linux-kernel - Re: [RFC PATCH 14/14] sched: implement usage tracking

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Message-ID: <1331737268.18960.125.camel@twins>
Date:	Wed, 14 Mar 2012 16:01:08 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org,
	Venki Pallipadi <venki@...gle.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Mike Galbraith <efault@....de>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ben Segall <bsegall@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 14/14] sched: implement usage tracking

On Tue, 2012-03-13 at 17:57 +0100, Vincent Guittot wrote:
> >  struct sched_avg {
> >        u64 runnable_avg_sum, runnable_avg_period;
> >        u64 last_runnable_update, decay_count;
> > +       u32 usage_avg_sum;
> 
> Why usage_avg_sum is 32bits whereas runnable_avg_sum and
> runnable_avg_period are 64bits long ? You are doing the same
> computation on these 3 variables. Only the computation need to be done
> in 64bits but the result could be saved in 32bits ? 

Since you can never use more than 100% of cpu time, usage_avg_sum is
bound to 100% of the period, which (assuming your period < ~4s) should
thus still fit in the ~4s u32 provides.

Runnable otoh is not bound by that and thus we cannot guarantee the
value stays within the ~4s value range.
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