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Date: Thu, 7 Jul 2011 16:51:59 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Turner <pjt@google.com>, linux-kernel@vger.kernel.org,
        Bharata B Rao <bharata@linux.vnet.ibm.com>,
        Dhaval Giani <dhaval.giani@gmail.com>,
        Balbir Singh <balbir@linux.vnet.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
        Srivatsa Vaddagiri <vatsa@in.ibm.com>,
        Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
        Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>,
        Pavel Emelyanov <xemul@openvz.org>, Hu Tao <hutao@cn.fujitsu.com>
Subject: Re: [patch 00/17] CFS Bandwidth Control v7.1
Message-ID: <20110707145159.GD19193@elte.hu>
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* Peter Zijlstra <a.p.zijlstra@chello.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