Subject: Re: [patch] scheduler: rebalance_tick interval update
From: Darren Hart <darren@dvhart.com>
To: Nick Piggin <piggin@cyberone.com.au>
Cc: lkml <linux-kernel@vger.kernel.org>
In-Reply-To: <4199957C.1020804@cyberone.com.au>
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Date: Tue, 16 Nov 2004 07:56:42 -0800
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> >This example didn't take into account:
> >	unsigned long j = jiffies + CPU_OFFSET(this_cpu);
> >Which, even if the last_balance's were equal and intervals were the same
> >(unlikely since each CPU has it's own domain and the intervals are
> >adjusted independently?),
> >
> 
> That is correct.
> 
> > would prevent them from both running on the
> >same timer tick.  So I don't think this example is accurate.  On the
> >other hand, even if it was valid, I would prefer this to running the
> >load_balance code on the same CPU and domain several ticks in a row
> >(which obviously accomplishes nothing).
> >
> 
> It is valid because the CPU_OFFSET basically just adds a constant amount
> to each CPU's 'jiffies'. My example took that into account already and
> used the absolute jiffies value. If that doesn't convince you, pretend
> that the delay were to occur to CPU0, whos CPU_OFFSET == 0.


Heh, I thought of exactly that case when I rolled out of bed this
morning.  Funny how that happens sometimes, you can be sitting in front
of the code and make what you think is a perfectly valid analysis and
then wake up the next morning with no context at all and realize you
were wrong.  That happens to me anyway :-), I can't be the only one...

Nevertheless, that is a pretty unlikely corner case.  They double up
only when last_balance+interval is the same and one of the CPUs involved
is CPU 0.

> 
> 
-- 
Darren Hart <darren@dvhart.com>

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