Date:	Mon, 21 Jul 2014 17:00:53 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Gioh Kim <gioh.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	'김준수' <iamjoonsoo.kim@....com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Michal Nazarewicz <mina86@...a86.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mel@....ul.ie>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, 이건호 <gunho.lee@....com>,
	'Chanho Min' <chanho.min@....com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration

On Mon, Jul 21, 2014 at 04:36:51PM +0900, Minchan Kim wrote:
> On Mon, Jul 21, 2014 at 03:16:10PM +0900, Gioh Kim wrote:
> > 
> > 
> > 2014-07-21 오전 11:50, Minchan Kim 쓴 글:
> > >Hi Gioh,
> > >
> > >On Fri, Jul 18, 2014 at 03:45:36PM +0900, Gioh Kim wrote:
> > >>
> > >>Hi,
> > >>
> > >>For page migration of CMA, buffer-heads of lru should be dropped.
> > >>Please refer to https://lkml.org/lkml/2014/7/4/101 for the history.
> > >
> > >Just nit:
> > >Please write *problem* in description instead of URL link.
> > >
> > >>
> > >>I have two solution to drop bhs.
> > >>One is invalidating entire lru.
> > >
> > >You mean? All of percpu bh_lrus so if the system has N cpu,
> > >it invalidates N * 8?
> > 
> > Yes, every bh_lru of all cpus.
> > 
> > >
> > >>Another is searching the lru and dropping only one bh that Laura proposed
> > >>at https://lkml.org/lkml/2012/8/31/313.
> > >>
> > >>I'm not sure which has better performance.
> > >
> > >For whom? system or requestor of CMA?
> > 
> > For system performance.
> > 
> > >
> > >>So I did performance test on my cortex-a7 platform with Lmbench
> > >>that has "File & VM system latencies" test.
> > >>I am attaching the results.
> > >>The first line is of invalidating entire lru and the second is dropping selected bh.
> > >
> > >You mean you did Lmbench with background CMA allocation?
> > >Could you describe in detail?
> > 
> > I'm sorry not to mention the background.
> > I did the test without CMA allocation because I wanted to check how it affects system performance.
> > 
> > The first test, invalidating entire lru, is adding invalidate_bh_lrus() at alloc_contig_range().
> > This is not affecting system performance because alloc_contig_range() is not called
> > for usual file-system management.
> > The resulf of the first test is the *default system performance.*
> > 
> > The second test, dropping all bh in lru, is adding drop_buffers().
> > Every call of drop_buffers drops all bhs in lru of every cpu.
> > It can affect system performance. *But* it does not affect system performance,
> > because it drops only bh of migrated pages.
> > 
> > 
> > >
> > >>
> > >>File & VM system latencies in microseconds - smaller is better
> > >>-------------------------------------------------------------------------------
> > >>Host                 OS   0K File      10K File     Mmap    Prot   Page   100fd
> > >>                         Create Delete Create Delete Latency Fault  Fault  selct
> > >>--------- ------------- ------ ------ ------ ------ ------- ----- ------- -----
> > >>10.178.33 Linux 3.10.19   25.1   19.6   32.6   19.7  5098.0 0.666 3.45880 6.506
> > >>10.178.33 Linux 3.10.19   24.9   19.5   32.3   19.4  5059.0 0.563 3.46380 6.521
> > >>
> > >>
> > >>I tried several times but the result tells that they are the same under 1% gap
> > >>except Protection Fault.
> > >>But the latency of Protection Fault is very small and I think it has little effect.
> > >>
> > >>Therefore we can choose anything but I choose invalidating entire lru.
> > >
> > >Not sure we can conclude like that.
> > >
> > >A few weeks ago, I saw a patch which increases bh_lrus's size.
> > >https://lkml.org/lkml/2014/7/4/107
> > >IOW, some of workloads really affects by percpu bh_lrus so it would be
> > >better to be careful to drain, I think.
> > >
> > >You want to argue CMA allocation is rare so the cost is marginable.
> > >It might but some of usecase might call it frequently with small request
> > >(ie, 8K, 16K).
> > >
> > >Anyway, why cannot CMA have the cost without affecting other subsystem?
> > >I mean it's okay for CMA to consume more time to shoot out the bh
> > >instead of simple all bh_lru invalidation because big order allocation is
> > >kinds of slow thing in the VM and everybody already know that and even
> > >sometime get failed so it's okay to add more code that extremly slow path.
> > 
> > There are 2 reasons to invalidate entire bh_lru.
> > 
> > 1. I think CMA allocation is very rare so that invalidaing bh_lru affects the system little.
> > How do you think about it? My platform does not call CMA allocation often.
> > Is the CMA allocation or Memory-Hotplug called often?
> 
> It depends on usecase and you couldn't assume anyting because we couldn't
> ask every people in the world. "Please ask to us whenever you try to use CMA".
> 
> The key point is how the patch is maintainable.
> If it's too complicate to maintain, maybe we could go with simple solution
> but if it's not too complicate, we can go with more smart thing to consider
> other cases in future. Why not?
> 
> Another point is that how user can detect where the regression is from.
> If we cannot notice the regression, it's not a good idea to go with simple
> version.
> 
> > 
> > 2. Adding code in drop_buffers() can affect the system more that adding code in alloc_contig_range()
> > because the drop_buffers does not have a way to distinguish migrate type.
> > Even-though the lmbech results that it has almost the same performance.
> > But I am afraid that it can be changed.
