Re: [PATCH] mm: cma: free cma page to buddy instead of being cpu hotpage

[Minchan Kim]

Hello,

On Tue, Oct 29, 2013 at 11:02:30PM +0800, Zhang Mingjun wrote:
> On Tue, Oct 29, 2013 at 8:27 PM, Mel Gorman <mgorman@xxxxxxx> wrote:
> 
> > On Tue, Oct 29, 2013 at 07:49:30PM +0800, Zhang Mingjun wrote:
> > > On Tue, Oct 29, 2013 at 5:33 PM, Mel Gorman <mgorman@xxxxxxx> wrote:
> > >
> > > > On Mon, Oct 28, 2013 at 07:42:49PM +0800, zhang.mingjun@xxxxxxxxxxxxxxx:
> > > > > From: Mingjun Zhang <troy.zhangmingjun@xxxxxxxxxx>
> > > > >
> > > > > free_contig_range frees cma pages one by one and MIGRATE_CMA pages
> > will
> > > > be
> > > > > used as MIGRATE_MOVEABLE pages in the pcp list, it causes unnecessary
> > > > > migration action when these pages reused by CMA.
> > > > >
> > > > > Signed-off-by: Mingjun Zhang <troy.zhangmingjun@xxxxxxxxxx>
> > > > > ---
> > > > >  mm/page_alloc.c |    3 ++-
> > > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > > index 0ee638f..84b9d84 100644
> > > > > --- a/mm/page_alloc.c
> > > > > +++ b/mm/page_alloc.c
> > > > > @@ -1362,7 +1362,8 @@ void free_hot_cold_page(struct page *page, int
> > > > cold)
> > > > >        * excessively into the page allocator
> > > > >        */
> > > > >       if (migratetype >= MIGRATE_PCPTYPES) {
> > > > > -             if (unlikely(is_migrate_isolate(migratetype))) {
> > > > > +             if (unlikely(is_migrate_isolate(migratetype))
> > > > > +                     || is_migrate_cma(migratetype))
> > > > >                       free_one_page(zone, page, 0, migratetype);
> > > > >                       goto out;
> > > >
> > > > This slightly impacts the page allocator free path for a marginal gain
> > > > on CMA which are relatively rare allocations. There is no obvious
> > > > benefit to this patch as I expect CMA allocations to flush the PCP
> > lists
> > > >
> > > how about keeping the migrate type of CMA page block as MIGRATE_ISOLATED
> > > after
> > > the alloc_contig_range , and undo_isolate_page_range at the end of
> > > free_contig_range?
> >
> > It would move the cost to the CMA paths so I would complain less. Bear
> > in mind as well that forcing everything to go through free_one_page()
> > means that every free goes through the zone lock. I doubt you have any
> > machine large enough but it is possible for simultaneous CMA allocations
> > to now contend on the zone lock that would have been previously fine.
> > Hence, I'm interesting in knowing the underlying cause of the problem you
> > are experiencing.
> >
> > my platform uses CMA but disabled CMA's migration func by del MIGRATE_CMA
> in fallbacks[MIGRATE_MOVEABLE]. But I find CMA pages can still used by

In that case, why do you want to use CMA?
It's almost same with resreved memory.

> pagecache or page fault page request from PCP list and cma allocation has to
> migrate these page. So I want to free these cma pages to buddy directly not
> PCP..

I know your goal and understand current problem it could make more number
of migration but how often it happens so that if we apply your patch,
how much is the gain? For example, you can get a number followin as.

1. old

getting XXXM contiguos memory area: 100ms
the number of migration : 200

2. new

getting XXXM contiguos memory area: 10ms
the number of migration : 0

It seems Mel want it and I'd like to see it to convince.
Of course, Andrew might want it, too.

> 
> > of course, it will waste the memory outside of the alloc range but in the
> > > pageblocks.
> > >

We need to know fundamental problem and number before you go with any method
so that we could judge if you approach is good.
Please add more detail explanation about current status in description.
It would be better to include more statistic data.

> >
> > I would hope/expect that the loss would only last for the duration of
> > the allocation attempt and a small amount of memory.
> >
> > > > when a range of pages have been isolated and migrated. Is there any
> > > > measurable benefit to this patch?
> > > >
> > > after applying this patch, the video player on my platform works more
> > > fluent,
> >
> > fluent almost always refers to ones command of a spoken language. I do
> > not see how a video player can be fluent in anything. What is measurably
> > better?
> >
> > For example, are allocations faster? If so, why? What cost from another
> > path is removed as a result of this patch? If the cost is in the PCP
> > flush then can it be checked if the PCP flush was unnecessary and called
> > unconditionally even though all the pages were freed already? We had
> > problems in the past where drain_all_pages() or similar were called
> > unnecessarily causing long sync stalls related to IPIs. I'm wondering if
> > we are seeing a similar problem here.
> >
> > Maybe the problem is the complete opposite. Are allocations failing
> > because there are PCP pages in the way? In that case, it real fix might
> > be to insert a  if the allocation is failing due to per-cpu
> > pages.
> >
> problem is not the allocation failing, but the unexpected cma migration
> slows
> down the allocation.

Okay, So how many? Need number. # of migration and time due to it.

> 
> >
> > > and the driver of video decoder on my test platform using cma alloc/free
> > > frequently.
> > >
> >
> > CMA allocations are almost never used outside of these contexts. While I
> > appreciate that embedded use is important I'm reluctant to see an impact
> > in fast paths unless there is a good reason for every other use case. I
> > also am a bit unhappy to see CMA allocations making the zone->lock
> > hotter than necessary even if no embedded use case it likely to
> > experience the problem in the short-term.
> >
> > --
> > Mel Gorman
> > SUSE Labs
> >

-- 
Kind regards,
Minchan Kim
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