Re: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration
From: Gioh Kim
Date: Mon Jul 21 2014 - 02:16:18 EST
2014-07-21 ìì 11:50, Minchan Kim ì ê:
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?
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.
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@xxxxxxx>
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@xxxxxxx>
Acked-by: Laura Abbott <lauraa@xxxxxxxxxxxxxx>
---
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.
And I think invalidate_bh_lrus() is general.
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.
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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