linux-kernel - Re: [tg_shares_up rewrite v3 09/11] sched: demand based update_cfs

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Date:	Fri, 12 Nov 2010 17:01:37 -0800
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Pierre Bourdon <pbourdon@...ellency.fr>,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Karl Rister <kmr@...ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [tg_shares_up rewrite v3 09/11] sched: demand based update_cfs_load()

On Fri, Nov 12, 2010 at 2:53 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Thu, 2010-11-11 at 19:24 -0800, Paul Turner wrote:
>> +#ifdef CONFIG_FAIR_GROUP_SCHED
>> +       cfs_rq->load_unacc_exec_time += delta_exec;
>> +       if (cfs_rq->load_unacc_exec_time > sysctl_sched_shares_window)
>> {
>> +               update_cfs_load(cfs_rq);
>> +               update_cfs_shares(cfs_rq, 0);
>
> Why not:
> +               cfs_rq->load_unacc_exec_time -= sysctl_sched_shares_window;
>
> (although you probably want to read the sysctl value into a local
> variable using ACCESS_ONCE() and use that for both cases).
>

I think this is hard to do in a clean fashion (without a strange arg
to update_cfs_load).

I'm also not sure it's worth synchronizing on a shares_window change
since, when we trigger an update from update_curr() is independent of
folding that time into the load average anyway.
(Note even: using sysctl_sched_shares_window is already a larger
window than update_cfs_load will normally fold at since we fold at
window/2 after the initial period, it just sets a grace period on
computation without updates in the busy case.)

>> +       }
>> +#endif
>>  }
>>
>>  static void update_curr(struct cfs_rq *cfs_rq)
>> @@ -713,6 +724,7 @@ static void update_cfs_load(struct cfs_r
>>         }
>>
>>         cfs_rq->load_stamp = now;
>> +       cfs_rq->load_unacc_exec_time = 0;
>
> and drop this one?
>
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