linux-kernel - Re: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration

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Date:	Fri, 18 Jul 2014 10:54:29 -0700
From:	Laura Abbott <lauraa@...eaurora.org>
To:	Gioh Kim <gioh.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	'김준수' <iamjoonsoo.kim@....com>,
	Minchan Kim <minchan@...nel.org>
CC:	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>
Subject: Re: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration

On 7/17/2014 11:45 PM, 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.
> 
> I have two solution to drop bhs.
> One is invalidating entire lru.
> 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.
> 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.
> 
> 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.
> 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>\

I'd prefer if you would remove my Acked-by line until I've actually
given it :)

> ---
>  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();
> +
>         ret = __alloc_contig_migrate_range(&cc, start, end);
>         if (ret)
>                 goto done;

I agree with the others that the if (...) check doesn't actually help
anything here and should probably be removed.

Thanks,
Laura

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