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From:   Zhaoyang Huang <huangzhaoyang@gmail.com>
Date:   Thu, 4 May 2023 14:23:01 +0800
Message-ID: <CAGWkznG5V+=onj9TsEhwh=Pxqt9+-KJAHLAoLJRMQ5-ysnwDgQ@mail.gmail.com>
Subject: =?UTF-8?Q?Re=3A_=E7=AD=94=E5=A4=8D=3A_=5BPATCH=5D_mm=3A_optimization_on_page_allocat?=
        =?UTF-8?Q?ion_when_CMA_enabled?=
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     =?UTF-8?B?6buE5pyd6ZizIChaaGFveWFuZyBIdWFuZyk=?= 
        <zhaoyang.huang@unisoc.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        =?UTF-8?B?546L56eRIChLZSBXYW5nKQ==?= <Ke.Wang@unisoc.com>,
        "fangzheng.zhang@unisoc.com" <fangzheng.zhang@unisoc.com>
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On Thu, May 4, 2023 at 12:30=E2=80=AFAM Roman Gushchin <roman.gushchin@linu=
x.dev> wrote:
>
> On Wed, May 03, 2023 at 03:58:21PM +0800, Zhaoyang Huang wrote:
> > On Wed, May 3, 2023 at 6:01=E2=80=AFAM Roman Gushchin <roman.gushchin@l=
inux.dev> wrote:
> > >
> > > On Tue, May 02, 2023 at 12:12:28PM +0000, =E9=BB=84=E6=9C=9D=E9=98=B3=
 (Zhaoyang Huang) wrote:
> > > > > Hi Zhaoyang!
> > > > >
> > > > > On Fri, Apr 28, 2023 at 07:00:41PM +0800, zhaoyang.huang wrote:
> > > > > > From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> > > > > >
> > > > > > Please be notice bellowing typical scenario that commit 1686766=
49
> > > > > > introduce, that is, 12MB free cma pages 'help' GFP_MOVABLE to k=
eep
> > > > > > draining/fragmenting U&R page blocks until they shrink to 12MB =
without
> > > > > > enter slowpath which against current reclaiming policy. This co=
mmit 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 detai=
ls?
> > > > I am trying to solve a OOM problem caused by slab allocation fail a=
s 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 phe=
nomenon 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. Ima=
gine 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 st=
ealing U&R pages(don't meet 1/2 criteria) without enter slowpath(zone_water=
mark_ok(WMARK_LOW) is true) until they shrink to 15MB. In my opinion, it ma=
kes 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 small=
er than CMA.
> > > > > Also, this is a hot path, please, make sure you're not adding muc=
h 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 ord=
er, int migratetype,
> > >         if (IS_ENABLED(CONFIG_CMA)) {
> > >                 /*
> > >                  * Balance movable allocations between regular and CM=
A areas by
> > > -                * allocating from CMA when over half of the zone's f=
ree memory
> > > -                * is in the CMA area.
> > > +                * allocating from CMA when over half of the zone's e=
asily
> > > +                * 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 =3D __rmqueue_cma_fallback(zone, order);
> > >                         if (page)
> > >                                 return page;
> > >
> > > Basically the idea is to keep free space equally split between cma an=
d 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.
>
> Can you then, please, explain in details what you're proposing instead
> and why it's better (not only in your case, but generally as well)?
> For the context, my original usecase was cma size under 10G and
> the total memory size between 32G and 128G.

Let us look at the series of scenarios below with
WMARK_LOW=3D25MB,WMARK_MIN=3D5MB(managed pages 1.9GB). We can know that
1. optimized 1/2 method start to use CMA since D which actually has
caused U&R lower than WMARK_LOW (this could be deemed as against
current memory policy, that is, U&R should either keeping stay  around
WATERMARK_LOW or enter slowpath to do reclaiming)
2. it keep using CMA as long as free pages keep shrinking (this is
against the original desire of balance cma & none-cma area)

free_cma/free_pages(MB)        A(12/30)     B(10/25)     C(8/25)
D(8/20)     E(8/16)     F(5/12)     F(5/10)   G(4/8)
optimized 1/2                                N                 N
         N             Y               Y                Y
Y            Y
!zone_watermark_ok                    Y                  Y
   Y             N               N                N            Y
     Y

>
> Looking at your original patch, I see that __if_use_cma_first() always re=
turns
> false if zone_watermark_ok() returns true. It worries me.
ok. we could deal with the scenario here when free pages is higher
than WMARK_HIGH
>
> Thanks!