zone, order);
> > +             if (migratetype == MIGRATE_MOVABLE) {
> > +                     bool cma_first = __if_use_cma_first(zone, order,
> alloc_flags);
> > +                     page = cma_first ? __rmqueue_cma_fallback(zone,
> > + order) : NULL;
> >                       if (page)
> >                               return page;
> >               }
> > --
> > 1.9.1
> >
> >