> > As you said if bh_lru size can be changed it affects more than now.
> > SO I do not want to touch non-CMA related code.
> 
> I'm not saying to add hook in drop_buffers.
> What I suggest is to handle failure by bh_lrus in migrate_pages
> because it's not a problem only in CMA.
> There is already retry logic in migrate_pages so I can think you could
> handle it.
> 
> > 
> > 
> > >
> > >>The try_to_free_buffers() which is calling drop_buffers() is called by many filesystem code.
> > >>So I think inserting codes in drop_buffers() can affect the system.
> > >>And also we cannot distinguish migration type in drop_buffers().
> > >>
> > >>In alloc_contig_range() we can distinguish migration type and invalidate lru if it needs.
> > >>I think alloc_contig_range() is proper to deal with bh like following patch.
> > >>
> > >>Laura, can I have you name on Acked-by line?
> > >>Please let me represent my thanks.
> > >>
> > >>Thanks for any feedback.
> > >>
> > >>------------------------------- 8< ----------------------------------
> > >>
> > >>>From 33c894b1bab9bc26486716f0c62c452d3a04d35d Mon Sep 17 00:00:00 2001
> > >>From: Gioh Kim <gioh.kim@....com>
> > >>Date: Fri, 18 Jul 2014 13:40:01 +0900
> > >>Subject: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration
> > >>
> > >>The bh must be free to migrate a page at which bh is mapped.
> > >>The reference count of bh is increased when it is installed
> > >>into lru so that the bh of lru must be freed before migrating the page.
> > >>
> > >>This frees every bh of lru. We could free only bh of migrating page.
> > >>But searching lru costs more than invalidating entire lru.
> > >>
> > >>Signed-off-by: Gioh Kim <gioh.kim@....com>
> > >>Acked-by: Laura Abbott <lauraa@...eaurora.org>
> > >>---
> > >>  mm/page_alloc.c |    3 +++
> > >>  1 file changed, 3 insertions(+)
> > >>
> > >>diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > >>index b99643d4..3b474e0 100644
> > >>--- a/mm/page_alloc.c
> > >>+++ b/mm/page_alloc.c
> > >>@@ -6369,6 +6369,9 @@ int alloc_contig_range(unsigned long start, unsigned long end,
> > >>         if (ret)
> > >>                 return ret;
> > >>
> > >>+       if (migratetype == MIGRATE_CMA || migratetype == MIGRATE_MOVABLE)
> > >>+               invalidate_bh_lrus();
> > >>+
> > >
> > >Q1. It's a only CMA problem? Memory-Hotplug is not a problem? Or other places?
> > >
> > >I mean it would be better to handle in generic way.
> > 
> > Only CMA and Memory-Hotplug needs it.
> 
> Memory-hotplug uses alloc_contig_range?
> You are adding the logic in alloc_contig_range and it is used for
> hugetlb and cma.
> 
> > And I think invalidate_bh_lrus() is general.
> 
> It couldn't handle memory-hotplug.
> 
> > 
> > >
> > >Q2. Why do you call it right before calling __alloc_contig_migrate_range?
> > >
> > >Some of pages will go bh_lrus by __alloc_contig_migrate_ranges.
> > >In that case, it is useless without caller's retry logic.
> > >Even you do it from caller's retrial logic, it's not a good idea because
> > >you makes new binding alloc_contig_range and uppder layer.
> > >
> > >So, IMHO, it would be better to handle it in migrate_pages.
> > >Maybe we could define new API try_to_drop_buffers which calls
> > >try_to_free_buffers and then only if the function fails due to
> > >percpu lru count, we could drain only the bh in percpu lru list instead of
> > >all bh draining. And places in migration path should use it rather than
> > >try_to_relese_page.
> > >
> > >But the problem from this approach invents new API which should be
> > >maintained so not sure Andrew think it's worth.
> > >Maybe we should see the code and diffstat.
> > 
> > I also consider to making new function, drop_bh_of_migrate_page in migrate_page(), just before unmap_and_move().
> > The migrate_page() has an argument reason that distinguish migrate-type, MR_CMA or MR_MEMORY_HOTPLUG or others.
> 
> Yes, that's what I suggested. If you see -EAGIN, maybe you could do it.
> Even, we could enhance it with extending target bh invalidation instead of
> all bhs invalidation so you could make two patches.
> 
> 1. use invalidate_bh_lrus in migrate pages
> 2. invalidate only failed bh intead of all CPU percpu bh_blrus flushing.

Otherwise,
  2-1. create try_to_drop_buffers and use it in migration path intead of
       try_to_release_buffers.

> 
> So, if guys hate 2 which is rather overdesigned, we could drop 2 but 1 is
> mergable still.
> 
> > 
> > But I DO NOT WATN TO touch non-CMA related code.
> > Current CMA and Memory-Hotplug code is not mature so that I am not sure it is ok to touch non-CMA related code for CMA/MemoryHotplug.
> > 
> > My point is:
> > 1. CMA/Memory-hotplug is rare and invalidating bh-lru is also rare.
> > 2. Only change CMA/Memory-hotplig related code.
> > 
> > >
> > >Overenginnering?
> > >
> > >>         ret = __alloc_contig_migrate_range(&cc, start, end);
> > >>         if (ret)
> > >>                 goto done;
> > >>--
> > >>1.7.9.5
> > >>
> > >>--
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> > >>the body to majordomo@...ck.org.  For more info on Linux MM,
> > >>see: http://www.linux-mm.org/ .
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> > >
> > 
> > --
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> 
> -- 
> Kind regards,
> Minchan Kim
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@...ck.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
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-- 
Kind regards,
Minchan Kim
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