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

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Date:	Thu, 15 Nov 2007 19:38:02 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	davem@...emloft.net
Subject: Re: [NET]: rt_check_expire() can take a long time, add a
 cond_resched()

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... 


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