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