linux-kernel - Re: [NET]: rt_check_expire() can take a long time, add a cond

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Date:	Fri, 16 Nov 2007 05:12:21 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	davem@...emloft.net
Subject: Re: [NET]: rt_check_expire() can take a long time, add a cond_resched()

Eric Dumazet a écrit :
> Arjan van de Ven a écrit :
>> On Thu, 15 Nov 2007 04:01:48 GMT
>> Linux Kernel Mailing List <linux-kernel@...r.kernel.org> wrote:
>>
>>> Gitweb:
>>> http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=d90bf5a976793edfa88d3bb2393f0231eb8ce1e5 
>>>
>>> Commit:     d90bf5a976793edfa88d3bb2393f0231eb8ce1e5 Parent:
>>> 66ba886254edbbd9442d30f1eef6f6fb0145027d Author:     Eric Dumazet
>>> <dada1@...mosbay.com> AuthorDate: Wed Nov 14 16:14:05 2007 -0800
>>> Committer:  David S. Miller <davem@...emloft.net>
>>> CommitDate: Wed Nov 14 16:14:05 2007 -0800
>>>
>>>     [NET]: rt_check_expire() can take a long time, add a
>>> cond_resched()     On commit 39c90ece7565f5c47110c2fa77409d7a9478bd5b:
>>
>>>     When the IP route cache is big, rt_check_expire() can take a long
>>> time to run.  (default settings : 20% of the hash table is scanned at
>>> each invocation)
>>>         Adding cond_resched() helps giving cpu to higher priority 
>>> tasks if
>>>     necessary.
>>>         Using a "if (need_resched())" test before calling
>>> "cond_resched();" is necessary to avoid spending too much time doing
>>> the resched check. 
>>
>> int __sched cond_resched(void)
>> {
>>         if (need_resched() &&  .....
>>
>> somehow I wonder why the second if() is useful at all; it's another
>> spot for a branch predictor to miss... and a void function call is
>> really really cheap... 
> 
> Its not that cheap. The ChangeLog included my own numbers, on a Pentium 
> M machine. (i686, 1.6 GHz, 1.5 GB ram)
> 
> Without "if (need_resched())" (so calling need_resched() X.XXX.XXX 
> times), each run takes 88ms
> 
> With the extra check (and *much* less function calls), each run takes 25ms
> 

Looking at cond_resched(), I think the extra cost comes from
"mov %esp,%edx ; and $0xffffe000,%edx" (current_thread_info())

I dont have oprofile numbers yet, but I suspect CPU may have some delays
to compute this pointer value, since %esp is probably 'busy' because
of the preceding "call"

(In the case the "if (need_resched())" is done in rt_check_expire(),
compiler moves this pointer computation (current_thread_info()) out of the loop)

c055f926 <cond_resched>:
c055f926:       89 e2                   mov    %esp,%edx
c055f928:       81 e2 00 e0 ff ff       and    $0xffffe000,%edx
c055f92e:       8b 42 08                mov    0x8(%edx),%eax
c055f931:       a8 04                   test   $0x4,%al
c055f933:       74 1a                   je     c055f94f <cond_resched+0x29>
c055f935:       f6 42 17 10             testb  $0x10,0x17(%edx)
c055f939:       75 14                   jne    c055f94f <cond_resched+0x29>
c055f93b:       83 3d 00 80 7c c0 01    cmpl   $0x1,0xc07c8000
c055f942:       75 0b                   jne    c055f94f <cond_resched+0x29>
c055f944:       e8 2b 80 bb ff          call   c0117974 <__cond_resched>
c055f949:       b8 01 00 00 00          mov    $0x1,%eax
c055f94e:       c3                      ret
c055f94f:       31 c0                   xor    %eax,%eax
c055f951:       c3                      ret
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