linux-kernel - Re: [patch 00/17] CFS Bandwidth Control v7.1

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Date:	Thu, 7 Jul 2011 16:51:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Hu Tao <hutao@...fujitsu.com>
Subject: Re: [patch 00/17] CFS Bandwidth Control v7.1


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Thu, 2011-07-07 at 13:23 +0200, Ingo Molnar wrote:
> > 
> > The +1.5% increase in vanilla kernel context switching performance is 
> > unfortunate - where does that overhead come from?
> 
> Looking at the asm output, I think its partly because things like:
> 
> @@ -602,6 +618,8 @@ static void update_curr(struct cfs_rq *c
>                 cpuacct_charge(curtask, delta_exec);
>                 account_group_exec_runtime(curtask, delta_exec);
>         }
> +
> +       account_cfs_rq_runtime(cfs_rq, delta_exec);
>  }
> 
> 
> +static void account_cfs_rq_runtime(struct cfs_rq *cfs_rq,
> +               unsigned long delta_exec)
> +{
> +       if (!cfs_rq->runtime_enabled)
> +               return;
> +
> +       cfs_rq->runtime_remaining -= delta_exec;
> +       if (cfs_rq->runtime_remaining > 0)
> +               return;
> +
> +       assign_cfs_rq_runtime(cfs_rq);
> +}
> 
> generate a call, only to then take the first branch out, marking that
> function __always_inline would cure the call problem. Going beyond that
> would be using static_branch() to track if there is any bandwidth
> tracking required at all.

Could jump labels be utilized perhaps?

Thanks,

	Ingo
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