linux-kernel - Re: [PATCH] kfree_rcu() should use the new kfree_bulk() interface for freeing rcu structures

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Date:   Tue, 19 Dec 2017 11:42:35 -0800
From:   Rao Shoaib <rao.shoaib@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
        brouer@...hat.com, linux-mm@...ck.org
Subject: Re: [PATCH] kfree_rcu() should use the new kfree_bulk() interface for
 freeing rcu structures

On 12/19/2017 11:12 AM, Matthew Wilcox wrote:
> On Tue, Dec 19, 2017 at 09:52:27AM -0800, rao.shoaib@...cle.com wrote:
>> This patch updates kfree_rcu to use new bulk memory free functions as they
>> are more efficient. It also moves kfree_call_rcu() out of rcu related code to
>> mm/slab_common.c
>>
>> Signed-off-by: Rao Shoaib <rao.shoaib@...cle.com>
>> ---
>>   include/linux/mm.h |   5 ++
>>   kernel/rcu/tree.c  |  14 ----
>>   kernel/sysctl.c    |  40 +++++++++++
>>   mm/slab.h          |  23 +++++++
>>   mm/slab_common.c   | 198 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
>>   5 files changed, 264 insertions(+), 16 deletions(-)
> You've added an awful lot of code.  Do you have any performance measurements
> that shows this to be a win?
I did some micro benchmarking when I was developing the code and did see 
performance gains -- see attached.

I tried several networking benchmarks but was not able to get any 
improvement . The reason is that these benchmarks do not exercise the 
code we are improving. So I looked at the kernel source for users of  
kfree_rcu().  It turns out that directory deletion code calls  kfree_rcu 
to free the data structure when an entry is deleted. Based on that I 
created two benchmarks.

1) make_dirs -- This benchmark creates multi level directory structure 
and than deletes it. It's the delete part  where we see the performance 
gain of about 8.3%. The creation time remains same.

This benchmark was derived from fdtree benchmark at 
https://computing.llnl.gov/?set=code&page=sio_downloads ==> 
https://github.com/llnl/fdtree

2) tsock -- I also noticed that a socket has an entry in a directory and 
when the socket is closed the directory entry is deleted. So I wrote a 
simple benchmark that goes in a loop a million times and opens and 
closes 10 sockets per iteration. This shows an improvement of 7.6%

I have attached the benchmarks and results. Unchanged results are for 
stock kernel, Changed are for the modified kernel.

Shoaib

Download attachment "make_dirs.tar" of type "application/x-tar" (10240 bytes)

Download attachment "tsock.tar" of type "application/x-tar" (10240 bytes)