linux-kernel - Re: [PATCH] kprobe: increase kprobe_hash

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Date:	Fri, 07 Nov 2008 19:18:54 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	David Miller <davem@...emloft.net>,
	LKML <linux-kernel@...r.kernel.org>, maneesh@...ibm.com,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] kprobe: increase kprobe_hash_table size

Hi Andrew,

Andrew Morton wrote:
> On Fri, 07 Nov 2008 18:44:30 -0500 Masami Hiramatsu <mhiramat@...hat.com> wrote:
> 
>> Increase the size of kprobe hash table to 512. It's useful when hundreds
>> of kprobes were used in the kernel because current size is just 64.
>>
> 
> "useful" is a bit vague.  How big is the problem which this solves, and
> how well did it solve it?

For example, when probing enters and exits of syscall-related functions,
we need more than 500 probes. In that case, each hlist would have 8
elements in average. With this patch, the hlist would have 1 element in
average.

I agree that there may be many opinions about what is the best suited size.
Why I chose 512 was that I thought the table (byte) size was less than or
equal 4096 even on 64-bit arch.

> See, someone (me) needs to decide whether to merge this and if so,
> whether to merge it into 2.6.29, 2.6.28, 2.6.27.x, 2.6.26.x and
> 2.6.25.x.  I'll need more information to make that decision, but I do
> not have it.

I think this improves performance just a bit.
So I think it would not be needed for 2.6.27.x or older kernel.

Thank you,

> 
>> --- 2.6.28-rc3.orig/kernel/kprobes.c
>> +++ 2.6.28-rc3/kernel/kprobes.c
>> @@ -49,7 +49,7 @@
>>  #include <asm/errno.h>
>>  #include <asm/uaccess.h>
>>
>> -#define KPROBE_HASH_BITS 6
>> +#define KPROBE_HASH_BITS 9
>>  #define KPROBE_TABLE_SIZE (1 << KPROBE_HASH_BITS)
> 
> 

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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